EEVblog #1169 - TI 1972 Computer Interfacing - Page 1
Forum section: electronics / general

author: EEVblog
author_rank: Unknown
timestamp: January 15, 2019, 08:49:45 pm
content: How did you connect to a time sharing computer in 1972?Teardown of the Texas Instruments TI Silent 700 model 745 terminal with acoustic coupler modem.

author: texaspyro
author_rank: Super Contributor
timestamp: January 15, 2019, 10:05:32 pm
content: When I was co-op engineer in college, we used these terminals. Ours were the version with two tape drives. The application was to generate configurations for microwave relay system racks. One drive would prompt the operator for various options. The responses were written to the second drive. That tape would then be downloaded to a Univac 1108 mainframe that would punch a card deck that was sent to an IBM mainframe that produced another card deck of all the parts that would be needed to build the system.The problem was that if any operator errors or bad or conflicting entries were done, the whole process would have to be repeated. It could take an engineer a week to generate a valid rack configuration. So, I changed the program (without asking anybody) to skip the tape drive step and do an interactive configuration dialog with instantaneous error checking and punch the first deck directly. It reduced the time to generate a valid (and error-free) configuration to under 30 minutes.Another change was to read the parts database directly. The original programs would query the database in a way that simulated reading punch cards... which the Univac charged around 25 cents per "card." I changed the way the data base was read which bypassed the card charge (which could be around 1000 "cards" per rack.Surprisingly management was rather furious, but got over it after realizing the savings and god-like praise from the configuration engineers. Over several years there was not a single report of the system generating a bad rack configuration (and anybody collecting the bounty offered for finding a bug that would generate an improper configuration)... it saved the company millions of dollars.

author: jancumps
author_rank: Unknown
timestamp: January 15, 2019, 10:10:31 pm
content: I would have drooled all over that service manual non-stop. My first stereo came with the schematics and all the corners are torn from constantly looking at it.

author: floobydust
author_rank: Super Contributor
timestamp: January 15, 2019, 10:11:44 pm
content: Model 745, 1972 $1,995 in today's dollars is about $12,000 !I see a lot of expensive innovation in it. The SMPS, a portable 8080 device, no ink ribbon (thermal printing tech?).Looks like an 8-track or cassette tape motor for the fan.

author: coppice
author_rank: Super Contributor
timestamp: January 15, 2019, 10:19:23 pm
content: The slow mode was 110 bps, for compatibility with the dominant ASR33 of the day. The models with an RS232C socket on the back worked up to 1200bps on that port. The acoustic modem was limited to 300bps.The "here is" key sent a break - basically a UART framing error, which the receiving end would register without decoding the serial stream, and wake up.The version in the picture with 2 cassette tape drives was one of the mainstays of early microprocessor system development. We used them a lot when developing assembly language code, before people like Intel and Motorola got their 8" floppy disk based development systems working.

author: Jr460
author_rank: Regular Contributor
timestamp: January 15, 2019, 10:38:34 pm
content: None standard DB-25.... Dave, shame on you.RS232 on DB25 was the standard, the DB15, or DB9 were not.Anyway, used one in the mid 80s. You would use it as the console of larger system so if it crashed you had the info on what happened on the console. Also when doing a special install you wanted that rather than CRT so you had a good record of what you did.But beyond that for many years we would use a DEC LA120 for the console of systems.

author: texaspyro
author_rank: Super Contributor
timestamp: January 15, 2019, 10:39:31 pm
content: Quote from: floobydust on January 15, 2019, 10:11:44 pmCost of a print head replacement was around $500...

author: texaspyro
author_rank: Super Contributor
timestamp: January 15, 2019, 10:41:27 pm
content: Dave, your printing problems could just be old paper. Try some new paper... but you might void your warranty.

author: Fungus
author_rank: Super Contributor
timestamp: January 15, 2019, 11:20:07 pm
content: Is that a Motorola transistor in a TI power supply?

author: texaspyro
author_rank: Super Contributor
timestamp: January 15, 2019, 11:42:57 pm
content: Quote from: Fungus on January 15, 2019, 11:20:07 pmAhhh, yes. Motorola devices. The company that I worked for got a HUGE contract to build a state of the art telecommunications/phone network for a large middle eastern country. They had to remove the Motorola logo from every single device in the system... zillions of them... picture a room full of women with electric erasers. Why, you ask? Well, I was told it was because a Jewish person was on Motorola's board of directors! No idea if they had to also remove the "MC" from the device numbers.

author: wilfred
author_rank: Super Contributor
timestamp: January 16, 2019, 12:06:12 am
content: I have one of those somewhere that I picked up for $10 on Ebay a few years ago. Mine is much smaller because it doesn't have an acoustic coupler. I must fish it out and try it. Mine will probably have the same print issues. So I was hoping to see you attempt to resolve them.According to the manualhttp://www.decadecounter.com/vta/pdf/TI%20Silent%20700%20Model%20745%20Operating%20Instructions.pdfthe HERE-IS key transmits the contents of the optionalanswer-backmemory. Maybe that is the empty socket.That same website had a document about the ACCUTRON mechanism that might interest some.http://www.decadecounter.com/vta/pdf/accutron_chronometric.pdfAlso there were manuals on the HP 2114 computer, various valves and HP calculators. Not saying it can't be found elsewhere but who knows.

author: chickenHeadKnob
author_rank: Super Contributor
timestamp: January 16, 2019, 01:10:31 am
content: Quote from: texaspyro on January 15, 2019, 10:39:31 pmAnd that's why they were never very popular in my neck of the woods, compared to say LA DECwriters.Dave gains extra street cred with his eagle eyes and laser like focus for noticing those odd-ball sockets. Those were made by TI! Yes Texas Instruments made dip sockets. I have a new-oldstock narrow dip 20 pin wire wrap socket in my hands right now and they don't have any insignia or markings to give that away. The 600mil sockets have a small Ti on the underside. I always kept a few of these wire wrap length wide dip sockets for a special end use. Alas they have gone to the same place as the dodo and all of those useful and unique jfets from that era.Gather around the campfire boys and girls while Grampa explains what they are good for. You see with the wipers oriented in the 'wrong' way they are the only sockets into which you can plug a 3M ZiF socket directly, without bother. Well there is a small amount of effort as you have to remove the plastic top cover of the TI socket with your fingernail or exacto knife.You see the ZiF socket pins are also oriented-flat in the wrong way. Match made in heaven, the 3M sockets fit snug. They can not be plugged into a machine pin socket which was always my first choice in sockets..You need a (TI) socket like this if you are making a DIY eprom programmer and want a simple and convenient way to connect the ZiF socket pins to the underlying circuit through an enclosed case. What you do is solder the TI socket to your board in such a way that the top sits just proud of the surface of your case. Then you just plug the ZiF socket into that after the case is assembled. Neat and easy-peasy. This is useless knowledge now that the TI sockets are unobtainium.pictures to followwhenif gramps can get his phone to cooperate.

author: texaspyro
author_rank: Super Contributor
timestamp: January 16, 2019, 01:42:28 am
content: Quote from: chickenHeadKnob on January 16, 2019, 01:10:31 amBut DECwriters are not very portable... the Silent 7xx machines were meant to be easily carrierd.. except for the desktop models.Also, the paper for the Silent 700'd was also rather expensive. The printheads seemed to last for a case or two of paper. I think that is why most of them were leased (with maintenance contracts). I watched our TI guy come in and replace a printhead. I shoulder surfed the number that he dialed to their test system. He left his login details on the new roll of paper he installed... password was not obscured. That made it easy to test the machine later

author: EEVblog
author_rank: Unknown
timestamp: January 16, 2019, 03:39:52 am
content: Quote from: jancumps on January 15, 2019, 10:10:31 pmI had the service manual for my Tandy 1000 with all the schematics and datsheet, it was paper gold.

author: EEVblog
author_rank: Unknown
timestamp: January 16, 2019, 03:42:47 am
content: Quote from: chickenHeadKnob on January 16, 2019, 01:10:31 amWow!

author: EEVblog
author_rank: Unknown
timestamp: January 16, 2019, 03:47:37 am
content: Quote from: Jr460 on January 15, 2019, 10:38:34 pmIt's a DB15 and I said non-standard

author: johnlsenchak
author_rank: Frequent Contributor
timestamp: January 16, 2019, 04:08:54 am
content: What you have there is a over priced typewriter that connects the serial port to a distance mainframe .Think of the computer technology today that you can get for $1995 compared to that archaic piece of junk

author: helius
author_rank: Super Contributor
timestamp: January 16, 2019, 05:38:33 am
content: Quote from: wilfred on January 16, 2019, 12:06:12 amThat's correct! This was used to identify the location of the connected teletype ("Building 100, terminal pool 6") in response to a ^E from the host. This could also be used to tell the host something about the terminal's capabilities (later, the ANSI X3.64 standard supported specific capability information).https://en.wikipedia.org/wiki/Enquiry_character

author: helius
author_rank: Super Contributor
timestamp: January 16, 2019, 06:07:28 am
content: Dave, you didn't mention the real reason that acoustic couplers were used. In the 1960s, there were no standard modular phone connectors, and the only devices that could legally be connected to the phone system were rented by the phone company (including phones, special multiline systems, and modems, all of which were billed at monthly rates). It was only after theCarterfone decisionthat you could privately own a modem electrically connected to the network.

author: boffin
author_rank: Unknown
timestamp: January 16, 2019, 06:46:43 am
content: Quote from: EEVblog on January 16, 2019, 03:47:37 amSurely you mean DA15? The 2nd letter is the case size, so in the 'normal' dual row config D shell connectors are:DE9DA15DB25DC37DD50Also, IIRC it's 20mA loop, and not RS-232. Yes you read that right, it pre-dates RS-232 !!!!!As for the "Here Is" key, it sends the programmed answerback, which can also be sent by the remote side sending a Control-E (ENQ - Enquire). It was a cheap/easy way for a remote side to detect a device, or send perhaps a short loginID or something like that. Terminals like the VT100 also had a builtin answerback (which we would abuse in college by setting it to "^log^m"). That empty socket is for a PROM it would be burned onto.FAST = 300 baud = 30 characters/sec of (start + 8 data + stop)SLOW = 110 baud = 10 characters/sec of (start + 8 data + 2 stop)and the 8 data could be 7 data+parityI last used one of these in the mid 1980s, it was still the cheapest, most portable printer you could find. Sure an LA120 was faster, but it was huge, and decidedly non-portable.Lastly to the loser that in here said "archaic piece of junk", we wouldn't have the stuff we do, without innovative stuff like the Silent 700.

author: blacksheeplogic
author_rank: Frequent Contributor
timestamp: January 16, 2019, 07:03:50 am
content: No need to mock the device, for some applications a physical print was preferred. For example systems I worked on often the console was a printer/keyboard. Early on I had to send programs for data entry and most processing was batch anyway.

author: coppice
author_rank: Super Contributor
timestamp: January 16, 2019, 12:36:48 pm
content: Quote from: blacksheeplogic on January 16, 2019, 07:03:50 amThe Silent 700 was a thing of beauty in its day. That's because it was the key alternative to an ASR33 teletype, which was a truly awful clunky noisy thing to have to use all day.

author: SeoulBigChris
author_rank: Regular Contributor
timestamp: January 16, 2019, 02:28:20 pm
content: I used this at Ga Tech in the very early 80s to connect to the campus computer mainframe. We had it in the ham radio club, and used it for homework and sometimes for logging radio contests.

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: January 16, 2019, 02:57:49 pm
content: I think we tend to forget just how insanely expensive it was to do anything with computers back in those days. At uni we got to use the mainframe for small jobs, but we were always reminded that it cost £5 a MINUTE to run the thing. Anyone whose program got stuck in a loop was not popular. (Don't ask me how I know...)

author: texaspyro
author_rank: Super Contributor
timestamp: January 16, 2019, 04:37:51 pm
content: Quote from: boffin on January 16, 2019, 06:46:43 amA lot of 110 baud systems used 1.5 stop bits...

author: boffin
author_rank: Unknown
timestamp: January 16, 2019, 04:54:33 pm
content: Quote from: texaspyro on January 16, 2019, 04:37:51 pmIndeed, but I didn't want to complicate things further.... After posting last night I went and did some reading about 20mA loop; as I had completely forgotten about it.Now the big question, how many remember using a TELEX ? (FIGS+WHO ARE YOU) -- how did I get so old ?

author: bsudbrink
author_rank: Frequent Contributor
timestamp: January 16, 2019, 04:57:03 pm
content: Quote from: boffin on January 16, 2019, 04:54:33 pmYou haven't died yet.My father (age 90) always says that he hates his birthday but that "It beats the alternative".

author: HwAoRrDk
author_rank: Super Contributor
timestamp: January 16, 2019, 04:58:22 pm
content: Quote from: blacksheeplogic on January 16, 2019, 07:03:50 amI recall a blog post from Microsoft's Raymond Chen talking about how they used to use serial terminals like these when doing kernel-level debugging on Windows 3.x and 9x. The advantage being that you could spit out debug information that wouldn't need to be logged in memory and would persist (because it's on paper) in the event of a crash.Perhaps I can find the link later.Couldn't find it.

author: WN1X
author_rank: Unknown
timestamp: January 16, 2019, 06:23:23 pm
content: Quote from: texaspyro on January 15, 2019, 10:05:32 pmAhh, the old daysI spent many hours writing code and running it on a Univac 1108 for the Navy. 36 bit words, core memory, tape drives and drum (not disk) storage.

author: HKJ
author_rank: Super Contributor
timestamp: January 16, 2019, 09:33:57 pm
content: Quote from: WN1X on January 16, 2019, 06:23:23 pmThe 36 bit meant 6 bit characters in filenames. It could also use disk storage, big removable disk stacks. It may have been a later model, I am sure it had solid state memory and it could handle about 100 users with 512k memory (I believe). Input was mostly from VDU's, but some people did also use punched cards with output on a some line printers.

author: texaspyro
author_rank: Super Contributor
timestamp: January 17, 2019, 12:59:13 am
content: Quote from: WN1X on January 16, 2019, 06:23:23 pmThey also used disk drives.Anybody experience the subtle wonders of a bearing going out on a FASTRAND drum? A couple of tons of spinning iron running at around 1000 RPM... bearing goes out, drum launches through concrete walls... great fun. Unless they were firmly bolted to the floor, the first FASTRAND systems would walk around the room as the earth rotated under them. Later units had two drums spinning in opposite directions.I have a couple of core memory stacks from the 1108.

author: helius
author_rank: Super Contributor
timestamp: January 17, 2019, 02:40:18 am
content: Seehttps://multicians.org/low-bottle-pressure.htmlThis site has lots of great stories from early mainframe engineers.

author: bsfeechannel
author_rank: Super Contributor
timestamp: January 17, 2019, 05:40:47 am
content: Quote from: boffin on January 16, 2019, 06:46:43 amTrue.This terminal is more generically called a teleprinter. Teleprinters were invented long before computers.Dave got amazed that the terminal didn't even have a screen, but still in 1972, computers were expected to print out their results. Video terminals became an affordable alternative a few years later.We owe to this "archaic" technology the existence of the Web.Tim Berners-Lee created the first Web browser, but it could only run on NeXT OS. So he commissioned Nicola Pellow to write a portable version: the Line Mode Browser. it emulated the behavior of a teleprinter, but it is what made the Web known to the general public for the first time.

author: DTJ
author_rank: Super Contributor
timestamp: January 17, 2019, 11:10:19 am
content: I used on of these Silent 700's as a 12 year old in around 1978. My father would borrow it from work for me to play around with. I'd put together programs in BASIC and run them and chew through a pile of paper roll.I think it was a PDP 11/70 that I used to log into.The home phone was near the kitchen.When using it after dinner cutlery being clinked together in the kitchen would be picked up by the acoustic coupler and cause random character to be generated.

author: coppice
author_rank: Super Contributor
timestamp: January 17, 2019, 11:39:00 am
content: Quote from: bsfeechannel on January 17, 2019, 05:40:47 amIn 1972 even the most basic video displays made the silent 700 look cheap. It was very rare to see one, and when you did it only displayed 8 or 12 lines of upper case only text. In 1975 DEC launched the VT52, which was one of the first decent 80x24 video displays, capable of displaying upper and lower case text. It was rather large, ran hot, and cost a fortune. It was largely made practical by advances in semiconductor RAM. In 1978 DEC launched the VT100. By this time they were putting some sophistication, like smooth scrolling, into the terminals, and they were much more compact. They were still several thousand dollars, though.

author: GeorgeOfTheJungle
author_rank: Super Contributor
timestamp: January 17, 2019, 01:36:37 pm
content: Dave clean the printhead with alcohol, the side that touches the paper, with a cotton swab.

author: boffin
author_rank: Unknown
timestamp: January 17, 2019, 04:39:22 pm
content: Quote from: coppice on January 17, 2019, 11:39:00 amAhh the VT52, great device, and one of the best keyboards ever. The keyclick was generated by a big relay that just clunked closed when you typed. The baud adjust was two multiway switches underneath which was annoying, but most of the time ours were 2400 or 9600 fixed.The VT100 on the otherhand was much more modern, but the keyclick/feel and the smooth scrolling were just crappy by comparison. The VT102 is when DEC started to really integrate everything on a single board.

author: floobydust
author_rank: Super Contributor
timestamp: January 17, 2019, 06:34:54 pm
content: 8080 development started late 1972 and it was released in 1974. Not sure why the product ads seem prematureDot-matrix impact-printheads were king in this era. Ribbon and ink everywhere, quite the mess fixing them. A silent thermal printer is way ahead of its time here.I wouldn't expect any of the electrolytic/tantalum capacitors to be working. It looks like it runs, the CPU. Wow.

author: German_EE
author_rank: Super Contributor
timestamp: January 17, 2019, 06:52:54 pm
content: At the first place I worked at we needed a serial terminal but the Texas Instruments devices were way too expensive. So, the command from on high was to build one.We started with an Epson MX-80 which was programmed via its DIP switches to print one character at a time and not do a line feed and carriage return afterwards, this had a parallel input. We then found a keyboard in the junk pile that had an 8-bit parallel output. Finally we raided stores for a pile of 74LS devices including lots of shift registers and spent a week wiring it all together including RS-232 done through transistor level shifters.Now for a lesson in economics.One nearly new Epson MX-80 printer plus four man weeks of time probably cost more than one of these Texas Instruments terminals. We tried to tell them before we even started but they got their 'terminal' anyway and the men in suits were happy.

author: Silveruser
author_rank: Regular Contributor
timestamp: January 17, 2019, 07:59:08 pm
content: Quote from: boffin on January 16, 2019, 04:54:33 pmNot only do remember TELEX, spent a few years fixing them. Also worked on 720C, a version of the Silent 700 used for CPIC ( Canadian Police Information Computer). It was a much heavier version full die cast case. Here is was coded on a plug in board with a diode matrix - you'd cut the leads on the diodes to code it. Weak points were the print head and end of line mech which used grain of wheat bulbs. The 720c were replaced with an early 8080 based Inteligent Terminal pre PC days for sure.

author: Quarlo Klobrigney
author_rank: Super Contributor
timestamp: January 18, 2019, 11:16:54 am
content: I used one of those back around 1982-83 to send in field reports from equipment dumps (via the modem) in far flung remote monitoring sites.I later found out about BBS's. I would log in, and a menu system would print out. The bugger of it all is that if you selected a menu item, and if you went back to the main screen, it would print it all over again. Roll after roll of thermal paper that I'm sure was not cheap back then. The fact I could speak to a mainframe 3000 miles away was sheer magic.So after all these years, I get to see what's under the hood. I was really afraid to open it as seen by the price of it back then. Old happy memories of being a self-autonomous employee. Show up at the site once in a while, make sure nothing caught fire or blew up, and send the reports and all was good.

author: westfw
author_rank: Super Contributor
timestamp: January 19, 2019, 03:21:02 am
content: QuoteHuh. If the first silent700s shipped in 1972, well before the 8080 was available, then I think that implies that as a product line, this "terminal" went through quite a bit of product redesign over its lifetime. 1972 till "mid 1980s" is actually a pretty impressive life for a computer product during that timeframe (pre-8080 to post IBM-PC!)QuoteHey, me too! Though more like 1976 and 16 years old. I also played a lot of Star Trek. Used to be something like $4/hour for the phone call (which dad complained about. $4 was a lot in 1976. Never complained about the paper use, though. Oil Companies! It didn't come home THAT often, though.)QuoteIt was "standard practice" to have a printing terminal connected to the "console" of your mainframe, well into the 1980s (until mainframes themselves started to die out, essentially.)QuoteIn those days, print was pretty much all that was available. What would you count as the first portable "screen-based" thing? The Osborne 1 (in 1981) - no modem, a 52 column display, and $1795? Maybe the TRS80 Model 100 (1983) - 8x40 display WITH a modem - $1100? Maybe the Toshiba T1100 in 1985 (Full 80x25 display!)QuoteQuite a few years later.QuoteQuoteYeah, that. Lots and lots of that! $1995 was quite a bargin for a Terminal AND a MODEM. Contemporary CRTs ran over $1000, plus close to another $1000 for the MODEM. Display cards for CPM systems that did 80 columns (the "standard" from punch cards, you know) were expensive, and required special high-resolution CRTs with high-resolution phospors that were difficult to find and quite pricey. It wasn't until after widespread IBMPC Clones that "computer monitors" became common. (Original IBM PC Monochrome display adapter + (12inch) hidef green monitor was about $700.)

author: texaspyro
author_rank: Super Contributor
timestamp: January 19, 2019, 03:40:39 am
content: I still have and use a Toshiba T1000 and know a company that still runs a production line with over 20 of them.

author: westfw
author_rank: Super Contributor
timestamp: January 19, 2019, 06:09:43 am
content: QuoteHeh. I bought one when they got cheap. The battery died a long time ago, it wouldn't run without it, replacement batteries were expensive and/or hard to obtain, and I never did get around to hacking it. I think it's still in my garage, though...

author: floobydust
author_rank: Super Contributor
timestamp: January 19, 2019, 06:38:53 am
content: You'd splash on some Hai Karate, toss the TI terminal in your car and drive to work. Perhaps in a car of the era, a 1972 Mercury Monterey for example.Cost around $5,000 curb weight: 2,015kg/4,442lbs, 7L 429CID engine, and it had maybe 15 transistors.

author: texaspyro
author_rank: Super Contributor
timestamp: January 19, 2019, 06:54:26 am
content: Quote from: westfw on January 19, 2019, 06:09:43 amThe battery is just 4 NiCad cells in an open-frame plastic holder... trivial to rebuild. I switched to using LSD NiMH cells long ago. Every 2-3 years that company that runs a production line on T1000's opens them up, vacuums out the fluff, and swaps out the cells. The damn things are still going strong.

author: blacksheeplogic
author_rank: Frequent Contributor
timestamp: January 19, 2019, 07:25:42 am
content: Quote from: westfw on January 19, 2019, 03:21:02 amI'm old enough to remember using a lugable in the mid-80's commercially.My point was primary the way the was portrayed without any f'in idea of what it was to work in the industry at that time, the limitations/constraints, expense, the applications being developed & run and what was actually required/needed commercially. These did the job the needed to, were beautiful in their time and did not deserve the stupid ranting and laughter looking at it with 30+ years of technology in between.

author: westfw
author_rank: Super Contributor
timestamp: January 19, 2019, 09:21:31 am
content: QuoteDoesn't it have some sort of "Sense" terminal as well? (I'm pretty sure I've lost the actual pack.) I was disappointed that it failed to run with no battery installed :-(I would've mailed it to Dave for a tear-down, if it was such an expensive PITA to mail things internationally from the US. :-(

author: German_EE
author_rank: Super Contributor
timestamp: January 19, 2019, 09:25:38 am
content: Didn't Brother also produce a typewriter that was a serial terminal?Edit. found it, the Brother EP44https://typewriterdatabase.com/1984-brother-ep44.5403.typewriter

author: coppice
author_rank: Super Contributor
timestamp: January 19, 2019, 02:59:48 pm
content: Quote from: German_EE on January 19, 2019, 09:25:38 amThere were a lot of typrewriter + terminal machines, but not many portable ones. Quite early on IBM started making versions of their Selectric golf ball typewriters with a computer interface. These were mostly for mainframe console use, but offered one of the few high quality print options as well.That Brother machine was a later generation of portable devices. I think it dates from 1984.

author: GeorgeOfTheJungle
author_rank: Super Contributor
timestamp: January 19, 2019, 04:01:42 pm
content: The 3M Whisper Writer was a portable modem+terminal with a thermal printer (the printer was a trendcom 200 ~= Apple Silentype).https://duckduckgo.com/?q=3M+Whisper+Writer&atb=v134-6__&iax=images&ia=images

author: texaspyro
author_rank: Super Contributor
timestamp: January 19, 2019, 06:50:37 pm
content: Quote from: westfw on January 19, 2019, 09:21:31 amI just had a pack rebuilt a few months ago. I seem to remember there are two red and two black wires in parallel going to a 4 pin connector.

author: helius
author_rank: Super Contributor
timestamp: January 19, 2019, 07:27:44 pm
content: NiCd chemistry is very rugged and doesn't need balance sensing, it can just be float charged indefinitely and sheds the excess energy as heat. This is one reason why simple substitution of NiMH cells in older devices doesn't work.Sometimes the reason the machine won't run without batteries is that it also uses them to rectify the power input (I think certain HP calculators worked this way).

author: texaspyro
author_rank: Super Contributor
timestamp: January 19, 2019, 08:44:08 pm
content: If the old NiCd charger circuit was a trickle charger, you can often substitute NiMH cells. A 0.1C charger for 1500 mAh NiCd cells works out to a 0.03C 4500 mAh NiMH charger. Most NiMH cells will tolerate C/30 trickle charging just fine.

author: German_EE
author_rank: Super Contributor
timestamp: January 20, 2019, 09:38:32 am
content: "Ahh the VT52, great device, and one of the best keyboards ever. The keyclick was generated by a big relay that just clunked closed when you typed. "A BIG THANK YOU to user Boffin for this idea. I just tried it and it works remarkably well

author: bsfeechannel
author_rank: Super Contributor
timestamp: February 23, 2019, 09:51:11 pm
content: The solenoid, as it was called, was added for touch typists accustomed to the sound of the printer head. It could be enabled or disabled on the fly as this IBM 3278 demo shows. The solenoid basically hammered the casing of the keyboard which was built like a tank.