4k ROM ICs

[+mizapf]

Do you have an oscilloscope? Maybe you can try to check bus control lines and especially the READY line. As I said, when READY is stuck to 0, the processor will not process any command, and the power-on behavior would be a black screen with a more or less random sound from the sound generator. This would also happen if the processor sees no ROM or GROM.

[GDMike]

40+ Year old electronics. Capacitors could also play a role and the power supply. I know, this doesn't help a damn bit. But I was so careful pulling VDPs, I just can't understand what happened with my situation.. such a simple procedure too.

[Delta]

29 minutes ago, mizapf said:

Do you have an oscilloscope? Maybe you can try to check bus control lines and especially the READY line. As I said, when READY is stuck to 0, the processor will not process any command, and the power-on behavior would be a black screen with a more or less random sound from the sound generator. This would also happen if the processor sees no ROM or GROM.

I have ordered an oscilloscope... 

What can I check on the processor with just a multimeter to determine if it is (sort of) working?

[Delta]

21 minutes ago, GDMike said:

40+ Year old electronics. Capacitors could also play a role and the power supply. I know, this doesn't help a damn bit. But I was so careful pulling VDPs, I just can't understand what happened with my situation.. such a simple procedure too.

Yes, it is old electronics, but this fragile? Adrian (of Adrian's Digital Basement) is always replacing ICs on the Ti's he works on and doesn't seem to have this problem 😊

The power supply and capacitors were the first thing I checked and they seem to be fine. 

Maybe I did kill the VDP...but without another to test, how can I be sure? 

Edited May 1, 2023 by Delta

[GDMike]

What's the next step in the ROM after it turns on the sound does anybody know? Somewhere there's a flow chart I think tells us what happens or what's supposed to happen in sequence.. maybe someone can chime in

[Stuart]

It turns OFF the sound. The sound generator powers up in a noisy state so the processor silences it early on in the boot sequence. If the sound generator stays noisy, that indicates that the processor isn't getting very far, if indeed it is doing anything at all.

[GDMike]

So if the ROMs have been changed out then next step would be CPU. Gotcha!

That's why he needs an oscilloscope, to determine the state of the CPU would be ideal.

Thx @STUART.

 

[Delta]

6 minutes ago, GDMike said:

So if the ROMs have been changed out then next step would be CPU. Gotcha!

That's why he needs an oscilloscope, to determine the state of the CPU would be ideal.

Thx @STUART.

 

Is there no way to check the CPU without an oscilloscope? 

[+mizapf]

18 minutes ago, Delta said:

Is there no way to check the CPU without an oscilloscope? 

This is a bit like diagnosing a patient's mental state by a photograph. The problem is that you want to know whether the CPU is working, and this means to watch changes in the state of lines. As they happen in the scope of microseconds, a scope would be the logical tool to use.

[Delta]

11 minutes ago, mizapf said:

This is a bit like diagnosing a patient's mental state by a photograph. The problem is that you want to know whether the CPU is working, and this means to watch changes in the state of lines. As they happen in the scope of microseconds, a scope would be the logical tool to use.

I suppose I will have to wait...in the meantime, I could desolder the processor and get a socket in there...after all, what could possibly go wrong?! 

[Duewester]

@Delta, the power up / reset is generated by u601 (the other chip that should have been covered in paste). Pin 4 is the reset. Put your meter or logic probe on it and see if it changes state immediately after power on. Pin 62 on the processor is the Ready signal. Look for state change there.

I don't know if you have or have not seen the doc attached. It saved my butt.

When I get back to my desk I'll find my notes and forward them to you.

ti99_tech (2).pdf

[Stuart]

1 hour ago, Delta said:

I suppose I will have to wait...in the meantime, I could desolder the processor and get a socket in there...after all, what could possibly go wrong?! 

The processor is fairly reliable. The MCM6810 RAMs have been known to fail before - if just one goes duff then the processor isn't going to process properly. You might want to socket those next. Just cut them out and remove the pin remains one by one to save damaging the board, and buy replacements from a reliable source.

 

As Duewester said above, if you've got a logic probe then you can do some basic testing - /RESET line, the clocks, READY, check if you have activity on the address and data buses.

[+FarmerPotato]

19 hours ago, Delta said:

Is there no way to check the CPU without an oscilloscope? 

I encourage you to believe you can diagnose this without a scope. 

 

My first CPU project, I had only a logic probe to work with, like this one: https://shop.elenco.com/consumers/logic-probe.html

It tells you 1 or 0, blinking for pulse. It gives you an idea if it is a 50-50 pulse (square wave) or occasional 1 or 0.  You can do enough measurements to narrow down what's working and what is not. 

 

 

(assume you check the VCC pin first with a voltmeter. The probe will tell you "1" even if it's too low like 4.5V, because a 1 is anything over 2.4V..)

 

Most pins @Duewester , @Stuart have already suggested.  
 

I also look at the low address bits, like A14 on the CPU, which should be in use constantly. 

 

I would start first with the 9904. That way, you are checking  the main clocks from the origin. After that, the 9900 clock and pins that better not  be stuck (like READY stuck at 0 = bad
9919) or have  continual activity (chip selects, low address bits.) 

 

If they're not, check the continuity to the crystals' pins. (Note: poking with a probe will  disturb the crystal oscillation, and the 9900 will lock up and need a reset. So just check continuity with power off.) 

 

9900 CPU

READY line? good states are 1, or 1 with 0 pulses.  

RESET pin? good state is 1.

IAQ pin- good state is pulsing. (Instruction acquire, means it's fetching code!) 

 

Address pins: good state is A14 always pulsing.  Same for data pins. 

(Rare: Make sure LOAD is a good 1)

INTREQ: good state is 1 with 0 pulses. A 1 is good input for the 9900, but the 9901 and 9918 feed it 0 pulses .) 

 

6810 RAM

Just check the low address bits - they should be pulsing.

Locate the Chip Enable pin, it must be pulsing.

 

4732 ROMs

Same test as RAM

 

9901

The INTREQ pin again, which connects to the CPU.

INT2 - this should also be 1 with pulsing 0 if the 9918 is working. 

INT1 - should be 1.

IC0,1,2,3 are hardcoded to 0001.

 

 

Any errors are my own. I welcome corrections. My 9900 knowledge is theoretical.  My debugging experience is with 9995 and 99105. 

If someone already has a troubleshooting flowchart  for using logic probe--please link to it?

 

 

[GDMike]

12 hours ago, FarmerPotato said:

I encourage you to believe you can diagnose this without a scope. 

 

My first CPU project, I had only a logic probe to work with, like this one: https://shop.elenco.com/consumers/logic-probe.html

It tells you 1 or 0, blinking for pulse. It gives you an idea if it is a 50-50 pulse (square wave) or occasional 1 or 0.  You can do enough measurements to narrow down what's working and what is not. 

 

 

(assume you check the VCC pin first with a voltmeter. The probe will tell you "1" even if it's too low like 4.5V, because a 1 is anything over 2.4V..)

 

Most pins others have already suggested.   I also look at the low address bits, like A14 on the CPU, which should be in use constantly. 

 

9900 CPU

READY line? good states are 1, or 1 with 0 pulses.  

RESET pin? good state is 1.

IAQ pin- good state is pulsing.

Address pins: good state is A14 always pulsing.  Same for data pins. 

(Rare: Make sure LOAD is a good 1)

INTREQ: good state is 1 with 0 pulses. A 1 is ok for the 9900, but it means the 9918 is not working!

 

9904 - this is critical

All the clock output pins should be pulsing: they are 75% 0 with 25% pulses to 1.  See diagrams in here

If they're not, check the crystals' pins.

 

6810 RAM

Just check the low address bits - they should be pulsing.

Locate the Chip Enable pin, it must be pulsing.

 

4732 ROMs

Same test as RAM

 

9901

The INTREQ pin again, which connects to the CPU.

INT2 - this should also be 1 with pulsing 0 if the 9918 is working. 

INT1 - should be 1.

IC0,1,2,3 are hardcoded to 0001.

 

 

 

 

This is badass

[Delta]

13 hours ago, FarmerPotato said:

I encourage you to believe you can diagnose this without a scope. 

 

My first CPU project, I had only a logic probe to work with, like this one: https://shop.elenco.com/consumers/logic-probe.html

It tells you 1 or 0, blinking for pulse. It gives you an idea if it is a 50-50 pulse (square wave) or occasional 1 or 0.  You can do enough measurements to narrow down what's working and what is not. 

 

 

(assume you check the VCC pin first with a voltmeter. The probe will tell you "1" even if it's too low like 4.5V, because a 1 is anything over 2.4V..)

 

Most pins others have already suggested.   I also look at the low address bits, like A14 on the CPU, which should be in use constantly. 

 

9900 CPU

READY line? good states are 1, or 1 with 0 pulses.  

RESET pin? good state is 1.

IAQ pin- good state is pulsing.

Address pins: good state is A14 always pulsing.  Same for data pins. 

(Rare: Make sure LOAD is a good 1)

INTREQ: good state is 1 with 0 pulses. A 1 is ok for the 9900, but it means the 9918 is not working!

 

9904 - this is critical

All the clock output pins should be pulsing: they are 75% 0 with 25% pulses to 1.  See diagrams in here

If they're not, check the crystals' pins.

 

6810 RAM

Just check the low address bits - they should be pulsing.

Locate the Chip Enable pin, it must be pulsing.

 

4732 ROMs

Same test as RAM

 

9901

The INTREQ pin again, which connects to the CPU.

INT2 - this should also be 1 with pulsing 0 if the 9918 is working. 

INT1 - should be 1.

IC0,1,2,3 are hardcoded to 0001.

 

 

 

 

Thank you. I have replaced ordered a logic probe which should be here soon. Hopefully it is not being sent from China. 

[+arcadeshopper]

20 hours ago, Delta said:

Yes, it is old electronics, but this fragile? Adrian (of Adrian's Digital Basement) is always replacing ICs on the Ti's he works on and doesn't seem to have this problem 😊

The power supply and capacitors were the first thing I checked and they seem to be fine. 

Maybe I did kill the VDP...but without another to test, how can I be sure? 

he also spent like 3 hours running around in circles testing everything but the cpu.. to find out it was the cpu

[+FarmerPotato]

53 minutes ago, arcadeshopper said:

he also spent like 3 hours running around in circles testing everything but the cpu.. to find out it was the cpu

Urgh.... there is a logical order to test things in, we should find documentation, or else create it.

There's the 99/4 technical guide which has some troubleshooting procedures. 
 

There are living people who were 4A repair technicians at TI. I found this thread, last Friday, while wondering about a PG1996 diode in the blue 4A PEB Technical Data schematics:

 

https://www.vintage-radio.net/forum/showthread.php?p=1084936

 

[+FarmerPotato]

21 hours ago, GDMike said:

What's the next step in the ROM after it turns on the sound does anybody know? Somewhere there's a flow chart I think tells us what happens or what's supposed to happen in sequence.. maybe someone can chime in

The disassembly. Thierry made one. TI Intern first. (Craig Miller, in the Smart Programmer, even?) 

 

 

[GDMike]

59 minutes ago, arcadeshopper said:

he also spent like 3 hours running around in circles testing everything but the cpu.. to find out it was the cpu

3 hours ain't nothing.. since most were socketed..

Easiest firstest

[+FarmerPotato]

3 hours ago, Delta said:

Thank you. I have replaced ordered a logic probe which should be here soon. Hopefully it is not being sent from China. 

Great! 
 

For pinouts reference, I love the TI Tech pages made 20 years ago by Thierry Nouspikel (a giant in the TI-99/4A community!) (that's a link to the 4732 ROM page)

 

 

 

 

[Delta]

1 hour ago, arcadeshopper said:

he also spent like 3 hours running around in circles testing everything but the cpu.. to find out it was the cpu

You are quite correct about that! 

[+Lee Stewart]

22 hours ago, GDMike said:

What's the next step in the ROM after it turns on the sound does anybody know? Somewhere there's a flow chart I think tells us what happens or what's supposed to happen in sequence.. maybe someone can chime in

 

1 hour ago, FarmerPotato said:

The disassembly. Thierry made one. TI Intern first. (Craig Miller, in the Smart Programmer, even?) 

 

Try The TI-99/4A Operating System, which shows original code from TI programmers with their comments.

 

...lee

[Duewester]

@Delta first, make sure you have the chip installed in the correct orientation. Make sue all pins are actually in their sockets. I have more than once installed a chip with a pin bent in or out and not in the socket. Look out for possible lifted trace or pad(s). These old boards can easily be damaged by lifting a pad.

Me personally, based on past experience, would have started with the 6810P chips. If you botch them up, they are more easily replaced (no eeprom stuff to deal with), just install new 6810P. I got pretty lucky on my massive chip adventure and by the time I got around - read scared to death - to the 8249 and 8250 chips (which were not 49 and 50 chips but a TI chip and a mystery chip - same make as 49 but different numbers completely) I was pretty proficient at socketing my chips and getting the pins in the sockets.

If you're like me, some form of board swap with @arcadeshopper (the easy way out) just isn't in your DNA.

[Duewester]

My notes. A compilation

ti99 4a troubleshooting.docx

[GDMike]

7 hours ago, Duewester said:

My notes. A compilation

ti99 4a troubleshooting.docx 1011.91 kB · 5 downloads

Awesome stuff. Thx