No, I checked that and it isn't. It is an active low signal and it is high. Regards Rob duell CPU

No stuck, high. Regards Rob high. in it.

Yes there is, but it looks odd, and very different to when I checked the address pins on the working one. It looked like the voltage might be spiking upwards, but it is hard to tell because I don't have a working logic analyser. RD and WR are active low, and they are both high, so I suppose they could be shorted high, but I don't know. Me too, although I have checked multiple times. I am using this page to give me the pinout: http://scanftree.com/microprocessor/Pin-Diagram-of-8085-and-Pin-description…

I checked Vcc and Vss and they appeared to be fine (5V and 0V respectively). It will never be steady with a scope because the address lines won't be a regular waveform. That said they looked very different on the faulty board to how they looked on the good board. I have a desoldering gun now, so desoldering is no longer the problem it once was for me, but I would still prefer to avoid this if possible. I am not sure what else to check though.

Bruce a to around If I had one.... I did buy a cheap Thurlby LA160 recently. It is the type that uses your scope for its screen. It seems to work fine except that many of the membrane keys don't respond :-( Regards Rob

Guzis Analyzer a 450Mhz That isn't a bad idea. I may have the wherewithal to set that up if necessary. Thanks Rob

On 2015-01-18 23:04, Diane Bruce wrote: If the RD pin never gets active, the contents of the ROM will not even be presented to the CPU, so while it's possible the ROM is borked, that is not the problem we're trying to fix right now. Johnny -- Johnny Billquist || "I'm on a bus || on a psychedelic trip email: bqt at softjar.se || Reading murder books pdp is alive! || tryin' to stay hip" - B. Idol

On Mon, Jan 19, 2015 at 12:00:22AM -0000, Robert Jarratt wrote: ... > > > > > > A remote possibility is the boot ROM is b0rked and it's simply running > > > around like a chicken without a head. > > > That is something a good logic analyser could tell you. > > > If the RD pin never gets active, the contents of the ROM will not even be > > presented to the CPU, so while it's possible the ROM is borked, that is not the > > problem we're trying to fix right now. > > I just found something interesting, the pin activity on the address lines, RD, WR, ALE etc appears normal for a bit, and only after a few seconds does it all become inactive. > Perhaps this really is an indication that the ROM is bad, if it is getting into some kind of loop where it does no external activity. I don?t really know the 8085 to know whether that is possible. > Regards If the VT-100 used an EPROM and not a masked ROM I'd suspect bit rot. A quick glance at this http://tkc8800.com/images/vt100/VT100_Technical_Manual.pdf makes no mention of EPROM vs. masked ROM, but it looking at this manual it is possible you have bad ram as well. In that case, the LEDs should signal this. > Rob -- - db at FreeBSD.org db at db.net http://www.db.net/~db

I'm probably just exposing my ignorance of the 8085 here, but did it have anything like the prefix opcodes x86 has (repne et al)? I'm wondering if it could be stuck doing something like (to admittedly try to use syntax I don't really know) "repne cmp %ax,%ax" that would never occur normally but might compute forever without fetching anything further. Or, does it have a instruction prefetch buffer? (My guess would be it's too old for that, but I don't really know.) If so, a very tight loop might execute entirely out of the prefetch buffer.... /~\ The ASCII Mouse \ / Ribbon Campaign X Against HTML mouse at rodents-montreal.org / \ Email! 7D C8 61 52 5D E7 2D 39 4E F1 31 3E E8 B3 27 4B

What if (due to ROM or RAM problems maybe) the 8085 executed a HLT instruction? -tony

On 2015-01-19 09:04, tony duell wrote: I didn't know it had a halt instruction, which is why I speculated (negatively) about that in a previous reply. But if it do indeed have a halt, then this would be a very possible explanation. Johnny -- Johnny Billquist || "I'm on a bus || on a psychedelic trip email: bqt at softjar.se || Reading murder books pdp is alive! || tryin' to stay hip" - B. Idol

Sorry, I know, but the terminal controller board appears to be significantly different to the VT100, the VT100 does not use the 8085. I will have a read of the sections you indicate though. Some do, but not all, and I don't hear the beep from the keyboard. HLT is looking likely. I will reseat the only socketed chip on the board to make sure. Thanks for the suggestion. Regards Rob

On 2015-01-18 22:47, Robert Jarratt wrote: I can't remember (and haven't checked), but make sure that chip isn't one of those funny one that requires several clocks or power levels. It should be steady between clocks. You should essentially run up the frequency of the scope so that you can see each clock phase nice and wide. Checking address pins you should then also have the same nice long bits. You might see some initial ring when moving to a different level, but that should be very small and short. After that it should be level for the rest of the clock cycle. Ie. You need to look closer. Just just "it's some kind of activity going on". That do help. :-) But I understand you. Not much point in replacing components that turns out to be fine. Especially since it still might be destructive to remove them. Johnny -- Johnny Billquist || "I'm on a bus || on a psychedelic trip email: bqt at softjar.se || Reading murder books pdp is alive! || tryin' to stay hip" - B. Idol

Sadly, that is the one thing that does not seem to be available. If anyone does know of a print set for the VT101 please let me know. Thanks Rob

On Mon, Jan 19, 2015 at 09:26:57PM -0000, Robert Jarratt wrote: What ever happened to in-circuit emulators? I used an 8085A ICE in a microprocessors class at Boston U. in the 1980s and it was *magic*. I always wondered how they worked -- is it just an 8085A with a doctored clock, or some complete re-implementation of it, or what? Physically it was a 40-pin DIP plug with a thick cable leading off to a mass of electronics. It's just the kind of thing that would turn up on eBay, but only if you didn't need it. John Wilson D Bit

Bruce I suppose again it is likely that a logic analyser would help. I have just skip-read the power up and self-test section from the VT100 manual you linked for me, and realise it halts the processor if it detects a ROM checksum error or an error in the RAM. I am not sure if this explains why I get random characters displayed, but the halt is consistent with a RAM failure as you suggest. When I next get the chance I will probe the RAM chips to see if their behaviour looks OK, but I only have a scope so it won't be a very comprehensive test. It will be the weekend before I get the next opportunity. Regards Rob

Burke wires add right I like the idea of swapping the ROM for an EPROM (whether to write my own test code, or just to replace with a fresh ROM image - assuming I can find one). I'll probe the ROM pins at the weekend to see if it appears to be OK. Do you recall what EPROM you used and how its pinout differed from the ROM you were replacing? As far as I can tell there is only one ROM on the board, could it be both the character ROM and the firmware? Regards Rob

On 20/01/2015 05:06, Robert Jarratt wrote: I used a 2716, but you will probably need a 2764 for the ROM in question. If you look at the four ROMs on the VT100 schematic you will see that the chip select pins are different for each one. Mask ROMs can be manufactured with any combination of chip select required whereas EPROMs are fixed. This means that the four mask ROMs can be installed in any order. There should be a separate ROM on the board for the character generator. The part number will be something like 23-018E2-00. It's usually soldered in. Matt

On 2015-01-24 01:29, Robert Jarratt wrote: Given it's "8232", I could suspect that it would be a 4K rom... Also, looking at the VT100 and VT103, they have 2K roms, named 8316, which really reinforces my suspicion that this would be a 4K rom... Johnny -- Johnny Billquist || "I'm on a bus || on a psychedelic trip email: bqt at softjar.se || Reading murder books pdp is alive! || tryin' to stay hip" - B. Idol

On 24/01/2015 00:52, Johnny Billquist wrote: As close as you'll get in an EPROM, modulo chip select polarities. Someone on the list promised me a few some years ago, but I never got them :-( I don't, sorry, but if anyone get an image dumped, I'll host it alongside the others on my website. Nope, it's 8K. A DEC 23-xxxE4 part number specifies 8K ROM. I expect 8232 is the date code. -- Pete Pete Turnbull

On Sat, Jan 24, 2015 at 12:29:59AM -0000, Robert Jarratt wrote: Based on nothing more than the fact that I know the APL char gen ROM socket on the VT100's AVO board has jumpers to choose between various chip-select polarities, and that the default setting (for DEC's mask ROM) differs from a 2716 or whatever the similar-sized normal EPROM was (since I had to change them to make one of those work 1,000 years ago), I buy the theory that this thing may have non-standard chip-select pins/polarities. Reading as all ones certainly does sound like it's not being selected properly, just as much as it could mean a frotzed chip. Nope but I do have a VT101, so if you get stuck (and someone does know how to set the CS pin(s)), I'd be willing to try to read it out. I got the VT11 char gen PROM out using a LPT port and some perf board so it's OK if it doesn't correspond to any vanilla EPROM type, as long as the pinout can be figured out. John Wilson D Bit

duell is only one levels it is contents) to possible RST those ROM is You're right, I did think that it seemed odd that it would appear to be all 1's when the CPU does appear to do things, and that the behaviour is random, suggesting that the ROM can't possibly be all 1s. I was planning to put the ROM on a breadboard so that I can play with its connections to the programmer, but, overnight, the old PC I use to connect the parallel port programmer has developed what looks like a memory fault. I don't have any spare memory so it will be next weekend now before I can get new memory and try again. :-( Regards Rob

A little while back I posted about a possible 8085 CPU fault on my VT101 terminal controller. After a session with the logic analyser, courtesy of another member of this list, we determined that the 8085 was indeed bad. We substituted it with another 8085 and the terminal controller started working normally. However. This has revealed a fault on the monitor control board. There is the occasional vertical wobble. Sometimes the image is stable, then it will wobble up and down a bit, sometimes just a bit, sometimes more frequently, then it will stabilise for a few seconds, then it will wobble again, etc etc. I know it is the monitor control board because we sent the Video Out output to a separate monitor and it was perfect. I have checked the ESR on all the electrolytics, they are all fine. Prior to all this I had replaced C439 with a higher voltage rated component, as recommended here: https://trmm.net/VT100. I used a 100uF part, rated at 100V. I had also replaced C437 as that had a high ESR prior to seeing this problem. Is there anything else I should check? Regards Rob

On 2/15/2015 5:24 PM, Robert Jarratt wrote: ESR alone is not enough to validate capacitors. - J.

On 02/16/2015 12:38 PM, Robert Jarratt wrote: Start with the largest electrolytics. Also, check any power resistors--they sometimes do fail. --Chuck

I have mentioned that I kept my transcripts of my CompuServe surfing. I keep thinking about how I might adapt them to a web version to recreate forums. I'm thinking about how to parse the posts. I'm no doubt overthinking for a rare case, but does anyone remember how CIS handled or allowed quoting a post within a post? Posts looked like: #: 23540 S7/Programming 20-Jun-86 02:40:20 Sb: #23526-MenuEd Fm: Larry Phillips/ICUG 74025,636 To: David Milligan 70707,2521 One other thing I might add. It would be SUPER if those writing editors for gadgets, et al, if they allowed output in assembler code for those of us who never passed our Cryptanalysis 101 course. Did CIS reject quoting posts within posts, if only to prevent spoofing? - John

Burke to schematics I have been desoldering and checking all the electrolytics. I have set each one up on my bench PSU with the current limiter set, and taking them up to their rated voltage, or as near as I could because some are rated 50V and my bench PSU only reaches 30V. None of them seemed to show any leakage, but my bench PSU will not display currents below 10mA, I am not sure if that is enough to show leakage that matters. I also tested their capacitance values, a few are well above nominal value, but within the tolerance shown on the printset. The worst one is +41% when the range is +50-10%. I started checking some of the tantalum capacitors. They all looked fine, but I don't know what a sensible ESR value is for these. One of them (C315) is rated 1uF 35V and has an ESR of 3.4. Is that high for a tantalum? One respondent asked for a picture, the printset is here, http://manx.classiccmp.org/details.php/1,5422 and the relevant drawing is on page 58. Regards Rob

-> Longer term I will have to get one, but for now that is all I have. I do have a variac and a rectifier, so I could get it to 50V that way. So what sort of value is reasonable for leakage? By "worst" I mean the one that was furthest from its nominal value. I have posted a picture of it here: http://1drv.ms/1Lo1hWB

Burke this capacitor has I did pretty much what you say last night. All the caps settled down to a leakage of the order of 10-20uA after no more than a minute on their maximum voltage. Although my bench PSU does not go to the 50V needed for some of the caps. Was considering putting the board back in after having done some light reforming on them (ie not for more than a couple of minutes each), in case they just needed reforming. Regards Rob

On Sat, 21 Feb 2015, Robert Jarratt wrote: Is C441 really 10uF 25V? That is what both the parts list and schematic show, but even as old as a VT100 is, I wouldn't think a standard 10uF 25V part would be in an 18mm diameter can. I've had this discussion with several people recently, but why bother even attempting to reform the capacitors on this particular board? If you are going to desolder them anyway, why not just replace them and be done with it? From the high resolution photo, the original axial mount parts at C439 and C437 have already been replaced and almost all of these parts are cheap and very easy to obtain. The only exception I can see is C437, which is a 75uF 6V part. Even with that one, you can still get 75uF axial mount parts from Vishay in 25V and 50V which would probably fit the pad layout just fine. I dunno...maybe I just have a totally different way of rebuilding older gear. I'd rather replace any 20-30 year old aluminum electrolytics wholesale with known good modern parts (which as long as you stay away the originals) and then move on to testing and troubleshooting other stuff and not have to later return to troubleshooting a power supply or something else because of intermittent issues caused by old aluminum electrolytics. This is definitely the way things are done in the arcade and coin-op world (no point in troubleshooting faulty logic chips until you clean up any power supply ripple), and also the way things are usually done with vintage TVs and radios (vacuum tubes/valves), but I've seen pushback from some people in the vintage computing community to wholesale replacement of aluminum electrolytics which are long past their life expectancy and I just don't get it. In the vintage audio communities, there are of course a handful of "purists" who would rather have 40+ year old leaky (as in voltage) dried out paper caps in there audio gear than modern poly film parts, but why the reluctance to changing out aluminum electrolytics in things like DC power supplies? It's not like these parts wouldn't have been replaced already had a vintage computer still been in a production environment.

On Sun, 22 Feb 2015, Robert Jarratt wrote: Ah. That explains it then. I was beginning to wonder if the printset had a typo and it was supposed to be a 250V rated part. I didn't see any indication in in the parts manifest that it was a NP part either, but perhaps I overlooked it. Are any of the others NP parts? Well...I tend to look at replacing aluminum electrolytic capacitors as more of a maintenance or preventive maintenance thing vs a repair issue. If they are old and/or you have ripple, replace them and get that potential cause out of the way first and be done with it. Were you able to find the correct axial mount parts for the 22uF, 4.7uF, and 75uF capacitors? I did a quick search through some of my past projects for compatible parts and managed to work up a short list of part numbers.

Wilson clock, 40-pin just the kind Quite. That or a logic analyser. Regards Rob

Bruce time. Unfortunately not. I already have that print set, but it is for the VT100, this is a VT101 and the terminal controller board is different. Looking more closely at the VT100 basic video board, some of the ROM part numbers are similar to the ROM I have, but not the same. On the VT100 they seem to be 8316s which are mask programmable. I don't know if that applies to the ones used in the VT101. If they are mask programmed then there won't be much I can do if the ROM is bad, unless it is possible to find pin-compatible programmable parts and a ROM image from somewhere. Regards Rob

Bruce ? VT100, Sadly, I already have that too :-( It doesn't have the print set, but it has been very useful. Thanks for all the pointers though, I do appreciate you taking time to look :-) Regards Rob

On 01/19/2015 12:48 PM, Robert Jarratt wrote: If you can swap in an EPROM for the ROM, you might try coding a small program to use the RIM/SIM instructions to give you some sort of indicator of operation--you could even code a bit-bang serial interface. --Chuck