Woz IIgs ROM 01 System bad 06010000

[elemenoh]

I have a nice condition Woz edition IIgs with ROM 01 (342-00770-B) that fails its self test with "System bad 06010000". I understand that means there's a read/write issue to the serial IC.

The solder joints of the Zilog SCC looked good under a microscope but I flooded with flux and reflowed anyway. No change.

I tested pin 23 for clock signal with a scope and that looks okay too. Photo below.

There are no passives near the SCC on my board, but I did a cursory check of the electrolytic caps across the board with an ESR meter. They all seem fine.

Would the likely culprit be the SCC chip itself? Anything else to check first?

This IIgs already has a super serial card, so I suspect the original owner might have had the same issue. The computer is in such nice condition that I'd love to fix this and make it 100% perfect.

 

[elemenoh]

Would this be a good replacement for it? https://www.ebay.com/itm/392535795279

 

[cheesestraws]

The difference between the 85C30 and the 8530 is that the former is a CMOS device, whereas the latter is an NMOS device. I believe they are meant to be electrically compatible, but perhaps someone with more experience can chime in here.

The 85C30s are still a live part, Zilog still makes them, so there's no particular requirement to go digging through NOS on eBay unless you particularly want to .

 

[elemenoh]

I noticed the sound on this one was also really quiet. I replaced all the 22uf and 1uf caps on the MLB, but the volume of the power-on beep didn't change and the self test still fails with 06010000.

 

[cruff]

We've see this before, but haven't figured out what the cause is yet. According to a ROM source listing found at https://mirrors.apple2.org.za/Apple.../Computers/Apple II/Apple IIGS/Documentation/, the serial I/O chip test failure code 01 indicates a problem with the SCC register R/W test, which is testing some of the channel A and channel B registers. It tests registers 2, 12, 13, and some of the bits in register 15. Unfortunately it doesn't indicate which of the registers may have failed the test. The test appears to test all of the bit combinations in the registers.

 

[elemenoh]

@cruff would replacing the IC be the best next step?

 

[cruff]

I hesitate to suggest replacing the chip unless we can understand what exactly the self test is failing on. Do you have access to an oscilloscope? You could verify if the the two clock signals (PCLK related to register access from the CPU) and the baud rate clock are working correctly.

R2 is the SCC interrupt vector register, which is shared between the A and B channels, but when read only the A channel returns the value written unmodified while the B channel can have other status reflected in it depending on the state of some other register's bits. I don't know that it is used in the Apple IIgs, I don't recall the bus structure of the 65c816 supports vectored interrupts or not.

R12 and R13 are the baud rate generator time constants, and should return what were written into them. What I don't know is if the 8530's baud rate clock were to be non-functional, if these registers would be affected.

R15 has some interrupt enable bits in it.

 

[elemenoh]

@cruff not sure what is expected, but here are the various pins you mentioned with just system power on. Should RX/TR be doing anything while the system is idle?

Pin 23 PCLK


Pin 13 RTxCA

Pin 32 RTxCB


Pin 15 TRxCA


Pin 30 TRxCB

 

[cruff]

PCLK looks ok, that would be timing the access to the internal registers of the 8530. It looks like the frequency is correct (3.58 MHz), as the rest of the IIgs seems to be working.

From the trace of the pin 13 RTxCA signal (and pin 32), it should be 3.6864 MHz, so should look similar in frequency to the PCLK signal, but will probably look like a sine wave since it is an analog signal through the crystal. I'm guessing that either crystal Y2 has failed, or the driving circuitry in the 8530 is bad. Those crystals should be fairly inexpensive, so changing it with a new one would probably be the next step.

 

[cruff]

You might try a quick reheat of the pins for Y2 and the pins 12+13 on the 8530 just to see if it is a cold solder joint.

 

[Verault]

Any word on this repair elemenoh?