
-Paul G. Crumley

-Information Technology Center
-Carnegie-Mellon University
-Pittsburgh, PA 15213
-	 PC-Jr Internal Diskette Drive

-GContents


-G Acknowledgements ....................................... 1

-G Introduction ........................................... 2

-G Requirements ........................................... 3

-G Description of Modification ............................ 4

-G Parts List ............................................. 5

-G Modification Instructions .............................. 8

-G Debugging Tips ........................................ 20

-G Card Map .............................................. 21

-G Feedback .............................................. 22

-Contents	ii


-	 Internal Use Only
-	 PC-Jr Internal Diskette Drive

-GE-ACKNOWLEDGEMENTSHF-


-X-acto is a registered trademark of X-acto Corporation.

-Radio Shack is a registered trademark of Tandy -Corporation.

-,  PC and PC-Jr are registered trademarks of -International Business Machines.

-TEAC is a registered trademark of TEAC Corporation of -America.

-Tandon is a registered trademark of Tandon Corporation.

-Qume is a registered trademark of International Telephone -and Telegraph.

-CDC is a registered trademark of Control Data Corporation.

-Apple is a registered trademark of Apple Computer -Incorporated.

-Atari is a registered trademark of Warner Communications -Incorporated.

-Acknowledgements	1


-	 Internal Use Only
-	 PC-Jr Internal Diskette Drive

-GE-INTRODUCTIONHF-


-This document provides a set of instructions for modifying -the  PC-Jr Internal Diskette Drive Adapter to allow the -connection of a third diskette drive to the PC-Jr after a -second diskette drive has been added by following the -instructions presented in the document

-" PC-Jr Internal Diskette Drive
-	Hardware Modification for Second Diskette Drive"

-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  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	2


-	 Internal Use Only
-	 PC-Jr Internal Diskette Drive

-GE-REQUIREMENTSHF-


-HFþ Hardware

-	HF- PC-Jr with a second diskette installed as described
-	in the above cited document.

-HFþ Software

-	HF-	DOS 2.1

-	The software patches provided in this document will
-	only work with DOS 2.1.  If you need to use some
-	higher level	of DOS	please	contact me	for
-	information on how to patch your software.

-	HF- A diskette with a copy of the file 3RDDISK.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	3


-	 Internal Use Only
-	 PC-Jr Internal Diskette Drive

-GE-DESCRIPTION OF MODIFICATIONHF-


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

-HFþ Alter the cable made when connecting the second
-	diskette drive so that a third diskette drive may be
-	connected.

-HFþ Make a few connections to existing circuits on the
-	diskette adapter.  No existing connections are altered,
-	only new connections are made.

-HFþ Assemble the third 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 third
-	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 and third diskette drives.  -Unfortunately, in order for the BIOS to automatically know -about the existence of the second and third diskette -drives, 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 and -third diskette drives.  Since the support for the -additional diskettes 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	4


-	 Internal Use Only
-	 PC-Jr Internal Diskette Drive

-GE-PARTS LISTHF-


-The modification requires the following parts:

-HFþ 4 ft Wire-Wrap wire.  30 or 32 gauge Wire-Wrap wire
-	works well for these connections.

-HFþ	1	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 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)

-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.

-Parts List	5


-	 Internal Use Only
-	 PC-Jr Internal Diskette Drive

-HFþ a knife such as an X-acto modelling knife.

-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.  Electronics "supermarkets" such as -Radio Shack have a variety of soldering irons.  Radio Shack -item number 64-2070, a 25 Watt soldering iron for about -$5.00, should be fine for this and other small projects.  -If you go and buy a soldering iron don't forget solder.

-The edge connectors should be available for a number of -companies.  If the company that you decide buy the diskette -drive from does not sell the connectors you should be able -to find two at a Radio Shack or some similar electronics or -computer shop.

-There are a number of companies that make  PC compatible -drives.  TEAC, Tandon, Qume, and CDC (Control Data -Corporation) 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 make sure that the enclosure and -power supply are fused and 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.

-	NOTE:

-Parts List	6


-	 Internal Use Only
-	 PC-Jr Internal Diskette Drive

-	I would discourage the use of home-built power supplies
-	for those that do not have a large amount of experience
-	with the design and construction of electronic
-	circuits.  There are a number of safety issues that you
-	must be concerned with when constructing such a power
-	supply.  In addition, there are some typographical
-	errors in the PC-Jr technical reference manual which
-	show incorrect power connections on the power supply
-	card.

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

-Parts List	7


-	 Internal Use Only
-	 PC-Jr Internal Diskette Drive

-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 assumes you have already upgraded your -PC-Jr to a two diskette system using the instructions -provided in the document " PC-Jr Internal Diskette Drive --- Hardware Modification for Second Diskette Drive." If -you are adding the second and third diskette drives at the -same time you might wish to use the document " PC-Jr -Internal Diskette Drive -- Hardware Modification for Second -and/or Third Diskette Drive." This document is available -in the JRDISKS PACKAGE from CMUPGC at PGHVM1.

-It is assumed that you have a copy of the first document -available as a reference while you are doing this -modification.  You might like to read over the instructions -in the second diskette drive before you start this -modification to refresh your memory.

-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.  Modify diskette adapter card.

-	For review, remember that IC1 is the 7438 and IC2 is a
-	74LS175.  These two chips were installed when the
-	second diskette drive was installed.  IC3 is a 74LS245
-	that is already installed on the diskette adapter.  A
-	connection was made to this chip when the second
-	diskette drive was installed.

-	No new chips need to be added to install the third
-	diskette drive.  You will simply add wires to the
-	card.

-	HFa.  Unplug the PC-Jr from the wall outlet, open the
-	case, and remove the diskette adapter from the
-	system.  Simply disconnect	the	cable from the
-	diskette adapter and leave the internal diskette
-	drive and external diskette drive connected to the
-	cable for now.

-	HFb.  Locate the 74LS245 at position IC3.  Connect pin 16
-	of IC3 to pin 5 of IC2, the 74LS175.  You are now
-	done with IC3.

-Modification Instructions	8


-	 Internal Use Only
-	 PC-Jr Internal Diskette Drive

-	HFc.  Connect pin 7 of IC2 to pins 9, 10, 12 & 13 of the
-	7438 located at position IC1.  You are now done
-	with IC2.

-	HFd.  Cut two pieces of wire about 12 inches long.
-	Solder one end of one of these wires to pin 8 of
-	IC1 and solder one end of the other wire to pin 11
-	of IC1.  You are now done with IC1.

-	HFe.  At this point all connections to the chips should
-	be completed.  You have attached one new wire to
-	IC3, the 74LS245.  This wire was attached right
-	next to the wire that was attached when the second
-	diskette drive was installed.  You have attached
-	two new wires to IC2 and you have made connections
-	to 6 pins on IC1.

-	Each pin on IC1 should have some wire attached to
-	it while the following pins on IC2 should be
-	unused:

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

-	HFf.  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	!
-	!
-	1 o 3 o 5 o 7 o o o o o o o o o o	!
-	!
-	o o	o	o	o	o	!
-	!
-	o o o o o o o o	o o o o o o o	!
-	!


-	HFg.  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 were used
-	when the second diskette drive was installed.  We
-	are going to attach the signals needed for the
-	third diskette drive to the pins labeled 1 and 3 in
-	the above diagram.  Connect one of the 12 inch
-	wires to pin 1 and connect the other to pin 3.  It
-	doesn't matter which wire goes to 1 and which goes
-	to 3.  What does matter is that you make the
-	connections to pins 1 and 3.  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 Instructions	9


-	 Internal Use Only
-	 PC-Jr Internal Diskette Drive

-	modification.

-	HFh.  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.

-	HFi.  Test that the adapter it is still operating
-	correctly as a two diskette adapter by installing
-	it in the PC-Jr and reattaching the cable that
-	leads	to the	internal and	external diskette
-	drives.  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.

-HF4.  Alter the cable so that the third diskette drive can be
-	attached.

-	As before I am assuming you have done the modification
-	to attach a second diskette to the PC-Jr so I will now
-	go into great detail about how to "press" the
-	connectors on to the cable.

-	HFa.  Once the diskette adapter has been tested turn off
-	the PC-Jr and the external diskette cabinet and
-	remove their power cords for the wall outlet.  Now
-	remove the diskette cable so it can be modified to
-	accommodate the third diskette drive.

-	HFb.  The cable should currently look something like
-	this:


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


-	HFc.  The new connector will be installed near the end of
-	the cable.	(see the diagram below) The exact
-	distance will have to be determined by you.  Select
-	the distance so that the cable will easily fit onto
-	the two diskette drives when they are mounted in
-	the external enclosure.  This new connector will
-	connect to the B: drive while the old connector

-Modification Instructions	10


-	 Internal Use Only
-	 PC-Jr Internal Diskette Drive

-	will become the new C: drive connector.

-	HFd.  Once the correct distance has been determined
-	install the new connector on the cable.  The twist
-	should be toward the center of the cable.  The
-	cable between the new and endmost connector should
-	be flat.  You may have to make the slits in the
-	cable a bit longer to allow the new connector to be
-	placed at the proper distance from the end of the
-	cable.  Remember to keep the pin 1 identifiers
-	correct.  The cable should now look like this:


-	__	--	__
-	!  !	!  !	!  !
-	_	!  !	!  !	!  !
-	! !----------!__!-----------------------------<===>--!__!-----!__!
-	! !
-	!_!<-- 8" -->!<------------- about 2' -------------->!<- 5" ->!


-	HFe.  Now the signals that were placed on the cable by
-	making connections at the points shown as 1 and 3
-	in the diagram used to modify the diskette adapter
-	must be connected to pins 12 and 16 of the endmost
-	connector.  These signals are now on pins 2 and 6
-	in the cable.

-	HFf.  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 isolate the
-	pin 2, 6, 12 and 16 conductors from the cable.  Use
-	the modelling knife to separate these conductors
-	form the cable.

-	HFg.  The conductors for pin 2 and pin 6 should be cut
-	off near the endmost connector while you should cut
-	the conductors for pin 12 and pin 16 near the new
-	connector.

-	HFh.  Now you should solder the pin 2 conductor to the
-	pin 12 conductor and solder the pin 6 conductor to
-	the pin 16 conductor.  The completed connection
-	should look something like this:


-	---	---
-	----------------------------!	!--------------------------!	!
-	pin 1	!	!	!	!
-	!	!  pin 2 =====\\	==== !	!
-	!	!  pin 6 ==\\ \\	==== !	!
-	!	!	\\ \\	!	!
-	!	!	\\ \\	!	!
-	!	!  pin 12 ==	\\ \\==== !	!

-Modification Instructions	11


-	 Internal Use Only
-	 PC-Jr Internal Diskette Drive

-	!	!  pin 16 ==	\\======= !	!
-	!	!	!	!
-	!	!	!	!
-	!	!	!	!
-	!	!	!	!
-	!	!	!	!
-	!	!	!	!
-	!	!	!	!
-	pin 34	!	!	!	!
-	----------------------------!	!--------------------------!	!
-	---	---

-	HFi.  When the wires are soldered wrap the exposed
-	conductors with electrical tape.  Make sure any
-	exposed conductors are well covered with tape so
-	there is no possibility of these connections coming
-	in contact with any parts of the external enclosure
-	or the	printed circuit cards on the diskette
-	drives.


-	NOTE:

-	I am currently looking for a better way to describe
-	this.  Many of you will probably notice that by
-	using pins 2 and 6 we could do something similar to
-	what was done with the twisted cable to make the
-	signals attach to pins 12 and 16.

-	This will require an additional connector but will
-	make for a neater job.

-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:

-	HFb.  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.

-	HFc.  Set both of the new diskettes in front of you with
-	the printed circuit cards facing up.

-	HFd.  Remove the terminating resistor packs from the
-	drives.  These are small packages of resistors that
-	look like an integrated circuit chip.	Usually
-	these parts are not the same color as the other
-	chips on the card.  Common colors for the resistor
-	packs	include white, orange, brown, blue	and
-	yellow.  Unfortunately, some are black in color
-	thus finding them can be a little difficult.  There

-Modification Instructions	12


-	 Internal Use Only
-	 PC-Jr Internal Diskette Drive

-	are three other clues to finding these resistor
-	packs.  First, these parts are almost always near
-	the edge connector where the cable attaches to the
-	diskette drive.  Second, the resistor packs are
-	socketed.  Third,	they usually don't have part
-	numbers like 74XXXX.

-	Once these resistor packs are found (there is one
-	on each diskette drive) they should be removed by
-	simply sliding the blade of a small screwdriver
-	under the chip and gently prying it out of its
-	socket.

-	NOTE:

-	It is possible that you might get a diskette
-	drive that has no resistor packs in place.  If
-	you got a used diskette drive the previous
-	owner may have	removed the resistor pack.
-	Though it is very uncommon for a company to
-	ship new diskette drives without the resistor
-	packs installed it is possible.  If you see an
-	empty socket near the edge connector and can
-	find no chip that looks like a good candidate
-	for being a resistor pack assume the resistor
-	pack has already been removed.

-	HFe.  Set the diskette address jumpers to indicate the
-	diskette drive should respond to the DRIVE SELECT 1
-	signal.  Each diskette can be set up to respond to
-	one	of	4	drive	addresses.	Most	diskette
-	manufacturers label the jumpers on their drives
-	DS0, DS1, DS2 & DS3.  You may wonder why you are
-	setting	both drives to	respond	to the same
-	address.  The cable is built so the proper signals
-	are directed to each drive so that all drives
-	should respond to the DS1 address.

-	Most drives that are advertised as being  PC
-	compatible will already have the jumpers set up so
-	the drive will respond to DRIVE SELECT 1 and will
-	not require you to move any of the jumpers on the
-	drive.

-	NOTE:

-	It is unfortunate that there is no standard
-	method of implementing these DRIVE SELECT X
-	jumps on diskette drives.  Many drives have
-	have nice, well labeled jumpers or switches
-	while others have no labels and the jumpers are
-	simply pieces of wire connecting pin positions
-	in standard chip sockets.  I would recommend
-	talking to a technically oriented person at the
-	company where you are thinking about purchasing
-	the diskette drives before placing an order.

-Modification Instructions	13


-	 Internal Use Only
-	 PC-Jr Internal Diskette Drive

-	Make sure these	jumpers	are	either well
-	labeled, preset to DRIVE SELECT 1, or well
-	documented in a manual that is shipped with the
-	drive.

-	HFf.  Install the drives in the cabinet and connect the
-	power supply to the drives.  Be careful that none
-	of the moving parts on the diskette drives will
-	come in contact with any of the mounting hardware
-	or cables in the cabinet.  When the drives are
-	positioned such that they are not going to come in
-	contact with anything that would restrict their
-	operation fasten them in place so they will not
-	move.  Make sure the diskette drive is securely
-	mounted so that the connections will be less likely
-	to vibrate loose with use.

-	HFg.  Attach the cable to the diskette drives.  The
-	connector at the end of the cable will be attached
-	to the drive that will be known as C:  (this used to
-	be attached to B:) and the new connector will be
-	attached to B:.

-	HFh.  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.

-	HFi.  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.

-	HFj.  Place the cable on the modified diskette adapter
-	card and insert the adapter card in the PC-Jr.

-	HFk.  Attach the cable to the internal diskette drive.

-	HFl.  Recheck these connections.

-	HFm.  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 and
-	third diskette drives' power supply into the wall
-	power outlet, turn on the new diskette drives, 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.

-Modification Instructions	14


-	 Internal Use Only
-	 PC-Jr Internal Diskette Drive

-	HFa.  The DOS boot record must be altered so that BIOS
-	will be	told that three 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
-	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
-	3RDDISK.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
-	3RDDISK.DBG you will have to enter the	DEBUG
-	commands yourself.  I will first describe how to
-	use the file 3RDDISK.DBG.

-	HF1) First, you must get the file 3RDDISK.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
-	3RDDISK.DBG will	be shown as ('s and )'s
-	below.)

-Modification Instructions	15


-	 Internal Use Only
-	 PC-Jr Internal Diskette Drive

-	NOTE:

-	DEBUG	uses	square	brackets.	These
-	characters	will	be	represented	as
-	parenthesis in this document.

-	HF2) You must copy the file 3RDDISK.DBG to the new
-	DOS diskette in the same manner you copied
-	DEBUG.COM.  Use the command "COPY B:3RDDISK.DBG
-	A: /V".

-	HF3) Next, use the command "DEBUG < 3RDDISK.DBG" to
-	run the DEBUG program.  DEBUG will take all of
-	its commands from the file 3RDDISK.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),80
-	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
-	091A:19FB 49	DEC	CX
-	091A:19FC 42	INC	DX
-	-A 19F8
-	091A:19F8 OR BYTE PTR (0410),80
-	091A:19FD JMP 1A26
-	091A:19FF
-	-N FORMAT3.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 FORMAT3 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.

-Modification Instructions	16


-	 Internal Use Only
-	 PC-Jr Internal Diskette Drive

-	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),80
-	JMP 012E

-	W 100 0 0 1
-	N FORMAT.COM
-	L
-	U 19F8 L 5
-	A 19F8
-	OR BYTE PTR (0410),80
-	JMP 1A26

-	N FORMAT3.COM
-	W
-	Q


-	HF3) If this works correctly you have just made a
-	new DOS diskette.  A new version of the FORMAT
-	command called FORMAT3 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 three 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.

-Modification Instructions	17


-	 Internal Use Only
-	 PC-Jr Internal Diskette Drive

-	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 diskettes in their cabinet
-	then turn on the PC-Jr.  After DOS boots you should
-	have the use of all three 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 added
-	diskette drives 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  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 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 does not have any
-	kinks or strange twists in it.  If you must,
-	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 drives.

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

-	HFd.  Well, this is the final test.  Plug the added
-	diskette drives 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

-Modification Instructions	18


-	 Internal Use Only
-	 PC-Jr Internal Diskette Drive

-	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	19


-	 Internal Use Only
-	 PC-Jr Internal Diskette Drive

-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 -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 problem is that of a
-	solder bridge forming and shorting 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 or third drive connected.

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

-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 or one of the resister packs was
-	not removed.

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

-Debugging Tips	20


-	 Internal Use Only
-	 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	21


-	 Internal Use Only
-	
 PC-Jr Internal Diskette Drive


