## PPM radio control IRQ interface

The PPM IRQ interface allows the PC to read the PPM signal from a Radio Control transmitter fed to the IRQ line (pin 10) on the parallel port. No interface diagram is presented here since it is rather specific to the transmitter used. A web search should yield good results.

By default PPJoy assumes the following timing characteristics about the transmitter:

- 1.0ms minimum PPM pulse length. Any pulse shorter than this will produce the minimum axis value.
- 2.2ms maximum PPM pulse length. Any pulse longer than this, but shorter than the minimum sync, will produce the maximum axis value.
- 3.5ms min sync PPM pulse length. A pulse longer than this is taken to mean the start of a new frame.

You can change these values if they are not suitable for your transmitter. See Changing timing parameters on how to change these values and PPM R/C transmitter timing parameters for their meanings.

PPJoy contains some debug code to help setup the PPM interface - PPJoy can cause the PC speaker to click every time an interrupt occurs. First step is to configure the PPM interface in PPJoy, then enable the debug code via a registry entry. You will need to restart your computer for the registry to take effect.

Then try connection and disconnecting pin 10 (IRQ) on the parallel port to pin 18 (Ground). Each time you make or break the connection you should hear one or more clicks. Now you know that PPJoy is properly hooked up to the IRQ.

Now connect your transmitter interface to the parallel port. You should be hearing a series of clicks (it might sound like a continuous tone). This means the parallel port can detect the signal from your TX. If not the voltage from your TX interface might not be enough and you may need some form of amplifier in your interface.

The enable the debugging code you need to copy the text below to a file with a .reg extension and then run the .reg file using explorer:

    REGEDIT4

    [HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\PPortJoystick\Parameters]
    "RuntimeDebug"=dword:00000001

To turn off the debugging code, open registry editor, and change the value of "RuntimeDebug" to 0. At some later stage this functionality will probably be built into the PPJoy control panel applet.

### Notes

- This interface is only supported on Windows 2000/XP. Windows 98/Me does not provide the support functions that PPJoy requires.
- General notes about interface construction.

### Credits

Diagrams and information for this interface can be found at http://www.csd.net/~cgadd/fms/fms_instruct2.htm#AnschlusseinerFernsteuerungandenParallelport, http://myweb.absa.co.za/eric.brouwer/interface.htm or http://www.heliguy.com/nexus/fmsinterface.html.