Refurb weekend: Texas Instruments Silent 700 Model 745 teletype

The first terminal I ever used was a teletype. Somehow my buddy when we were in high school got a hold of this weird "printer typewriter" which was none other than one of the famous Texas Instruments Silent 700 series.

For those of you unfamiliar with teletypes, which are more properly known as teleprinters, the concept is exactly like it sounds: they're terminal devices ("tele") that use paper ("type") instead of a screen. Everything you receive or type (though some models lack a keyboard and were receive-only) is dumped in hard copy. To the Teletype Corporation's everlasting chagrin the term "teletype" became genericized, much like Kleenex or Xerox, and the abbreviation "TTY" is a contraction which survives to this day as a terminal identifier in pretty much every Un*xy thing. While TTYs have been replaced as consoles by so-called "glass" terminals, or by client computers with serial ports, they are still used occasionally in the airline industry and as assistive devices in the form of TDD/TTYs. (Teletype Corporation, for its part, was absorbed into its parent — AT&T, by way of Western Electric — after AT&T was broken apart in 1982 and became "AT&T Teletype" before full dissolution in 1990.)

Unlike classic units such as the Teletype Model 33, probably best known today in its "automatic send and receive" variant as the Model 33 ASR or ASR-33, which were essentially automatic typewriters with noise, size and weight to match, the Silent 700 was indeed comparatively silent because it uses a thermal printhead instead of impact printing. It prints five pixel by seven pixel dot matrix characters in 80 character lines. Texas Instruments released the first Silent 700 models in 1971.

The initial generation of Silent 700s are shown above. By 1972 TI offered 14 separate configurations with varying feature sets. Among others, the Model 710 was the basic Keyboard Send-Receive "KSR" model, effectively a clone of the IBM 1051/1052 teletype; the Models 721, 722 and 731 were all receive-only devices; and the Model 725 was the deluxe luggable unit with a spiffy black hardcase and a built-in acoustic coupler.

Remember that this was the era of the Bell System where hooking up your own devices directly to a consumer telephone line was a no-no, so you just took your hotel phone handset, jammed it right into the microphone and speaker cups and manually dialed in. While this got around the Bell System's notorious anal-retentiveness, it also limited data rates to around 300 baud, though this was not much handicap at the time. However, the Model 720 was the one most of us would recognize as a "modern terminal" with a newfangled EIA RS-232B connector, ASCII support (as opposed to Baudot), and a riproaring speed of 30 characters per second. Some units like the Model 723 were parallel instead of serial. Lowercase was an optional addon, though as the printout above shows, on these units TI just used smaller versions of uppercase letters (now you know where the TI 99/4 got that from).

In 1973 TI simplified its product line, reducing it to the baseline Baudot Model 732 and US-ASCII Model 733 in two flavours: KSR, the traditional terminal, and a new ASR "Automatic Send-Receive" version, which bolted twin cassette decks to the unit for automatically sending and recording data (much more convenient than paper tape, and a 1975 Computerworld ad directly said so). The tape decks featured automatic search and duplication capabilities and each cassette stored 144,000 characters per side. Both KSRs and ASRs could be optionally equipped with the acoustic coupler which fit into the unoccupied space on the right side of the terminal. This second generation had a new keyboard as well with colour-coded keys as opposed to the uniform grey of the first generation models.

TI expanded the ASR further in 1974 into the Model 742 Programmable Data Terminal ("PDT") which could be programmed in a simple form entry language called TICOL. During data entry, the format is printed line-by-line and the operator fills in the blanks provided which the program can check for type, size or range, or perform simple calculations or comparisons. TICOL could also operate on data received from a remote site once it had been downloaded. The original Model 742 had 2K of RAM and offered TICOL I, which had 74 instructions; it was upgraded in 1976 to 4K of RAM and TICOL II with an additional 56 instructions. Programs were stored on tape and could automatically chain to the next module. An optional polling system could call out to remote 742s and pull data locally.

And, just to complete the product line, TI introduced a reengineered portable unit the same year for $2595 (about $13,560 in 2022 dollars). Not much smaller than the first generation units, the Model 735 was a trim 25 pounds, comfortably luggable by data entry specialists on anabolic steroids, or anyone whose medical plan included a free hernia truss. It came with the acoustic coupler and RS-232C connection standard and ran up to 300 baud. It could fit under airline seats of the time, but remember that you could fit a lot more under airline seats of the time than today. (Heck, I could probably have fit under one.)

The Model 735 must not have been too popular because TI replaced it in short order with a third-generation portable in 1975, the smaller, lighter (13 pound) and cheaper ($1995) Model 745. This was possible because of its 2MHz TMS 8080 microprocessor (a clone of the Intel 8080 TI produced to quickly get into the CPU market) and other, smaller chips replacing the bulkier discrete logic in earlier units. TI paired it with the Model 743 KSR, mostly the same unit, but with a different keyboard, ASR-33 current loop option, and optional modem to connect to a 110/300 baud Bell CDT-type DAA ("data access arrangement," used in those days as an isolating device from the phone line) instead of the 110/300 baud acoustic coupler. The Model 743 did not come with a case and despite being slightly smaller and even a couple pounds lighter was intended only for desktop use. Both the Model 743 and 745 could be equipped with different keyboards, including APL, ASCII and katakana (!), or no keyboard at all to turn them into receive-only devices. TI dropped the 742 PDT around this time, but kept the 733 ASR in the product catalogue along with a new Model 750 RO "receive-only" based on the 743 but in a different, even smaller form factor. Interestingly, parity was set permanently at the factory (even was standard, but odd and mark were BTO options).

My personal device, and the one I first used way back when (we have no idea where it went, sadly), is a Model 745. I picked this one up to play with but had yet to do a great deal with it.

The slim case with a nice padded grip (could even fit under today's airplane seats) clips onto the bottom panel with storage for the power cord in the top. Very thoughtful.

And here's the unit itself. It's a bit dirty and slightly yellowed, and there's a crack in the keyboard frame which was bugging me, but these were cosmetic. Since right now I'm busy inventorying and moving items into storage, however, I tested it out to see how well it was working. The unit powers up and responds to keys, though compared to a glass terminal the Silent 700 isn't all that silent due to the power supply hum (normal, per the service manual) and internal cooling fan.

Still, the electronics seem to work, but the print is very faint (on 8.5" thermal paper) and the printhead is clearly dirty. I think we'll want to give this unit a little TLC before finding it temporary storage. That means, after all that long wind up, it's time for ... a Refurb Weekend!

The 743/745 service manual is available from Bitsavers. Incredibly, the official process for cleaning the printhead is to soak bond paper in alcohol and print a few lines with it, so I dug out the 91% isopropanol and took a sheet out of the laser printer.

After a little trouble getting the wet paper to feed in (kept slipping), I banged out the invisible Great American Novel and blotted the platen dry, then refed back in the thermal paper.

Definitely cleaner now, but still faint.

While the service manual explains how the contrast can be adjusted, it only does so with the top case off. However, the contrast adjust is perfectly accessible from the side.

A flashlight shows the pot which can be manipulated with a jeweler's flathead screwdriver. I cranked it a bit to the right.

Excellent! Still, we'll have to take the top case off to get at the broken keyboard frame, so we undo the four flathead screws under the machine. One was a bit tough going and wouldn't come out of the case. We'll come back to this.

With the top case and the paper door off, we can see the paper feed mechanism and the keyboard.

The top half of the unit contains several very large capacitors and the cooling fan (not surprising, it can draw about 75 watts). Don't put your finger anywhere near this area if you have it plugged in and especially running: if you don't shock yourself on the electrolytics, you'll cut yourself on the blades.

The warning about the high voltage is hidden by the wires, but the 2A fuse rating should tip you off, and if it doesn't, those 200 microfarad capacitors certainly would. The acoustic coupler in the top case is connected by the grey wire and twin yellow wires on the right. Under those cables is the 15-pin EIA connector, which can be passively turned into a more typical DE-9 RS-232 connector. TI sold an official connection kit but I've got some modular connectors around here that will do just fine. The nearby jumper may need to be disconnected to enable this.

The broken frame just comes off by freeing its side clips, showing the top white connector wired to the mainboard, and the black retaining clips holding the keyboard in. There are three on the top and bottom, left, right and middle. The service manual says the keyboards can be swapped out by pushing the keyboard towards the rear until it is free of the front clips. This is actually quite difficult to do (and even harder to get back in) but I wanted to have a quick look at the electronics while I had it open.

The underside of the keyboard with obviously hand-routed traces (nice rounded arcs). The logic board is now exposed.

I blew a little dust out of here but the mainboard was well-preserved for all that. Oddly, this unit has an AMD 8080A (AM9080APC) CPU instead of the original TMS 8080, a drop-in replacement for the Intel 8080A, but we do see a TMS 5504 (TMS5504NL) I/O and timer chip. This device is one of the few public manifestations of Texas Instruments' TMS 5500 family which was supposed to be their first original 8-bit processor series. Instead, the product line got completely scotched to put resources into the TMS 9900 and beat Intel's new 16-bit 8086 design to market, which it did in 1976, but not without a lot of technical consternation (another story for another day). I'm not aware of the 5500 CPU ever being produced — let alone any shipping products that actually used one — but the presence of the 5504 suggests the Model 745 may have been initially designed around it. Both chips carry 1979 date codes, which probably explains why an AMD chip is here because Texas Instruments was likely not producing any more 8080s by that time. Nearby is the 4K ROM, a TMS 4732 (TMS4732NL) with a 1980 date code, and just below the 5504 is the TMS 4036 (TMS4036NL) 64x8 SRAM with a 1979 date code, providing a whopping 64 bytes of memory directly accessible to the CPU. A piezo beeper is at the far left. Earlier versions of the Model 745 had two 1K ROMs instead.

So, that weird screw. Turns out (I didn't do this) the metal crimped cap on the plastic tunnel for that screw snapped off. It took a little work to get it off the screw and back on the plastic, and it still doesn't align right, but at least the screw moves normally and no longer jams.

Cleaned and reassembled with the repaired keyboard frame back on. Another successful Refurb Weekend in the can!

As a postscript, TI introduced bubble memory and a larger keyboard to the line in 1977 with the Model 765 portable and Model 763 KSR, the first commercial products ever to have the famous storage medium. Bubble memory wasn't ever fast (in fact, the machine capped it at 2400 baud) but it didn't have to be for this application, and was non-volatile to boot, effectively making them smaller and shock-proof replacements for the older ASRs. The 763/765s were dramatically faster units that could accept serial rates from 110 baud to 9600 baud, though the Model 765's acoustic coupler was still 110/330 baud. A special "ASR mode" allowed them to record and playback keystrokes just like the old cassette system, but now entirely in solid-state. In these units the 8080 was demoted to servicing the keyboard and printer, and a TMS 9980 controller handled communications (via a TMS 9902) and bubble memory access, driving the 8080 over a 9600bps internal connection (the two CPUs did not share a bus and were otherwise independent). The bubble memory stored from 20,000 to 80,000 characters depending on configuration and could be edited as "files" in-place; the 763/765 could also accept operator commands typed on the console. However, as we've seen from the date codes, the older 743/745 units were still in production alongside.

Bubble memory didn't have a long lifespan as an industry storage fad, causing TI to replace the 760 series with the 780 series in 1980 alongside the new wide-carriage OMNI 800 family. Although technically descended from the 763/765, they were closer in architecture to the 743/745 in that the TMS 9980 was now entirely absent and the 8080 was once again the only CPU on board, which also made them cheaper. For the first time they also printed proper lowercase letters, though they lacked true descenders, and could print up to 140 characters a second. The Model 785 was the base portable model, with an integrated 1200 baud acoustic coupler; the 787 substituted a 1200 baud modem, while the 783 omitted either and served as the standard KSR. Although all of them printed "international character sets," only the Model 783 (and its receive-only descendent, the Model 781) had katakana support, and only Models 781 and 783 supported an additional 1000-character buffer. The OMNI came in the Model 820 and 825 KSRs along with receive-only Models 810, 820 RO and 825 RO.

The final generation of Silent 700s came out in 1983 and were newly based on an 8MHz TMS 7041 microcontroller, part of the 8-bit TMS 7700 family with integrated RAM, 4K ROM and onboard UART, thus jettisoning the older 8080-based architecture completely. The base unit was the new ultraportable 5.9-pound Model 707, which had a built-in 300 baud modem, cartridge expansion for built-in programs (including "auto access cartridges" with preprogrammed credentials for answerback), and a 132-column carriage on standard 8.5" paper using a compressed printhead. A lead-acid battery pack and external acoustic coupler were optional. The Model 703 was the spiritual KSR variant of the 707 with a 9600bps serial port instead of the internal modem; the 1985 703 RO and 707 RO were receive-only devices. The last Silent 700 was the 700/1200 BPS in 1987, which merged the 703's serial port with the 707's modem and upgraded it to 1200 baud. Sales advertisements for these final Silent 700s were seen as late as 1990.