TI-99/4A ROM Replacement (Part 2)
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- Опубліковано 2 чер 2024
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The EPROMs have finally arrived, so let's try replacing the suspect faulty ROM with an EPROM. Along the way we'll run into multiple problems, like rebadged chips, incompatible EPROM programmers, and different chip sizes. Let's see if we can finally get it up and running!
Chapters:
00:00 Intro
00:14 Recap
01:41 Testing bus with ROM removed
02:26 Using a 2732A EPROM
05:15 Backup plan: 27C128
07:12 Building an adaptor for the EPROM
12:21 Testing new EPROM
18:51 Testing without the SRAM
20:11 Reducing the stack size
21:13 Conclusion
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#ti994a #texasinstruments #repair - Наука та технологія
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These videos are like ASMR, your voice is so relaxing. They're perfect after a hard day at work (fixing computer problems, strangely enough).
Haha, I'm glad! 😀
Agree, just like ASMR. And that's a real big plus!
ROMs rarely fail, although they can it doesn't happen often. If the RAM is bad however, and not responding properly to the chip enable (maybe enabled all the time) a bad RAM chip could cause the system to fail to read the ROM properly. Different ROM chips might even react to that environment differently. It would be interesting to see the systems behavior with the original ROM and the RAM removed or replaced.
Good point. I'm planning on going back and trying back the original chips once I get it to a working state. I don't know about the TI99, but I've seen plenty of ROMs fail in other systems, so it seems definitely very possible.
@@NoelsRetroLab
Usually when ROMs fail, they are EPROMs. The TI uses mask programmed ROM chips. They can fail, but RAM fails much more frequently.
Your perseverance is legendary. I love this channel :)
Thank you! I appreciate that!
Excellent video series. I know it been over a year but you should know this...
In the 1980s and early to mid 1990s TI never made a 27xx-series EPROMs. They made a 25xx-series EPROMs which has a different programming voltage and a slightly different Pin Outs. They also had a raised window section. You can take a microscope to read the die to see what kind of chip it is through the glass. With the 25xx-series EPROMs you could have put them into the ROM socket directly as that is what they were designed for. After the mid 1900s TI could have made a 27xx-series EPROMs but I doubt it and do not take my word for it.
Love all your repair videos, they are great!
Thanks for making the video! Really like the effort you're putting in here. I am the author of the first FPGA implementation of the TI-99/4A: github.com/Speccery/EP994A so I can say that I know this computer quite well. My FPGA design implements all the chips in the computer. While I have built an add on cartridge and expansion bus devices for the real iron, I haven't debugged faulty TI's too much (my QL is another story). With regards to the different voltage levels you're seeing on the data bus, one potential source could be the GROM chips which have weak drivers. These should be easy to see with a scope, since they operate very slowly and have many wait states. I haven't yet watched part 1, but you probably are aware that the TMS9900 CPU implements its register file in RAM. The CPU only has three on-chip registers. If you replaced the other ROM chip with an EPROM, it would be a simple matter to create for example a debug ROM consisting only of NOPs (and a jump to beginning of ROM at the end). This could be used to see if the CPU is correctly executing through the entire ROM space.
Thanks for sharing those insights, Erik! As you can see, I'm on the opposite end from you: I'm fairly familiar repairing computers, but the TI is completely new to me 😀 I was aware that the CPU uses the RAM as its registers, but I like your idea of filling the ROM with NOPs or even some simple loop to make sure that the CPU is behaving correctly. I think that if the SRAM replacement doesn't work, I'll try that (replacing the other ROM) before moving on to using a logic analyzer. I'm definitely learning a lot about this architecture as I go. Fascinating how different it is from most other 80s computers!
Hey Erik, I tried sending you a message through FB (or at least I assumed it was you). Could you check it or email me here at the email in the channel? (About page). I'd love to ask you a couple technical questions about the data bus and that simple program. Thanks!
I laughed out loud when I read your t-shirt.
LOL... Me too!
Yeah, that's the mood I was in while tacking this repair 😀
The scorch mark on the rightmost 2732A leads me to believe they are counterfeit because it was probably caused by blasting the die with a programming voltage while the die wasn't designed for that (or wasn't even an EEPROM to begin with).
Yeah, I agree. I only noticed the scorch mark afterwards but that doesn't look good at all.
@@NoelsRetroLab I've worked with an incredible number of EPROMs and have ~40K of them in stock. That font used on those is one I do not recall seeing on older TI EPROMs.
Excellent work my friend! Keep up the great/fascinating work.
Thank you!
Hey Noel!
Like your channel very much, lots of unique technical stuff and our beloved good old machines. Featured your channel on mine!
Keep them coming!
Thanks! Glad you're enjoying it!
Great video Noel hope all is well.
Thanks!
Love the channel. Please keep up the good work.
Thanks, will do! 👍
Hello, I recently discovered your channel, it is excellent! Greetings from Argentina
Thank you! Me alegro de que te gusten 😀
Thank you for the video. Question. What tool are you using at 1:09 when you desolder the ROM chip?
Just a random memory of my TI frying twice in 1982/83 - the box cycled through sounds and colours, and while correlation doesn't equate to causation, it seemed to be somehow related to alpha lock being on when accessing the joysticks... might have been a quirk of my TI, and the report is best taken with a pinch of salt, since it's over a gap of 38 years. It's how I remember it happening.
Oh wow! So you had to replace the SRAM twice back in the day? That seems very fragile. I wonder what the fault rate is on them.
Very interesting series!
Thanks! It's always a challenge to deal with a computer with so many problems that I'm not intimately familiar with.
Strange, I was subscribed and then noticed I wasn't??? Great video =D
It was the UA-cam gremlins 😀 Thanks for the comments. Means a lot coming from you! 👍
I got the exact same programmer - apparently ours are both "fake" judging from the PCB visible from the vents, that's what XGecu say anyway - and also was disappointed to learn that it cannot program 21V old eeproms.
Can't wait to see the next chapter, thanks for the video!
Really??? I did read some stuff about fake programmers, but I didn't realize I had one of those. Do you have a link where they explain that? I've been able to upgrade to the latest firmware for years without a problem. I'm currently looking for one that can handle 21V or even 25V too.
@@NoelsRetroLab I'm not sure if you have a counterfeit programmer but you can check out the official homepage of the XGecu programmer: www.xgecu.com/en/note.html
As you can update your programmer (I commented too quickly ;-) I wouldn't be too worried, though, as only some older versions apparently bricked counterfeit programmers.
Thanks for the video =)
My pleasure!
The rf shield touching the EEPROM will also act as a heat sink.
Nice video as always Noel, I would be interested in seeing a video about where you get those rom sockets from (ebay ?) and which ones to buy, also, nice to see you changed the wrag for clearing up the solder after cleaning with alcohol
Thanks! I was actually thinking of doing one better and making a video designing the PCB for such an adapter. But I would like to make it quickly to continue, so I might just buy one for now and save that for a future video.
The rags are still the same type. They're made from old shirts and I just cycle through them every few days. It's just that I have a lot of red ones so it seems it's always the same one 😀
Where can I get the file you used to burn the 27C128? I have a "good" ROM, but don't see the exact make/model in the software provided with my TL866 II Plus programmer, to read from it.
Legend. Great video!
By the way, with the TL866II you can pull out the Vpp leg and apply the programming voltage from an external source. I keep meaning to build an adapter, but it's easy enough. You just need to turn off pin detect.
I read about it here...
pinside.com/pinball/forum/topic/program-21v-25v-roms-in-lower-voltage-programmers
Funny, I did come across that thread when I was researching that, but I was a bit uneasy about applying that voltage all the time, and not just during the programming sequence. I suppose it doesn't matter. Still, I'm thinking about getting an EPROM programmer that supports those voltages natively. Do you have any recommendations, Mark?
@@NoelsRetroLab TBH, I just use the TL-866II and it works in most cases.
That programming voltage will get you every time on the old devices. I keep a Willem parallel port programmer and old laptop just for that purpose.
Yeah, it seems a lot of people do that. I need to get an old EPROM programmer, but they're all so expensive!
What's the deal with the 1-Jan-09 date on the Hantek; no battery backed clock? I noticed the time seemed to go backwards at edit points.
Either that, or I'm shooting my videos waaaaaaay ahead of time 🤣Yeah, apparently there's no battery for the clock. Every time I turn it on and off it restarts. I never noticed that until a few weeks ago actually, but it does make for some weird edits sometimes.
@@NoelsRetroLab I did actually wonder for for a split second "Did he record this in January? Wait... 09?". lol Thanks for the answer, and keep making these interesting videos! I'm learning a lot by your examples. You and a few other channels here have inspired me to pick up the iron, and I fixed my first old cart this week (broken via not conducting from one side to the other). Thanks again!
@@3vi1J That's awesome! I love hearing that people are getting inspired to start trying things on their own. Once you start... there's no turning back! 😀
2532 chips can be harder to program - they take 25v, so look for the 2532A which uses 21v. Also, you can program a 2532 as a 2732 with a socket adapter. You should look into some RAM testing gear like the Neoloch Inquisitor as it will test the 6810 RAMs used in the TI. It has become an indispensable piece of gear in my shop - Get all the blades for it. BTW: If you want to work on older gear, I'd suggest picking up an older programmer that can do the chips you'll commonly find. I have a Topmax and a Data I/O Series 22 which will let me do almost anything EPROM back to 2704 days plus bipolar PROMs, GAL/PALs, and some PLAs.
I'm surprised your video didn't show you reading the ROM from the TI in your programmer. You can use software like Hash Calc to generate a CRC-32 (if your programmer doesn't do this natively) and run the results through Google. If you have a good checksum you'll see ROM download sites in your search results. Now, this just tells you that the ROM is readable, it doesn't tell you if it will work correctly at speed in the circuit. I've run across that a couple of times over the years of fixing retro systems and old arcade game boards.
Good point. I believe I tried reading it and it didn't work, but then again 2532s aren't supported in my annoying EPROM programmer, so that didn't surprise me very much.
Yes, I see I'm going to need a different programmer. Are those two the ones you would most recommend? Do you know if they work with modern computers, or do they use an old PC with RS-232?
@@NoelsRetroLab Topmax is $$$ and requires XP with a built-in parallel port, not USB or PCI, but VERY well worth the money as it will do the 2532, non-JEDEC 1MB ROMs, 16-bit EPROMs, and the odd Motorola 68764/68766 ROMs. The Data I/O Series 22 is very old and hard to find. It will do a lot of really old arcane chips. Both are highly recommended by me.
@@Arcadecomponentscom Thanks! That's super useful info. I'll start looking for them.
I think that trying to fix things you have to feel a bit like Sherlock Holmes, identifying the culprit. In your case you have some evidences, but you may have to ask to more witnesses (or chips). With some of them you use the Truth Machine (Oscilloscope) ;)
Haha, so true! It is half detective work, half scientific work.
Use two scope probes with one on read not write to see if the data is going in or out of the CPU.
Yeah, that's a really good idea. I might try that before pulling out the data analyzer if things don't start working with the new SRAM.
That TI is a right pain. ANYTHING could be wrong with it. :)
Yeah, pretty much. And I'm not even familiar with the system, so it's the worst possible case. Oh well, hopefully people will learn or at least enjoy it as I'm learning my way through it.
Good lord
What a troublesome patient!
Instead of making an adapter for the computer you make an adapter to burn 2532 chips as 2732 in the mini pro. 2532a are not hard to find at least no more so than other vintage eproms
For some reason, every time I ordered 2532s I kept getting rebadged fakes, so I gave up on that.
I have a GQ brand GQ-4X4 programmer that has aux power so can program the older chips that need more voltages. If you wanted to send me those eproms I can test them out ?
Thanks for the offer, Lee! I think I should plan on getting an EPROM programmer that supports 21V or even 25V. That would really help me in future repairs. Are you happy with yours? Would you recommend it?
@@NoelsRetroLab I use the minipro first and then the GQ-4x4 if that does not support. You can request updated parts if they do no not currently support and it worked perfect for some really old eprom that needed 25v
@@leesmithsworkshop Thanks! I'll look into that one.
@Mr Guru Can I simply do that, or do I need to add some logic to only apply 21V when the pulse is active? I don't know the details of EPROM programming to know if it's OK to apply it all the time or not (or whether that even depends on the EPROM type). The other thing I was thinking about as a quick hack was to add an RS232 battery in series with the VPP, which would neatly bring the total but I have no idea if that will even provide the necessary current. Definitely going to look into getting some of those adaptor PCBs and see if I can get them before next video. Thanks!
I was checking out your designs and realized that they're slightly different adaptors. Do you know where I might be able to find the files for a 2532 to 27128 adaptor? It's simple enough that I could probably make it myself, but I'd rather get it as soon as possible for part 3. Or I would even buy a pcb directly if they're available somewhere. My first search failed me.
Duh.
EPROM Mfg page says data cannot be guaranteed past 10 years. So EPROM errors on old chips is to be expected.
Why hasn't someone created a dirt cheap USB programmable Atmel MCU-based universal EPROM/ROM replacement where the I/O and power pin configuration is programmed by specifying the device type to the MCU and the MCU's storage is used for the EPROM/ROMs data content? It could be so cheap that it could just be left in place.
Probably because there isn't enough demand since most of the time EPROMs work just fine, but it's not a bad idea!
@@NoelsRetroLab Yes, but everyone has a USB port to use, but not an EPROM programmer/burner. Plus, the AUTHENTIC antique ICs are only going to get rarer with time and the market further polluted by unethical Chinese sellers. 60MHz STM32 Arm Cortex MCUs are US$0.80 in 2.5k quantity and that's not even directly from ST Microelectronics. Just off the top of my head, this wouldn't be a very difficult thing to design. But I don't need the money that might be made in that limited market before the Chinese ripped it off to sell clones on eBay, and it's not of huge interest to me anyway. So, if it's actually feasible and worth doing, it would be great to see someone else run with it.
Hi, Noel. I got the same problem for my Apple //e. Since I want to keep my original EPROM from wear out, I go straight ahead to adapter from: www.arcade-cabinets.com/board_hacks/24U/index.shtml. They're great and easy to use, the only trade-off is they're double the height, may not fit in all of the chassis, for Apple //e and //c it's fine, but not sure TI 99/A since it seems slim, I own it 35 years ago and it's been a long time.
Yes, those little adapter PCBs are great and much more elegant than my stacks of sockets 😃
@@NoelsRetroLab Your little socket is simple and handy, I've tried to solder thin copper wire with socket, looking is far easier than really work on it.
Retro computing at its finest
Thank you! 😀👍
Oh, are you running the programmer software under WINE or something?
No, the other way around: I'm running WSL on Windows 10, which gives you access to a nice shell.
@@NoelsRetroLab Doh! I was getting my hopes up then... =)
I keep a Windows drive in one of my old ThinkPads pretty much just to run the programmer at the moment, so I was hoping I could do away with that and use the laptop for something else.
@@Xoferif There is open source software for the minipro programmers..
@@donpalmera was going to suggest the same. I have one of those programmers. The Windows software is dreadful, but minipro on the commandline is a good alternative. gitlab.com/DavidGriffith/minipro/
@@Xoferif There is the Minipro app for Linux that you could use: gitlab.com/DavidGriffith/minipro. I've installed it, and have seen other UA-camrs (Ben Eater, maybe?) using it. Another alternative would be to set up a VM with a minimal Windows install. VirtualBox and other VM software will let you redirect USB devices directly to the VM. I've used that technique in the past when I needed to use Windows utilities to update firmware on GPS and other devices via USB.
i need a Amstrad 40007 replacement...anyone know if its possible with an STM32 or the like? (non-fpga!)
Uff, that's the Gate Array isn't it? I'm afraid you can't yet. Only thing you can do is get it from another Amstrad. We need to do a similar reverse-engineering job with it as they did with the Spectrum ULA before we can start manufacturing new ones.
@@NoelsRetroLab bugga! i just brought a 464 board and its missing the gate array...i know the 40010 used in the 664 and 6128 isnt pin compatable, but could probably make an adapter like you have in this vid....
btw.. nice vid!.. they seem to be getting better all the time..keep it up!
@@WacKEDmaN Thank you! That's a total bummer that it came without the chip! You're sure it's not one of those 464 boards that has room for both the 40007 and 40010, right? It might seem that it's missing the Gate Array, but it has the old version in it.
@@NoelsRetroLab seems to be an original 1983 version 1 board.. i.ebayimg.com/images/g/pDQAAOSw0c9fFuoJ/s-l1600.jpg
looks like im outta luck...but im sure somethin can be hacked up to get a 40010 running..they seem to be a bit more common than the 40007
BTW, someone in the a 464 Facebook group was selling all the chips they pulled out of an Amstrad CPC 464, including the 40007. Check it out if you're interested.
Are you going to replace the other ROM chip because if one is bad then the other ROM is bad.
Not yet. It's kind of a pain to replace it, so unless I suspect it's defective, I'd rather not. At least all the data lines are looking good with it, so I'll wait for now. If after replacing the SRAM it's still not booting up, then I might replace it just to eliminate easy candidates.
There is a real problem with fake/bad 24 pin EPROMs. I bought 40 TMS2532 EPROMs and only 4 of them would program.
Yikes! There's a big problem in general with fake chips. That's what my next video is going to be about actually.
hi, i had the same problem programming russian 2716 clone with the minipro, needs 25v, so i pull out the vpp pin and applied 25v from external power supply by touching the pin,and it works, strangely the minipro don't sense the missing pin. (minipro tl866cs), you had nothing to lose,burn them with 25v and see what happend.
Yes, good tip. I read that in a forum as well, so I tried it after making this video and it didn't work. I suspect the EPROMs I got were not genuine.
change sram, it will work
Good prediction! 😃