Re: [M100] Model 100 - LCD Shows Pixels Only
You can check each address line at the ROM to see they are all being twiddled and that those connections are good. This is not fool proof as a bad bus latch or any chip on the address bus can much up one of more lines. The data lines are more complicated. If you test at the ROM you will see them twiddling but you need to look at the ROM chip select at the same time to make sure you are seeing data from the ROM. Then you need to look for the same thing back at the processor to see if all the lines are good back to there. If you have more than 8 channels on your LA it will be much, much easier. As I recall I used channel 0 for the /WR had the Saelee software filter out all data there that bit was not low. Then I had to guess at what bit 0 might have actually been by looking at the disassembly. A lot of work but I learned a lot on the process. Good luck, Jeff Birt From: M100 On Behalf Of Eric Sent: Thursday, December 28, 2023 12:00 PM To: m...@bitchin100.com Subject: Re: [M100] Model 100 - LCD Shows Pixels Only Thank you everyone for the quick response! When I power up the computer, all pixels are on and remain dark (LCD is entirely "black"). Contrast potentiometer: Seems to work as intended. The display will lighten or darken as the potentiometer is adjusted. LCD PCB: No signs of corrosion and/or damaged traces. ROM: I'll ohm out the traces to see if I can find any broken connections. Otherwise, are there any other troubleshooting steps I can take? RAM: Jeff, great video! Thank you for sharing. The M100 Test Harness sounds like an ideal tool. I have limited experience with a logic analyzer however it looks like a fun challenge. I'll try to duplicate your setup but I may need some tips on how to configure the Saleae software... Eric On Thursday, December 28, 2023 at 08:17:02 AM PST, Josh Malone mailto:josh.mal...@gmail.com> > wrote:
Re: [M100] Model 100 - LCD Shows Pixels Only
I had a RAM problem with my Model 100 when I first got it. It seemed to work fine initially, but attempting to paste text resulted in odd characters appearing, and the amount of free memory slashing drastically. Various people were very confused as to why this was happening, but a bit of experimenting also revealed the machine would crash with memory only partly full. This suggested corrupt RAM somewhere. My machine (UK spec, no modem) had a soldered RAM module and 2 socketed ones. By a process of trial and error, using it with 8K, 16K and 24K in all available permutations, I was able to (thankfully!) work out one of the socketed modules was the problem. It's now out the machine, and I've bought 2 modern modules to get it up to 32K. I've had no problems since. Lee On Thu, 28 Dec 2023, at 17:59, Eric wrote: > > Thank you everyone for the quick response! > > When I power up the computer, all pixels are on and remain dark (LCD is > entirely "black"). > > Contrast potentiometer: Seems to work as intended. The display will lighten > or darken as the potentiometer is adjusted. > LCD PCB: No signs of corrosion and/or damaged traces. > ROM: I'll ohm out the traces to see if I can find any broken connections. > Otherwise, are there any other troubleshooting steps I can take? > RAM: Jeff, great video! Thank you for sharing. The M100 Test Harness sounds > like an ideal tool. > I have limited experience with a logic analyzer however it looks like a fun > challenge. I'll try to duplicate your setup but I may need some tips on how > to configure the Saleae software... > > Eric > > > On Thursday, December 28, 2023 at 08:17:02 AM PST, Josh Malone > wrote: > > > On Thu, Dec 28, 2023 at 10:35 AM wrote: >> When LCDs with these types of LCD driers first power up they turn on all the >> pixels. You will see this even in brand new ‘modern’ dot matrix LCD module >> (on the new LCDs the multiline modules tend to only make the first row black >> on power up.) Only after the computer sends a rest command to the LCD >> drivers they will clear the screen. >> > > Ah! Okay, I misunderstood that process then. If the driver ICs themselves set > every pixel then I agree it sounds like the machine is not running code then. > Thanks for clarifying this, Jeff. I appreciate it. > > -Josh > > > Lee Osborne West Lothian, Scotland 07960 096282 leeosbo...@fastmail.co.uk www.journeyman.online/services
Re: [M100] Model 100 - LCD Shows Pixels Only
Thank you everyone for the quick response! When I power up the computer, all pixels are on and remain dark (LCD is entirely "black"). Contrast potentiometer: Seems to work as intended. The display will lighten or darken as the potentiometer is adjusted. LCD PCB: No signs of corrosion and/or damaged traces. ROM: I'll ohm out the traces to see if I can find any broken connections. Otherwise, are there any other troubleshooting steps I can take? RAM: Jeff, great video! Thank you for sharing. The M100 Test Harness sounds like an ideal tool. I have limited experience with a logic analyzer however it looks like a fun challenge. I'll try to duplicate your setup but I may need some tips on how to configure the Saleae software... Eric On Thursday, December 28, 2023 at 08:17:02 AM PST, Josh Malone wrote: On Thu, Dec 28, 2023 at 10:35 AM wrote: When LCDs with these types of LCD driers first power up they turn on all the pixels. You will see this even in brand new ‘modern’ dot matrix LCD module (on the new LCDs the multiline modules tend to only make the first row black on power up.) Only after the computer sends a rest command to the LCD drivers they will clear the screen. Ah! Okay, I misunderstood that process then. If the driver ICs themselves set every pixel then I agree it sounds like the machine is not running code then. Thanks for clarifying this, Jeff. I appreciate it. -Josh
Re: [M100] Model 100 - LCD Shows Pixels Only
One thing about the LCD; if the lcd driver write acknowledge signal is broken, the cpu will be caught in an endless loop waiting for thr drivers to write successfully. In this state the cpu would be running but in a software loop polling the driver write ack signal. Sounds like a possibility here. On Thursday, December 28, 2023, Josh Malone wrote: > On Thu, Dec 28, 2023 at 10:35 AM wrote: > >> When LCDs with these types of LCD driers first power up they turn on all >> the pixels. You will see this even in brand new ‘modern’ dot matrix LCD >> module (on the new LCDs the multiline modules tend to only make the first >> row black on power up.) Only after the computer sends a rest command to the >> LCD drivers they will clear the screen. >> > > Ah! Okay, I misunderstood that process then. If the driver ICs themselves > set every pixel then I agree it sounds like the machine is not running code > then. Thanks for clarifying this, Jeff. I appreciate it. > > -Josh > >
Re: [M100] Model 100 - LCD Shows Pixels Only
On Thu, Dec 28, 2023 at 10:35 AM wrote: > When LCDs with these types of LCD driers first power up they turn on all > the pixels. You will see this even in brand new ‘modern’ dot matrix LCD > module (on the new LCDs the multiline modules tend to only make the first > row black on power up.) Only after the computer sends a rest command to the > LCD drivers they will clear the screen. > Ah! Okay, I misunderstood that process then. If the driver ICs themselves set every pixel then I agree it sounds like the machine is not running code then. Thanks for clarifying this, Jeff. I appreciate it. -Josh
Re: [M100] Model 100 - LCD Shows Pixels Only
When LCDs with these types of LCD driers first power up they turn on all the pixels. You will see this even in brand new ‘modern’ dot matrix LCD module (on the new LCDs the multiline modules tend to only make the first row black on power up.) Only after the computer sends a rest command to the LCD drivers they will clear the screen. Since you have already confirmed PCR (Power Clock Reset) and the beep test does not work it would tend to indicate the computer is not booting up properly. There can be multiple reasons for this. 1. Bad ROM or broken lines to ROM. This can cause the computer to run random garbage code, but will sometimes result in it running a halt command which you can see as the processor will stop. 2. Bad RAM, specifically the #0 RAM module. This is where the stack gets set up and if the stack area of RAM is bad it will crash on the first bit of code that tries to pop anything off the stack (because it gets nonsense back). I have experienced both problems multiple times. The RAM issue is harder to find without a diagnostic ROM. I have a M100 test harness and the first things its diagnostic ROM does is test the RAM. If it finds a RAM module with a large enough working section for the stack it will continue testing. If not, it stops. The first M100 I worked on had a bad RAM module #0 which allowed the stack to be overwritten by writes to other addresses. On a cold boot / hard reset up the part of the initial start-up sequence is to copy some code from ROM to RAM (which is a self-modifying bit of code, thus needs to be in RAM.) The copying of the ROM into RAM overwrote the start and thus the return from the jump to that memory address went to never-never-land. I found it with an 8-challen logic analyzer by recording ROM accesses and following along with an annotated disassembly. It was a very painful and slow process. The diagnostic ROM/test harness makes this a doddle. Here is a video I did on the first M100: https://youtu.be/hbLWk7ir9sI Jeff Birt From: M100 On Behalf Of Eric Sent: Wednesday, December 27, 2023 11:56 PM To: m100@lists.bitchin100.com Subject: [M100] Model 100 - LCD Shows Pixels Only Hi, I'm working to troubleshoot a non-functional Model 100 that only displays pixels when powered up. Seems these computers occasionally have RAM issues but I'm not sure how to identify faulty RAM that has not been removed. The RAM modules are soldered in place and I would like to troubleshoot the board to avoid unnecessary removal of parts. If anyone has other troubleshooting steps, I'm happy to try them out. I have access to a logic analyzer (Saleae software), oscilloscope, and DMM. Issue: When powered up, LCD shows pixels only (black screen w/adjustable contrast) Troubleshooting Performed: New NiCad installed and charged (memory switch on). Board traces near battery corrosion have good continuity. All capacitors were replaced. Board traces near leaking capacitors have good continuity. Cold Start and "BEEP" attempted. Beep was not heard. DC/DC Converter Circuit meets specs per section V of Service Manual 26-3801/3802 Reset Signal meets specs per section V of Service Manual 26-3801/3802 Logic Circuit meets specs per section V of Service Manual 26-3801/3802 CPU, ROM, Ram all have activity on the scope. Clock signal looks acceptable. Eric
Re: [M100] Model 100 - LCD Shows Pixels Only
On Thu, Dec 28, 2023 at 1:13 AM Eric wrote: > Hi, > > I'm working to troubleshoot a non-functional Model 100 that only displays > pixels when powered up. Seems these computers occasionally have RAM issues > but I'm not sure how to identify faulty RAM that has not been removed. The > RAM modules are soldered in place and I would like to troubleshoot the > board to avoid unnecessary removal of parts. If anyone has other > troubleshooting steps, I'm happy to try them out. I have access to a logic > analyzer (Saleae software), oscilloscope, and DMM. > > Issue: When powered up, LCD shows pixels only (black screen w/adjustable > contrast) > Hi Eric, Just to clarify, the LCD is entirely "black" right? All pixels are on? I think this is an issue with the LCD board, and here's why: On power-up, the LCD will very briefly display random data where the state of each pixel is random based on contents of the controller IC RAM. Shortly after this, the "boot" sequence will "clear" the screen which causes all pixels to turn on (black) and then off (white). The main CPU will actually wait for this to happen before proceeding with the power-on sequence. So, I suspect that the command to complete the "reset" is not completing. The failure of the "blind BEEP" test is consistent with this. The CPU can run code, but is hanging during the power-on sequence. I would carefully inspect the LCD PCB for corrosion and/or damaged traces. Possibly re-flow the controller chips if you feel comfortable working with those. If you happen to have another working LCD board, you can swap them and make sure the problem follows the LCD board. Good luck! -Josh
Re: [M100] Model 100 - LCD Shows Pixels Only
Could be a bad contrast potentiometer. On Thu, Dec 28, 2023, 1:13 AM Eric wrote: > Hi, > > I'm working to troubleshoot a non-functional Model 100 that only displays > pixels when powered up. Seems these computers occasionally have RAM issues > but I'm not sure how to identify faulty RAM that has not been removed. The > RAM modules are soldered in place and I would like to troubleshoot the > board to avoid unnecessary removal of parts. If anyone has other > troubleshooting steps, I'm happy to try them out. I have access to a logic > analyzer (Saleae software), oscilloscope, and DMM. > > Issue: When powered up, LCD shows pixels only (black screen w/adjustable > contrast) > > Troubleshooting Performed: > New NiCad installed and charged (memory switch on). Board traces > near battery corrosion have good continuity. > All capacitors were replaced. Board traces near leaking capacitors > have good continuity. > Cold Start and "BEEP" attempted. Beep was not heard. > DC/DC Converter Circuit meets specs per section V of Service Manual > 26-3801/3802 > Reset Signal meets specs per section V of Service Manual 26-3801/3802 > Logic Circuit meets specs per section V of Service Manual 26-3801/3802 > CPU, ROM, Ram all have activity on the scope. Clock signal looks > acceptable. > > Eric > >
