[Coco] DriveWire for TC-9/SWTPC/M68MM17

[Joel Ewy]

Just to confirm my understanding:  The CoCo's Bitbanger uses pins from 
PIA IC4.  DriveWire for the Dragon uses the same client driver with the 
addition of a little extra hardware to invert and possibly 
level-translate the same pins from its 6821 parallel port. Correct?  So 
with a similar adapter one should be able to use one of the TC-9 
Tomcat's PIAs for DriveWire, and the same should be true of a 6821 board 
on an SWTPC, GIMIX, Motorola Micromodule 17, S-100 6809, or SBC, 
assuming you are using the driver appropriate to your CPU type and clock 
speed, and you have a descriptor pointing at the right address.

Actually, as I think about it, the clock speed might be a problem. The 
DW code is very, very tight and relies on instruction timings that are 
only going to be valid for the exact clock speed of the CoCo.  So if you 
have an SWTPC running at 2 MHz it won't be able to run an unmodified 
DriveWire client written to work at 1.78MHz, will it?  Since the TC-9 
uses the GIME it is probably running at CoCo 3 speed, though I think I 
remember FHL claiming some kind of speed-up.  How hard would it be to 
modify the DW client code to run at 2 MHz or 1 MHz?  Or would it be 
worth it to replace the crystals and drop 10-20% of the CPU clock to get 
DriveWire?  Hmm.

JCE