• Yay Bodge Wires

    David Kuder • 07/28/2018 at 23:51 • 0 comments

    I managed to track down enough components to get my MiSTDeX card working fully, but not without a ton of bodges.  Always check your schematics over and over again, trace through your address logic, and make sure you can actually get the right parts.

    First problem I ran into was not having enough 74_574's on hand to make the address latching circuit for the VDP's memory.  Turns out you can use a 74_573 in place of the 74_574 that buffers the VDP's data output into the SRAM data bus if you tie the 573's latch enable high.  You could also use other bus drivers in a pinch, but the 573's pinout is close enough to make it a single pin bodge.

    Next, I had ordered the only 10.738MHz crystal I could track down, but it was the wrong type (series vs parallel) for the TMS9918A.  This necessitated building a 74_04 based clock generator circuit, and outputting opposing phases to the TMS9918A.  It is close enough to get a picture, but the colorburst is just off enough to result in a black&white picture on the TV.  I'll keep trying to track down the correct crystals, or a small variable capacitor to tune the frequency of my bodge.

    This brings us to the address logic.  Turns out I swapped A4 and A6 on the address logic for the PSG and VDP.  This results in a horrible mess trying to figure out which jumper to use for a given address.  And I then noticed that I had made several other mistakes in the PSG address section.

    Now the good news, I have managed to get RomWBW to work with the TMS9918A, and the included chiptune player works on the AY-3-8910.  I've also gotten J.B.Langston's demos working on my hardware.