
-Paul G. Crumley

-Information Technology Center
-Carnegie-Mellon University
-Pittsburgh, PA 15213

-GContents


-G Introduction ........................................... 1

-G Requirements ........................................... 2

-G Description of Modification ............................ 3

-G Parts List ............................................. 4

-G Modification Instructions .............................. 7

-G Debugging Tips ........................................ 22

-G Special Thanks ........................................ 24

-G Card Map .............................................. 25

-G Feedback .............................................. 26

-Contents	ii


-GE-INTRODUCTIONHF-


-This document provides a set of instructions for modifying -the IBM PC-Jr Internal Diskette Drive Adapter to allow the -connection of a second diskette drive to the PC-Jr.

-This modification requires alterations to the diskette -adapter which will certainly void your warranty.  Though I -have tested this modification extensively and have not had -any reported failures to date (yes, some people have had -problems but so far all the diskette drives have ultimately -worked) there are no guarantees.  Also, this modification -should be treated as IBM Internal Use Only.  I believe that -it would be inappropriate to pass this on to the outside -world.  This modification is NOT supported in any formal -manner and is NOT endorsed or approved by anyone or -anything.  If you have any questions about the use of this -modification please direct your questions to me, Paul -Crumley, CMUPGC at PGHVM1.

-Enough of this, you get the idea.  The adventure begins!

-Introduction	1


-	IBM PC-Jr Internal Diskette Drive

-GE-REQUIREMENTSHF-


-HFþ Hardware

-	HF-	PC-Jr

-	HF- 1 internal diskette drive and adapter

-HFþ Software

-	HF- DOS 2.1 or higher

-	HF- A diskette with a copy of the file 2NDDISK.DBG

-	This is not required.  This is only needed if you
-	want the patches to DOS to be done automatically
-	for you.	This must be	downloaded in	BINARY
-	format.  I uploaded it using E78COPY with the /B
-	option.

-Requirements	2

-GE-DESCRIPTION OF MODIFICATIONHF-


-By following this set of instructions you will make the -following modifications to the IBM PC-Jr Diskette Drive -Adapter:

-HFþ Make a new cable to connect the second diskette drive
-	to the PC-Jr diskette adapter.

-HFþ Make the following modifications to the diskette
-	adapter:

-	HF- Add a latch that will provide the drive select
-	logic for the second diskette drive.

-	HF- Add a buffering chip that will drive the needed
-	signals on the cable that will attach to the second
-	diskette drive.

-HFþ Assemble the second diskette drive in its case and
-	attach it to the PC-Jr diskette adapter.

-HFþ Patch the boot record of your DOS diskette so the
-	second diskette drive is recognized and used.

-This modification should be compatible with almost all -software.  The PC-Jr BIOS is designed to access up to 3 -diskette drives.  This hardware modification allows I/O -ports that are already defined for BIOS to be used to -enable the second diskette drive.  Unfortunately, in order -for the BIOS to know about the existence of the second -diskette drive, more hardware is needed than this simple -modification.  It would be very difficult to modify the -present diskette adapter to supply this hardware solution.  -For that reason I have chosen to have you patch your DOS -boot record so that BIOS will be told to use the second -diskette drive.  Since the support for the second diskette -is contained in the BIOS and not in DOS there should be -very few problems with compatibility.

-If you desire, you can also patch the FORMAT command that -comes with DOS 2.1 so that all diskettes that you format -will have this patch applied automatically.

-Description of Modification	3



-GE-PARTS LISTHF-


-The modification requires the following parts:

-HFþ 1 74LS175.

-HFþ 1 7438.

-HFþ 1 .1 uF disk capacitor rated at 15 volts or greater.

-HFþ 1 16 pin socket for the 74LS175.  Use solder tail
-	rather than Wire-Wrap sockets.  Get the best sockets
-	you can find.  Gold plated if they are available.

-HFþ 1 14 pin socket for the 7438.  As above.

-HFþ 6 ft Wire-Wrap wire.  30 or 32 gauge Wire-Wrap wire
-	works well for these connections.

-HFþ 3 ft 34 conductor ribbon cable.  The grey kind if
-	fine.  No need for the expensive multicolored stuff.

-HFþ 2 ft 34 conductor edge connectors.  If you look at
-	the original cable that connects the PC-Jr diskette
-	adapter to the diskette drive the edge connector is at
-	the end that attaches to the diskette drive.

-HFþ 1 ft 34 conductor Berg connector.  If you look at the
-	original cable that connects the PC-Jr diskette adapter
-	to the diskette drive the Berg connector is at the end
-	that attaches to the diskette adapter.

-HFþ solder.  Use the thinnest solder you can find.

-HFþ masking tape.  Just a couple of inches is all you
-	need.

-HFþ glue or silicone bathtub sealer.  These may be needed
-	to hold some of the wires down on the card so they are
-	not touching the case or other parts.  The silicone
-	bathtub sealer usually works best.

-HFþ 1 double sided diskette drive.  See below.

-HFþ 1 enclosure and power supply for the diskette drive.
-	See below.

-HFþ 1 double sided diskette.  This is used to make a new,
-	patched version of the DOS boot diskette.

-The tools required include:

-HFþ soldering iron.  (25 Watts if you have only one.  see
-	below)

-Parts List	4


-HFþ unsoldering bulb.

-HFþ diagonal snippers.

-HFþ needle-nose pliers.

-HFþ wire strippers that work with the small gauge wire.

-HFþ flat blade screwdrivers -- a small and a medium sized
-	one.

-HFþ vice or clamps to hold the card while you work.

-HFþ magnifying glass.

-HFþ old toothbrush.

-HFþ a ruler.

-HFþ scissors that can cut the ribbon cable to the correct
-	length.

-HFþ a knife such as an X-acto modelling knife should be
-	fine.

-HFþ a pen that will write on the masking tape.

-HFþ vise or other pressing device to use to squeeze the
-	connectors onto the ribbon cable.

-HFþ any tools that will be needed to assemble the case and
-	power supply you are going to use.  Things like nut
-	drivers and pliers might be useful.

-Prose descriptions of some of the above:

-The choice of soldering iron is a very personal matter.  I -have three different ones, 12, 25 & 50 Watts, so I can -match the iron to the job at hand.  If you plan on having -only one, a 25 Watt iron is probably the best choice if it -has a sharp, clean tip.  A 12 Watt iron with a sharp tip is -probably the best to use for the soldering process while -the 50 Watt monsters should be avoided for soldering but -can come in handy for unsoldering parts.

-One of the inexpensive unsoldering bulbs available at -places like Radio Shack will come in quite useful when you -must remove the solder from some holes along one side of -the card to install the sockets.  More expensive and exotic -unsoldering tools should not be needed for this task.

-When selecting the sockets I recommend using the best you -can find.  Mechanical connections are the first place to -look when there is trouble.  Use good parts and save -yourself some grief later on.

-Parts List	5



-You should purchase an extra 74LS175 and 7438 just in case -one of these parts is damaged in shipping or when it is -being installed.  Extra sockets might also be a good idea.

-These parts should be available from a number of -companies.  A good place to look is in the back of -magazines such as Byte or Radio Electronics.  Many of these -companies have a minimum order so it might be worth looking -around for a place where you can get all of the parts you -need so you will be above the minimum order.

-There are a number of companies that make IBM PC compatible -drives.  TEAC, Tandon, Qume, and CDC come to mind quickly.  -Basically, if the diskette drive works in a PC or PC/XT you -should have no problem using it with the PC-Jr.  Watch out -for Apple and Atari compatible models.

-When selecting an enclosure and power supply for the -diskette drive make sure the power supply has both 5 and 12 -volts available.  You might be able to find the diskette -drive and the enclosure/power supply as a "package deal" if -you shop around a little bit.  I would recommend looking -for a enclosure and power supply that is sold as a unit -rather than buying them separately.  If they are bought as -a unit you can be sure everything will fit together well -and there should be fewer "surprises."

-Make sure that the enclosure and power supply come with the -proper power cable needed to attach to the diskette drive -and also make sure that a 120 volt wall plug cable is -included.  You should ask to make sure that the enclosure -and power supply are fused and if they are U. L.  and CSA -approved.  Anything that has 120 volts going into it can be -dangerous so buy a quality enclosure and power supply and -save yourself future problems.

-Once all of this is collected you are ready to proceed.

-Parts List	6



-GE-MODIFICATION INSTRUCTIONSHF-


-This modification is carried out in a series of steps.  -There are checkpoints along the way to help isolate errors -to a small number of connections thus providing a more -manageable debugging process if something goes wrong.

-This modification is geared to someone that has experience -soldering and working with electronic parts.  If you don't -feel comfortable with a soldering iron you may like to ask -a friend to help you with the soldering steps of this -modification.  If you work slowly and carefully you should -set aside two to three hours to complete this -modification.

-You also must know how to identify pin 1 of IC chips.  If -you look at the top of chip you will see a notch or dot at -one end.  If you point that end of the chip towards you, -the closest pin on the right is pin 1.  The pin numbers go -down the right side then back the left.  The numbering for -16 and 20 pin chips is similar.  The following diagram -should help:


-	---------------
-	/	/ !
-	/	/ !
-	/	#	/ /!
-	/	/ ! !
-	/	t	/ /! !
-	/	r	/ ! ! !  <---- pin 7
-	/	a	/ /! ! !
-	/	P	/ ! ! !
-	/	/ /! ! !
-	/	__	/ ! ! !
-	/	/ /	/ /! ! !
-	/	/ /	o / ! ! !
-	------------ / /! ! !
-	!	! ! ! !
-	------------ /! ! !
-	!	! !
-	!	! !
-	!	!
-	pin 14 ----> !	! <---- pin 1


-The method I present has the nice property of being easily -removed.  Nothing is cut.  Parts are only added to the -diskette adapter card.  If you must remove this -modification you can restore the card to its original -state.

-Modification Instructions	7



-Get out your soldering iron, here we go!

-HF1.  Read ALL the instructions before starting!!!!!

-HF2.  The second time through this list check off each step
-	as it is completed so that you are certain that all the
-	the steps are done properly.

-HF3.  Make and test a new cable.

-	HFa.  Make sure the diskette drive and adapter work.
-	This shouldn't be a problem with old PC-Jrs.  You
-	know they work if you use them all the time.  If
-	you just bought a new PC-Jr I would recommend using
-	it for a couple of days to make sure everything
-	works correctly.  This modification will void your
-	warranty on the diskette adapter and maybe on the
-	whole PC-Jr.  By running the PC-Jr for a few days
-	you	should flush out most	of	the	possible
-	manufacturing defects.  Do try to use the machine
-	for at least 10 to 15 hours as 10 minutes of
-	service won't bring out the problems too well.
-	After testing, turn off the PC-Jr and remove the
-	power cord from the wall.

-	HFb.  Open the case of the PC-Jr by gently prying up at
-	the rear of the seam around the top of the case.
-	The back of the cover will will lift up while the
-	front of the cover is held in place by a lip
-	attached to the front of the PC-Jr case.  After the
-	back of the cover has started to lift up pull the
-	cover towards the rear of the PC-Jr.  This will
-	free the front of the cover and the cover can be
-	removed.

-	HFc.  Locate the Diskette adapter by finding the wide
-	grey cable that is attached to the diskette drive.
-	Follow that cable to towards the left of the case.
-	The diskette adapter is at the left end of this
-	cable.

-	HFd.  Remove the diskette adapter by grasping both the
-	front and rear of the card at the same time and
-	gently pulling the card up.  Pull gently as the
-	card is still attached to the cable.

-	HFe.  Remove the cable from the diskette adapter by
-	firmly gripping the connect (not the cable) and
-	pulling the connector straight away from the card.
-	Be careful to not bend the small pins on the card
-	when you remove the cable.

-	HFf.  Set the diskette adapter aside in a safe place for
-	now.

-	HFg.  Remove the other end of the cable from the diskette

-Modification Instructions	8



-	drive using the same technique as was used to
-	remove it from the diskette adapter.  You don't
-	have to be quite as gentle as with the card since
-	there are no little pins to bend but be gentle
-	anyway.

-	HFh.  The new cable will look something like this:


-	__	__
-	!  !	!  !
-	_	!  !	!  !
-	! !----------!__!--------------------------------------<===>--!__!
-	! !
-	!_!<-- 8" -->!  !<------------------- 2'-2" ----------------->!


-	HFi.  Using the old diskette cable as a model make the
-	left part of the new cable to the same dimensions
-	as the old cable while leaving the excess cable
-	hanging off toward the right.  The connector for
-	the second diskette drive will be placed at the
-	right end of this cable.  The left most connector
-	will	be installed	first.  This	is the	Berg
-	connector not one of the edge connectors that
-	attaches to the diskette drives.

-	HFj.  If you look at the connector carefully you should
-	be able to find one end of the connector marked
-	with pin 1 or a small arrow will be etched into the
-	connector shell.  Line this end of the connector up
-	with the striped side of the cable.  This will help
-	you keep the connectors oriented correctly.  The
-	connectors are attached to the cable by first
-	lining up the bulges in the insulation of the cable
-	with the teeth of the connector.  The connector
-	should have a channel the correct width for the
-	cable	to fit	in making the	bulges line up
-	correctly.

-	HFk.  After the cable is positioned correctly place the
-	back part of the connector over the cable.  This is
-	a bit hard to describe but I think it should be
-	pretty easy to figure out	how all this fits
-	together.  Place this assembly in a vise or other
-	type of clamp and slowly press the back of the
-	connector into place.  The teeth will be cutting
-	into the cable's insulation.  Go slowly and be
-	careful	that	everything	is	going	together
-	properly.

-	HFl.  Repeat this operation for the second connector
-	making sure you have the pin 1 ends of both
-	connectors pointing in the same direction.  This
-	will be the edge connector that is attached to the
-	middle section of the cable.

-Modification Instructions	9



-	HFm.  Now, using the old cable as a model, make a crease
-	in the new cable so it will fit in the PC-Jr
-	correctly.

-	HFn.  Test this new cable by plugging it into the
-	diskette drive and the diskette adapter, replacing
-	the diskette adapter in the PC-Jr, plugging the
-	PC-Jr into the wall, and turning on the PC-Jr.
-	There is no need to put on the top completely.
-	Just	set it on the	PC-Jr so you will	not
-	accidentally	touch the	power	supply or drop
-	anything inside the PC-Jr while the power is turned
-	on.  After you have verified the cable works turn
-	off the power and unplug the PC-Jr.  Remove the
-	diskette adapter and this new cable and prepare to
-	attach	the connector for the	second diskette
-	drive.

-	HFo.  If the PC-Jr's diskette didn't work correctly with
-	the	new cable try the old one to make sure
-	everything is hooked up correctly.  If the old
-	cable works, carefully inspect the new cable to
-	make sure that	the	connectors	are	attached
-	correctly.  look for twisted or misaligned parts of
-	the ribbon cable in the area where the teeth cut
-	through the insulation.  You should be able to pry
-	the backs off the connectors if you exercise some
-	care.  With three feet of ribbon cable you can cut
-	the used section off and try again if you have to.

-	HFp.  This	is	perhaps the	hardest	part	of	the
-	modification task so take	your time and work
-	carefully.	To	make the	drive select signals
-	connect to the correct pins you have to make two
-	slits in the cable	forming three sections of
-	conductors.  The center section is twisted one half
-	turn and then this resulting group of 34 conductors
-	is attached to the rightmost edge connector.  The
-	completed connection should look something like
-	this:


-	--
-	===========================================!  !
-	1	! 1!
-	!  !
-	!  !
-	!  !
-	9	! 9!
-	===-------========!  !
-	10\	/	16 !10!
-	cable	11 \	/	15 !11!  connector
-	\ /	!  !
-	!	!  !
-	!	!  !

-Modification Instructions	10



-	/	!  !
-	15 /	\	11 !15!
-	16/	\	10 !16!
-	===-------========!  !
-	17	!17!
-	!  !
-	!  !
-	!  !
-	34	!34!
-	-------------------------------------------!  !
-	--

-	HFq.  If you are familiar with the PC's diskette cable
-	you have a better picture of what you are making
-	than I can represent here.  If available, use the
-	PC cable as a guide.  Starting with the conductor
-	for pin 1 count over 9 conductors and slit the
-	ribbon	cable	between	the	ninth	and	tenth
-	conductors.  Make the slit about 6 inches long
-	being careful to not damage the insulation.  Count
-	over 7 more conductors and slit the ribbon cable
-	between conductors 16 and 17.  COUNT CAREFULLY.

-	HFr.  The cable will now be in three sections.  Twist the
-	center section and press the last edge connector
-	into place leaving the outer sections of ribbon
-	cable flat.  It can take a bit of work to get
-	everything into alignment before the connector is
-	pressed together so take your time and get it
-	right.

-	HFs.  Test the cable again in the same manner as you
-	tested it above.

-HF4.  Modify diskette adapter card.

-	HFa.  Clean away some workspace, put down some old
-	newspaper if it is needed to protect your work
-	surface, and plug in your soldering iron.  Get a
-	drink (alcohol and hot soldering irons don't mix!)
-	and relax while you are waiting for the iron to
-	heat up.

-	HFb.  Set the adapter in front of you oriented as is
-	shown in the card map located near the back of this
-	document.  Along the right side of the adapter you
-	should see two vertical rows of solder dots.  These
-	dots are holes that are filled with solder.  The
-	new parts that are needed for the second diskette
-	drive will be placed in these holes.

-	HFc.  Before the sockets for the new parts can be
-	installed, the holes that the pins for these socket
-	will use must be cleaned out.  This is done with
-	the unsoldering bulb.  Mount the adapter in your
-	vise or other type of clamp so that it is fairly

-Modification Instructions	11



-	vertical rather than flat.  If the two rows of
-	solder-filled holes are at the top it might be
-	easier for you to do the next few steps.  The
-	procedure for removing the solder from the holes
-	is:

-	HF1) Grab the unsoldering	bulb with	one hand.
-	(usually the left of right-handed persons, the
-	right for left-handed ones.)

-	HF2) Hold the hot soldering iron with the other
-	hand.  (yes, you have to put down that drink
-	now!)

-	HF3) Squeeze the bulb and place its tip near one of
-	the solder-filled holes.

-	HF4) Place the tip of the soldering iron on the same
-	hole, but on the opposite side of the card.

-	HF5) Wait for the solder to melt and then quickly
-	release the bulb thus sucking the solder out of
-	the hole.

-	HF6) You may have to repeat the above two steps for
-	some holes to get them cleaned out.

-	HF7) Move on to the next hole and repeat the above
-	three steps.

-	HF8) Continue this operation until all the holes
-	along that side of the adapter are clean.

-	HFd.  Use the magnifying glass and CAREFULLY check to
-	make sure that no small globs of solder are left on
-	the card or have slid under any of the other
-	chips.  Use the old toothbrush to scour away any
-	solder deposits that are stuck on the adapter.
-	When all the deposits are gone you may continue.

-	HFe.  It is now time to mount the sockets for the new
-	chips.  Look at the card map again.  It shows that
-	the 7438, a 14 pin chip, should be located near the
-	top of the card and the 74LS175, a 16 pin chip,
-	should be located at the bottom of the card.  Get
-	the 14 and 16 pin socket.  If you look at the
-	sockets closely you should see some type of marker
-	that is used to indicate which pin is pin 1.
-	Usually the pin 1 corner is bevelled.  Using the
-	card map as a guide place the make sure pin 1 is in
-	the proper corner.  When you place the sockets,
-	position them at the extreme ends of the rows of
-	holes.

-	HFf.  Position the .1uF disk capacitor between the two
-	chips so that one of the leads in in one row of

-Modification Instructions	12



-	holes and one lead is in the other.  You may have
-	to bend the leads around a bit to make them fit.
-	Be gentle since they can break off if they are
-	handled too roughly.  You should have everything
-	placed something like this:


-	----------------------------------------------------------------
-	!	!
-	!  O	.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .	!
-	!	---------------------	---	------------------------	!
-	!	!	!	! !	!	!	!
-	!	!>	!	! !	!>	!	!
-	!	!o	!	! !	!o	!	!
-	!	--------------------	---	-----------------------	!
-	!	.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .	!
-	!	!
-	!	!
-	!	14 pin	16 pin	!
-	!	Socket for	.1 uF	Socket for	!
-	!	7438	Capacitor	74LS175	!
-	!	!


-	HFg.  Use a piece of masking tape to hold these parts in
-	place when the card is turned over to be soldered.

-	HFh.  Turn the card over.  If something shifts because
-	the masking tape was not tight enough go back and
-	make sure all the parts are pressed firmly against
-	the card.

-	HFi.  Solder each of the sockets and the capacitor to the
-	card.  For now, leave about 1/4 inch of the .1 uF
-	Capacitor sticking out the back of the adapter and
-	cut off the rest.

-	HFj.  Go back with the magnifying glass and check for any
-	solder globs that could be causing shorts between
-	the pins of the chip.

-	HFk.  If your pen will write on the adapter make a mark
-	at the pin 1 position of the two sockets.  This
-	will help you find the correct pin position when
-	you are connecting wires to the sockets.  If your
-	pen won't write on the adapter use a piece of
-	masking tape.

-	HFl.  Now you finally get to start connecting the new
-	chips to the circuit.  These connections should be
-	made on the back of the adapter.  You should be
-	VERY careful when locating the pins since the pins
-	are all in different places when view from the back
-	than they are when viewed from the front.  Recheck
-	each connection as it is made to make sure you have

-Modification Instructions	13



-	the right pins connected.

-	HFm.  Connect pin 8 of the 74LS175 chip, IC5 on the chip
-	map, to one lead of the .1 uF disk capacitor.
-	Connect this same lead to pin 7 of the 7438 socket,
-	IC1 on the card map, and pin 8 of the 74LS175
-	socket, IC2.	Make	these connections	to	the
-	capacitor as close to the card as possible.

-	HFn.  Connect pin 16 of IC5 to the other lead of the .1
-	uF disk capacitor.  Connect this same lead to pin
-	14 of IC1 and pin 16 of IC2.  Now cut off any extra
-	length of the leads on the .1 uF capacitor.

-	HFo.  Connect pin 1 of IC5 to pin 1 of IC2.  Connect pin
-	9 of IC5 to pin 9 of IC2.  You are now done with
-	IC5.

-	HFp.  Locate the 74LS245 at position IC3.  Connect pin 17
-	of IC3 to pin 4 of IC2.  You are now done with
-	IC3.

-	HFq.  Connect pin 2 of IC2 to pins 1, 2, 4 & 5 of IC1.
-	You are now done with IC2.

-	HFr.  Cut two pieces of wire about 12 inches long.
-	Solder one end of one of these wires to pin 3 of
-	IC1 and solder one end of the other wire to pin 6
-	of IC1.  You are now done with IC1.

-	HFs.  At this point all connections to the chips should
-	be completed.  IC4 was not touched, it was only
-	labeled to help you find IC3 and IC4.  You should
-	have made 4 connections to IC5 while IC3 had only
-	one connection. IC1 and IC2 have many unused pins.
-	Check to make sure that the following pins are have
-	no connections:

-	HFþ	IC1 -- pins 8, 9, 10, 11, 12 & 13

-	HFþ	IC2 -- pins 3, 5, 6, 7, 10, 11, 12, 13, 14, &
-	15

-	HFt.  Now we will make the connection to the ribbon cable
-	connector.  Orient the adapter so that the chips
-	are facing the table	and the	diskette cable
-	connector is in the far right corner.  You should
-	see something like this:


-	-------------------------------------------------------------
-	!
-	o-o---o-o-o-o-o-o-o-o-o-o-o-o-o-o	!
-	!
-	o o o o 5 o 7 o o o o o o o o o o	!
-	!

-Modification Instructions	14



-	o o	o	o	o	o	!
-	!
-	o o o o o o o o	o o o o o o o	!
-	!


-	HFu.  Notice that the far row of pins are all connected
-	together and the third pin from the left in that
-	row is missing.  The pins marked 5 and 7 are were
-	we are going to attach the signals we need to
-	control the second diskette drive.  Connect one of
-	the 12 inch wires to pin 5 and connect the other to
-	pin 7.  It doesn't matter which wire goes to 5 and
-	which goes to 7.  What does matter is that you make
-	the connections to pins 5 and 7.  You should cut
-	off most of the excess wire before making these
-	connections so that you don't have these wires
-	dangling	about when you are done	with	this
-	modification.

-	HFv.  Go back and check all of these connections again to
-	make sure they are correct.  Incorrect connections
-	can lead to ruined chips so be thorough.  Check
-	that the proper pins are connected and also check
-	the solder connections to be certain that none of
-	the adjacent pins are shorted to one another.  Look
-	for wires where much of the insulation has been
-	melted away from the pin connection.  If there is a
-	danger of the bare wire touch some nearby pin or
-	one of	the traces	on	the	board	redo the
-	connection.

-	HFw.  Test the adapter again as you have done in the past
-	to make sure it is still operating correctly as a
-	single diskette adapter.  If the PC-Jr works fine
-	continue.  If things don't go as planned look for
-	incorrect connections and soldering mistakes.  The
-	section with debugging tips may be helpful.

-	HFx.  Now get the 7438 and 74LS175.  Make sure you have
-	identified pin 1 for both chips and insert them in
-	their respective sockets.  the 7438 goes in socket
-	IC1 and the 74LS175 goes in socket IC2.  Be gentle
-	while inserting the pins in the socket.  The pins
-	are rather fragile and bend quite	easily.  If
-	disaster should strike and one of the pins does get
-	bent don't panic.  Simply use a small screwdriver
-	to pry the chip out of the socket,	use the
-	needle-nose pliers	to straighten	the	injured
-	appendage, and try again.

-	HFy.  With IC1 and IC2 installed test the adapter once
-	again.  If everything works correctly continue.  If
-	there are problems check the connections and solder
-	joints for problems.

-Modification Instructions	15



-	HFz.  When you are convinced the adapter is working
-	correctly you can unplug the soldering iron and set
-	it aside.  Hopefully it won't have to be plugged in
-	again for this project.

-	HFaa. Use some of glue or silicone bathtub sealer to
-	fasten the loose wires securely to the adapter.
-	This will help prevent accidental damage to the
-	connections by catching the wires on something and
-	pulling one of the wires loose.

-HF5.  Install the diskette in its enclosure and connect the
-	cables.

-	HFa.  It is difficult for me to give specific details on
-	installing the diskette drive in the enclosure and
-	connecting the power supply since I don't know what
-	kind of diskette drive or enclosure you will have.
-	Some general things that must be done include:

-	HFþ	Make sure	the diskette drive	is securely
-	mounted so that the connections will be less
-	likely to vibrate loose with use.

-	HFþ	Make sure that pin 1 on the ribbon cable is
-	connected	to the pin 1 end of	the edge
-	connector on the diskette drive.  Check this
-	twice just to be sure.

-	HFþ	Make	sure	the power	cable	is connected
-	properly.	If	the	4 conductor plug	came
-	preattached to the power supply this shouldn't
-	be a problem.  If you are using a "homebrew"
-	design be careful, mixing up the connections
-	will have disastrous results.

-	HFþ	Leave room for the ribbon cable to get out of
-	the cabinet.  Try not to bend the cable or
-	pinch	it too much at this could lead to
-	reliability problems later.

-	HFþ	If possible, get an enclosure and power supply
-	as a unit so you know everything will fit
-	together well.

-	HFþ	For safety reasons don't have the power supply
-	plugged into a wall power outlet when you are
-	working in the cabinet with the cover removed.

-	HFb.  After the new diskette drive is mounted and the
-	power cable and ribbon cable are connected you can
-	proceed to the next step.  Keep the cable routing
-	in mind as you install the diskette so you don't
-	have to change things around too much when the
-	covers are installed.

-Modification Instructions	16



-	HFc.  Place the new ribbon cable on the modified diskette
-	adapter card and insert the adapter card in the
-	PC-Jr.

-	HFd.  Attach the new cable to the internal diskette
-	drive.

-	HFe.  Recheck these connections.

-	HFf.  As you have done so many times before test the
-	PC-Jr to make sure it still works as a single
-	diskette system.  Test the PC-Jr first with the
-	second diskette drive's power supply disconnected
-	from	the	wall	power outlet.	If that works
-	correctly turn off the PC-Jr, plug the second
-	diskette drive's power supply into the wall power
-	outlet, turn on the second diskette, and turn on
-	the PC-Jr.  If everything still works correctly
-	continue	on	with	the	final	part	of	the
-	modification, modifying the software to use the
-	second diskette drive.  If there are problems go to
-	the section on debugging tips.

-HF6.  Patch the DOS 2.1 diskette.

-	HFa.  The DOS boot record must be altered so that BIOS
-	will be told that two diskette drives are available
-	rather than just one.  By patching the boot record
-	DOS does know that the BIOS data area has been
-	altered insuring	a	great amount	of software
-	compatibility.

-	HFb.  If these next instructions sound too simple minded
-	and you are offended please accept my apologies.
-	We hardware types like to leave nothing to chance.
-	"Better precisely wrong than vaguely right."

-	HFc.  Using the DISKCOPY command make a new DOS 2.1
-	diskette.  This is done by placing the original DOS
-	diskette	in	the internal diskette drive	and
-	rebooting the system.  After DOS has booted and you
-	get past the time and date questions type "DISKCOPY
-	A:  A:" and press the ENTER key.  The DISKCOPY
-	should prompt you from there.  Just remember that
-	the SOURCE diskette is the original DOS diskette
-	and the TARGET diskette is the new diskette you are
-	making.

-	HFd.  Next copy the DEBUG command to the new diskette.
-	Insert the new diskette in the internal diskette
-	drive and type "COPY B:DEBUG.COM A: /V" then press
-	enter.  When the system asks to have diskette B:
-	inserted USE THE INTERNAL DISKETTE DRIVE and press
-	ENTER.  The DEBUG program is on the Supplemental
-	Programs diskette that comes with DOS 2.1.  You
-	will be asked to place the target diskette in drive

-Modification Instructions	17



-	A:.  Put the new DOS diskette back in the internal
-	drive and press ENTER.  After the COPY command has
-	completed check that DEBUG has been copied by using
-	the "DIR A:" command.

-	HFe.  Now you have a choice to make.  If you can download
-	the file of DEBUG commands that I have sent you as
-	2NDDISK.DBG	you can	use	the Standard	Input
-	facilities of DOS 2.1 to have the new diskette
-	patched for you.  If you can not download the file
-	2NDDISK.DBG you will	have to enter the DEBUG
-	commands yourself.  I will first describe how to
-	use the file 2NDDISK.DBG.

-	HF1) First, you must get the file 2NDDISK.DBG on a
-	diskette.  This file	must be downloaded in
-	BINARY format.  I used E78COPY to upload this
-	command to VM.  Make sure you have a good copy
-	of the file before you do anything.  If you are
-	not sure if your copy is OK check it against
-	the instructions listed for those that can not
-	download files.  They should	be	a perfect
-	match.	(except	the	square	brackets	in
-	2NDDISK.DBG will	be shown as ('s and )'s
-	below.)

-	HF2) You must copy the file 2NDDISK.DBG to the new
-	DOS diskette in the same manner you copied
-	DEBUG.COM.  Use the command "COPY B:2NDDISK.DBG
-	A: /V".

-	HF3) Next, use the command "DEBUG < 2NDDISK.DBG" to
-	run the DEBUG program.  DEBUG will take all of
-	its commands from the file 2NDDISK.DBG and
-	display what it is doing on the screen.  You
-	should see the following scroll by on the
-	screen as DEBUG executes:


-	-L 0100 0 0 1
-	-U 0100 L 5
-	0906:0100 EB2C	JMP	012E
-	0906:0102 90	NOP
-	0906:0103 49	DEC	CX
-	0906:0104 42	INC	DX
-	-A 0100
-	0906:0100 OR BYTE PTR (0410),40
-	0906:0105 JMP 012E
-	0906:0107
-	-W 100 0 0 1
-	-
-	-N FORMAT.COM
-	-L
-	-U 19F8 L 5
-	091A:19F8 EB2C	JMP	1A26
-	091A:19FA 90	NOP

-Modification Instructions	18



-	091A:19FB 49	DEC	CX
-	091A:19FC 42	INC	DX
-	-A 19F8
-	091A:19F8 OR BYTE PTR (0410),40
-	091A:19FD JMP 1A26
-	091A:19FF
-	-N FORMAT2.COM
-	-W
-	Writing 1B00 bytes
-	-Q


-	HF4) If this goes as expected you are done making a
-	new DOS diskette.  A new version of the FORMAT
-	command called FORMAT2 has also been created on
-	your DOS diskette.  If you use this "patched"
-	version of the FORMAT command to make new
-	diskettes the	alterations	for the	second
-	diskette drive will be applied to the boot
-	record automatically.

-	HFf.  Even if you must type in the DEBUG commands
-	yourself you should not have too hard a time
-	altering your DOS diskette so that you can have
-	access to the second diskette drive.

-	HF1) Place the new diskette in the internal drive
-	and type "DEBUG" then press ENTER.

-	HF2) When DEBUG is ready to accept commands it will
-	display a dash ("-") character.  Simply type in
-	the following list of commands.  Remember that
-	the ('s and )'s are square brackets.  After
-	each line is typed press ENTER.  For the blank
-	lines just press ENTER.  The screen should look
-	just like	the script shown above for the
-	automatical DEBUG session.  This should allow
-	you to check your work.

-	L 0100 0 0 1
-	U 0100 L 5
-	A 0100
-	OR BYTE PTR (0410),40
-	JMP 012E

-	W 100 0 0 1
-	N FORMAT.COM
-	L
-	U 19F8 L 5
-	A 19F8
-	OR BYTE PTR (0410),40
-	JMP 1A26

-	N FORMAT2.COM
-	W
-	Q

-Modification Instructions	19



-	HF3) If this works correctly you have just made a
-	new DOS diskette.  A new version of the FORMAT
-	command called FORMAT2 has also been created on
-	your DOS diskette.  If you use this "patched"
-	version of the FORMAT command to make new
-	diskettes the	alterations for	the	second
-	diskette drive will be applied to the boot
-	record automatically.

-	HFg.  You should mark this diskette as a special PC-Jr
-	diskette that will only	work with a modified
-	PC-Jr.  If you try to use this modified diskette on
-	any PC that doesn't have two diskette drives DOS
-	will become rather confused.  There should be no
-	problem moving data with these diskettes, just
-	don't use them to boot the system.

-HF7.  Test your work.

-	HFa.  This is the time to pause for a moment and reflect
-	on your life, say prayers, perform superstitious
-	acts, etc.  This is a very personal step.  You know
-	what you have to do at this time so do it.

-	HFb.  Turn off the PC-Jr and place the modified DOS
-	diskette in the internal diskette drive.  Turn on
-	the power to the new diskette in its cabinet then
-	turn on the PC-Jr.  After DOS boots you should have
-	the use of both drives.  If there are any problems
-	continue on the the next section of debugging
-	tips.

-HF8.  Replace the covers.

-	HFa.  Turn off the power to the PC-Jr and the second
-	diskette drive and remove their power plugs from
-	the wall outlet.

-	HFb.  If you look at the top edge of the right side of
-	the PC-Jr case you will see that IBM thoughtfully
-	left a slot for this cable near the rear of the
-	PC-Jr case.  Fold the cable neatly, lay it in the
-	slot, and replace the PC-Jr's cover.

-	HFc.  Again, it is hard for me to know just how to tell
-	you about installing the cover(s) on your second
-	diskette drive cabinet.  Some general things to
-	keep in mind are:

-	HFþ	Make sure the ribbon cable is not being pinched
-	by any of the covers.

-	HFþ	Make sure the ribbon cable is does not have any
-	kinks or strange twists in it.  If you must,

-Modification Instructions	20



-	make a neat crease in the cable to allow it to
-	make its way out of the cabinet.

-	HFþ	Check the other wiring inside the cabinet to
-	make sure none of the wires or cables will come
-	in contact with any of the moving parts of the
-	diskette drive.

-	HFþ	Try not to block any ventilation slots when
-	routing the cables.

-	HFd.  Well, this is the final test.  Plug the second
-	diskette and the PC-Jr back into the wall power
-	outlet, make sure the modified DOS diskette is in
-	the internal diskette drive, turn them on, and....

-	HFe.  If everything goes well I recommend taking another
-	break, getting another drink and relaxing for a
-	moment before you start to clean up the mess you
-	made.  If there are problems I insist you take
-	another break and get a drink (remember about hot
-	soldering irons!) before continuing.  Many problems
-	look better after a little time away from the
-	project.

-Modification Instructions	21



-GE-DEBUGGING TIPSHF-


-If something doesn't work, experience with the modification -shows that almost all of the problems are with soldering.  -Make sure there are no shorts created by any of your solder -joints.  Check the 74LS175 very carefully as it is easy to -accidentally solder two pins together by accident.  Also -check the wires that attach to the connector to make sure -there are no shorts there.

-After that, recheck all the connections for accuracy.  -Check the connections made on the back side of the card -carefully since it is easy to get the pin numbers mixed up -when you are working on the back of the card.

-If you still have problems send a note.  I will help with -problems on a "when time permits" basis.

-Here are some simple tests you can do to help isolate the -problem.

-HF1.  If nothing happens when you turn on the switch
-	something big is wrong.  The first thing to check is
-	that the PC-Jr is plugged into the wall outlet and that
-	the transformer is still plugged into the back of the
-	PC-Jr.  These connections can get bumped loose with all
-	the movement that goes on.  If these are OK, remove the
-	adapter and try the PC-Jr again.  If everything works,
-	(and it should) try the adapter again.  If the adapter
-	fails like this the most common problems are:

-	HFa.  A solder bridge has formed shorting the power
-	supply.

-	HFb.  One or more chips are installed backward.  These
-	chips destroy themselves when power is applied and
-	short the power supply.

-HF2.  If the original drive doesn't work try using the
-	original cable to check that the new cable is wired
-	correctly.  If the original cable works OK try the new
-	cable without the second drive connected.

-HF3.  If the second drive doesn't work try installing the new
-	drive as the internal drive.  You don't have to
-	physically install the drive, just make the cable
-	connections.  This should test the second diskette
-	drive.

-HF4.  Sometimes either drive will work alone but the two
-	drives will not work when both are connected to the
-	cable.  This usually means that the cable is not
-	properly constructed or there is a problem with the
-	7438 and its wiring.

-Debugging Tips	22



-These tips should give you some idea of where to start -looking if you have a problem.

-Debugging Tips	23


-	IBM PC-Jr Internal Diskette Drive

-GE-SPECIAL THANKSHF-


-Bill Brown provided valuable information regarding the -proper buffering of the diskette control signals.  The -diskette drives require special signal conditioning and I -had neglected this need in my first set of instructions.

-Special Thanks	24


-	IBM PC-Jr Internal Diskette Drive

-GE-CARD MAPHF-


-This diagram points out the important areas of the card to -help aid in the locating of the connection points used in -this set of instructions.


-	-------------------------------------------------------------------
-	!	..............x..	-------------------	!
-	!	..........b.a....	!	765	!	o I:	!
-	!	o------------------	: C:	!
-	!	: 1:	!
-	!	--------- ------ -------	:  :	!
-	!	!  IC3 ! ! IC4! ! IC5 !	:  :	!
-	!	o-------- o----- o------	o I:	!
-	!	: C:	!
-	!	: 2:	!
-	!	:  :	!
-	!	!! :::::::::: !!	!
-	---------------------------- ------------ -----------------------

-	Chips towards you

-In the above diagram:

-HFþ The 765 is the biggest chip on the card.

-HFþ a and b are the pins to which you connect the signals
-	from the 7438.

-HFþ o indicates pin 1.

-HFþ The 74LS175 and 7438 are mounted in sockets you solder
-	into place.

-HFþ x is a pin that is missing.  It is used as a key on the
-	original cable.

-HFþ IC1 shows the placement for the new 7438.  The o
-	indicates pin 1.

-HFþ IC2 shows the placement for the new 74LS175.  The o
-	indicates pin 1.

-HFþ IC3 shows the placement for the 74LS245.  The o
-	indicates pin 1.

-HFþ IC4 shows the placement for the 74LS04.  The o
-	indicates pin 1.

-HFþ IC5 shows the placement for the old 74LS175.  The o
-	indicates pin 1.

-Card Map	25




