Equipment that used the 8275H CRTC? - Page 1
Forum section: electronics / repair

author: james_s
author_rank: Super Contributor
timestamp: February 25, 2017, 07:34:35 pm
content: I'm working on a CRT controller from an older Hurco CNC machine that is based on the P8275H programmable CRT controller. The monitor blew up resulting in 12V being fed back into the video output pins and fried several ICs on the controller board. I've repaired the monitor and replaced the bad TTL chips but I'm still not getting proper sync signals out of the thing. I've bought several 8275's from different sellers and a couple appear to be dead, one shows reasonable signs of life but still not quite right. What I'd like to do is find something else using this IC that I can use as a test bed, pop the chip into a known good board and see if it works. Anyone know of anything reasonably attainable that used these? Mass produced 8 bit computer, test equipment, etc? I'd also buy a known good tested working 8275, but I don't want to waste time/money on more random ebay pulls.Diagnosing this board has been tricky because I cannot find any schematics, even a guy I know who worked at Hurco talked to some guys he knows there and they don't have anything still for systems this old. The way the PCB is laid out makes it extremely tedious to trace out the circuit because there are vias everywhere, many underneath soldered in ICs.

author: Rasz
author_rank: Super Contributor
timestamp: February 25, 2017, 08:28:08 pm
content: this is a 40 year old chip, wouldnt it be easier, cheaper and overall better to upgrade whole controller to something from this decade or even century?

author: james_s
author_rank: Super Contributor
timestamp: February 26, 2017, 04:54:51 am
content: Not really. We've upgraded the controllers on a number of machines of this era (early 90s) in the past and it's a big and expensive project and not really worth it. The old controllers work well and are nicely integrated with the machines, and spending a few bucks and some time repairing this board is a lot more economical than spending $5k and several tens of man hours replacing the whole system on a machine that is probably not worth that much in working order. The old Hurco is sort of a toy over in the corner of the shop for tinkering and projects that are not worth tying up the $100k+ production milling centers.Besides, what's the fun in simply replacing something rather than repairing it? I like this old tech, it reminds me of working on my vintage computers and classic arcade machines.

author: Rasz
author_rank: Super Contributor
timestamp: February 26, 2017, 07:50:02 am
content: $150 and you go on with your life instead of playing archaeologyIf this was something actually culturally significant I would tell you to contact computer history museum (computerhistory.org), or Marc Verdiell (https://www.youtube.com/user/mverdiell/videos), but with cnc it would just waste their time

author: james_s
author_rank: Super Contributor
timestamp: February 26, 2017, 05:50:29 pm
content: You have not worked with one of these machines have you? We were doing the conversions years ago and it's a big job, you need a lot more than that dinky Chinese box. You need a big VFD for the 10HP spindle motor, you need new drivers for the servomotors, you need to work out a solution to drive all the solenoid valves for coolant, pneumatics, way lube pumps, all of the safety interlocks need to function properly to keep OSHA happy. I'm not sure you grasp the scale and complexity of the machine. There is also no way in hell I would trust a rinky dink $150 Chinese controller to run a machine that is spinning a cutter that can cost several times that and will tear parts off one's body without even slowing down.Look, we want to repair the existing control and keep this machine original, that is not under discussion and frankly I don't care what your opinion is on that matter. I like playing with old tech like this, the reasons why are irrelevant. If you have nothing useful to contribute to the discussion of repairing the vintage electronics then please don't clutter the thread with useless ideological debate.

author: helius
author_rank: Super Contributor
timestamp: February 26, 2017, 07:40:53 pm
content: You will have difficulty finding a machine that uses the 8275 CRTC.Like this one:ortheseboardsfor S-100 cabinets.

author: james_s
author_rank: Super Contributor
timestamp: February 26, 2017, 07:54:29 pm
content: Interesting, I had no idea these chips were so rarely used, I'd have thought they would have found their way into numerous 8 bit computers and video cards, dumb terminals, that kind of stuff. Looks like I'm going to have to hook it up to a microcontroller and write some code to put the chip through its paces, or hook up the logic analyzer and see if I can capture the initialization. Unfortunately the CRT is the only utilized output on the board which is actually a complete 8085 based computer so it's hard to tell if the rest is working properly. Wish it was a 6502, I'm a lot more familiar with those.

author: Armadillo
author_rank: Super Contributor
timestamp: February 26, 2017, 07:57:30 pm
content: I read the datasheet which indicate that the 8275H needs to be programmed with parameters otherwise;"Reference; Device programming"Quote:"After the reset command is written, DMA requests stop, 8275H interrupts are disabled, and the VSP output is used to blank the screen. HRTC and VRTC continue to run. HRTC and VRTC timing are random on power-up."Un-Quote:Maybe try setup or configuration again??

author: helius
author_rank: Super Contributor
timestamp: February 26, 2017, 07:57:46 pm
content: I think its architecture was just not well-suited to cheap high-volume computers. It needs a dedicated DMA controller, for instance.Here are other uses:Zorba CP/M,Robotron. It could be in some terminals.

author: Rasz
author_rank: Super Contributor
timestamp: February 26, 2017, 08:36:27 pm
content: Quote from: james_s on February 26, 2017, 05:50:29 pmClearly I didnt, because you1 didnt even bother to give full model number of the machine2 alluded $20 chip from ebay is too much3 directly stated its not worth that much4 and is a toyand now you attack me for concluding something build on 1970 technology not worth your $20 was a good candidate for a simple cheap controller swapQuote from: Armadillo on February 26, 2017, 07:57:30 pmare there even CRTCs that start with some sane default parameters? 6845 certainly doesntMAME lists few systems with P8275, all late 1970/early 1980, like Panasonic JD-700M. Good luck finding that ;~)

author: helius
author_rank: Super Contributor
timestamp: February 26, 2017, 09:00:09 pm
content: I think there arereputable storesselling them, that may be more reliable than ebay (lot of dubious remarked ICs there).But problems besides bad ICs could be at fault, like bad sockets, cold solder joints, cracked PCBs... it pays to cover all bases.

author: Armadillo
author_rank: Super Contributor
timestamp: February 26, 2017, 10:57:02 pm
content: JD700 service manual;

author: Cerebus
author_rank: Super Contributor
timestamp: February 27, 2017, 12:26:10 am
content: Quote from: james_s on February 26, 2017, 07:54:29 pmOh, they did, they did. I probably saw one in every other bit of computer kit I popped the top off back in the '80s. The thing is that all that stuff is now landfill or been recycled for scrap gold so trying to think of something that might still be around is a bit of a stretch. I mean, when was the last time most people saw a green screen terminal in actual production use? For me, it was in around '96.

author: james_s
author_rank: Super Contributor
timestamp: February 27, 2017, 01:05:50 am
content: Quote from: Rasz on February 26, 2017, 08:36:27 pm1. The model of the machine is irrelevant, I'm not asking about the machine here, or even the specific controller hardware, I was asking about a convenient way of testing the chip, which I've learned from other participants here is less common than I had assumed.2. $20 would be fine. $50 would be fine. What is not fine is buying multiple chips, waiting weeks for each to arrive, finding that they behave differently from one another and not knowing which if any are functioning correctly. Is the odd sync signals because the CRTC is bad, or because another fault is preventing the parameters from being set properly? I don't want to simply throw parts at it until it works, I want to understand how it works, why it doesn't work, and properly test the parts, most of which are easy to test. Even if I had a replacement board already I'd still want to determine what's wrong with this one.3. The machine worth anywhere from a few thousand up to maybe $5,000-$6,000 in working condition. That's dirt cheap compared to most of the machines in the shop but not so cheap that it's not worth repairing. If I'm going to go hang out and make some parts for myself I'm going to use a machine I can afford to replace if I break it rather than something where a mistake costs enough to buy a nice new car.Now just what is wrong with something built around 1970s technology if it works? Hurco was still building machines with this technology well into the 1990s, it's solid and time tested design that for the most part works very well. This is a conservative industry, like test equipment it doesn't change just for the sake of change. You can still easily pay $150K for a machine made a few years ago that is running Windows XP embedded, by computer standards that's ancient but these are tools, they don't need to be upgraded. In fact one nice thing about the older controllers is the firmware runs bare metal on the hardware with no OS, they never crash, they don't have to be properly shut down, they just work. There's a reason they made essentially the same control system for ~20 years.I think "attack" is a strong word here, I wasn't intending to attack, I simply get annoyed when I'm looking for specific information to solve a puzzle and get responses that are essentially "just throw it away and buy a new one", that isn't helpful at all. I'm fully capable of throwing something away and replacing it, I don't need to come here for that, I came here because I want to fix this thing and I want to understand it. Retro hardware is something that interests me a great deal and the disposable society with its attitude that old stuff is junk and not worth messing with by virtue of being old is something that has always rubbed me the wrong way.I have a whole room full of arcade machines from the late 70s-early 80s but none of them use a CRTC as far as I know. They have their own uniquely fascinating hardware though.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: February 28, 2017, 04:59:48 am
content: Darnit, the 8275 sounds familiar, but I can't rememberwhy!

author: james_s
author_rank: Super Contributor
timestamp: February 28, 2017, 05:49:20 am
content: In case anyone is curious I was able to fix this. Logic analyzer to the rescue I could see that the CPU seemed to be acting strangely, then traced that to a bad 74LS373 that handles the data and address bus multiplexing for the 8085 CPU that runs the video display. Honing in on that chip and looking at all of its inputs and outputs it was pretty clear something was not right. This supports my past experience that while it's natural to suspect the hard to test/exotic part(s) when something is not working, more often than not the real problem is something simple. Funny thing is I had checked that chip with a logic probe earlier and it looked like all the lines had activity, turns out those little glitches occurring on the dead lines were enough to blink the logic probe. The inconsistent behavior I was seeing with the 3 8275 chips I have comes down to the snippet Armadillo noticed in the datasheet about the timing settings being random at powerup.Anyway that was a nice satisfying puzzle.