Never Valid User Address

After finding out that my CMS 9619 Advanced Single Board MicroComputer was having problems accessing the EXORbus, I tried some trouble shooting. My MULTI-PLANE EXORbus backplane made it easy to accesss the various signals with an extra card edge receptacle attached. I tried pulling down various lines of the data bus with a resistor, and noticed that the 8 bits of data seemed to be responding appropriately. So, probably not the data bus tranceiver.

With little more to go on, I ended up buying an inexpensive USB logic analyzer and installing PulseView software for it. I was quickly able to identify that the bus's VUA (Valid User Address) signal was not rising when it was supposed to, even though it was receiving the correct signal from the PAL address decoder. Without this signal, the I/O card was never properly addressed, and the data bus was always floating. I tested another CMS 9619A board I have which displayed similar symptoms and found that it too was suffering from a failure of the same IC. Odd coincidence. So, after some desoldering, super-gluing a trace that was inadvertently partially pulled off the board, and re-soldering a new 74LS244 8-bit driver (U9), I am back in business. The chip came off of the other board much easier with the help of a heat gun.

I can't recommend these cheap logic analyzers enough to a hobbyist, although the first one I recevied was not working properly on some channels (make sure to test them before you rely on them for anything).

I also bought an EspoTek Labrador USB oscilloscope to try out, but I received, tested, replaced, and finally used the cheap logic analyzers from ebay while I was still waiting for the Labrador to arrive.

CMS 9619 Hardware Mods (continued)

Update: Since this post I have figured out how to change the address map using a GAL IC.

I recently posted some minor hardware modifications I made to my CMS 9619A Advanced Single Board Microcomputer to get it to use a 32K SRAM chip. Unfortunately, I had to disable the EXORbus data bus transciever to get it to work. So, although the board works properly as a single board computer, it cannot access the EXORbus. With NitrOS-9 up and running, I would really like to regain the ability to use external I/O cards.

To get this to work, the data bus transceiver (U11) needs to be re-enabled, but only during the External I/O address space at $FF80 to $FFBF (checkout the CMS 9619 memory map for details). This can be done by combining the normal data bus transceiver enable signal with the signal for address line A15. This works because the only time the transceiver is normally enabled in the high address space is in the External I/O address space (because everything else in that range is onboard the CPU module). All we should need to do is invert the normal data bus transceiver (U11) enable signal, and NAND it to the A15 address line.

Fortunately, there are a few ICs on the board that are not fully utilized. We can use the 1st gate (pins 1->2) on the U1 74LS04 inverter IC to invert the data bus transceiver enable signal, and the first gate (pins 1+2->3) on the U3 74LS00 gate IC to NAND the inverted signal to A15.

 

No traces visible to pins 1 or 2 on U1 or U3

This is almost too perfect that these logic gates are available, so I'm sure it won't work. Also, since the board is already bodged in this area with the factory's dead-bug'ed delay line, all of the modifcations are easily reversible with no traces to cut.

Before

After

After the modifcations, everything booted up properly and I was able to access my 32K SRAM at $0000-$7FFF However, I was unable to read any data from my known-good CMS 9650 8 port Serial I/O card. After some basic troubleshooting, I got to thinking... I don't think this processor card was EVER able to access the external bus- which is why I think I chose it for modfications. Major troubleshooting to come...