AT&T 5620 (and Related Terminals) Frequently Asked Questions


This is the Frequently Asked Questions List for AT&T 5620 and related terminals. Please send your contributions, comments, suggestions or frequently-asked-questions to eric@brouhaha.

  1. What is the history of the 5620 and related terminals?
  2. What is the history of layers?
  3. Where can I get software for the 5620 and related terminals?
  4. Where can I get schematics for the 5620 and related terminals?
  5. What are the most common hardware problems with the 5620 and related terminals?
  6. How do you fix an unresponsive mouse?
  7. What kind of memory is used in the 5620?
  8. How to I open the case?
  9. What do I do if I'm having trouble with communications between the terminal and the host computer?
  10. What kind of keyboard does a 5620 use?
  11. Where can I get the games I've heard were written for the 5620?
  12. Is there a replacement for the red mouse? (draft)
  13. Who still supports the 5620 and related terminals?

Subject: 1 What is the history of the 5620 and related terminals?

(By David Dykstra and Fred Salomon at AT&T)

In the beginning was the Blit terminal. It was controlled by a Motorola 68000 processor. It had a big green screen. The terminal hardware was designed primarily by Bart Locanthi. Most of the software was written by Rob Pike at AT&T research.

Teletype took that and built on the Blit software and made the 5620 in 1984, changing the processor to a Bellmac 32000 and making a real heavyweight with a big green screen. The monitor had persistent phosphors and 1024x800 pixels. Version 2 of the 5620 was released in 1985 which included the following firmware enhancements:

The successor to the 5620 was the 630 which came out 1987. The processor changed back to the Motorola 68000 which the developers had wanted all along. Most of the 630 monitors were amber, although white and green monitors were also available. The monitors had a non-interlaced 1024x1024 pixels and did not have slow phosphors. A second RS232 port was included with optional SSI (3270 connectivity) and 512K RAM cards.

In 1989 new firmware for the 630 came out and the name was changed to the 730. This firmware supported 3 RS232 ports, LAN cards, built-in 4014 emulator, and enhanced PF-keys. Options to the 730 included the SSI card (which added the 3rd RS-232 port), ISO and TCP LAN cards, and an X-cartridge. The LAN cards supported from 512K to 4Meg of RAM. In 1990, the 730+ came out with a faster 68000, more EPROM space and RAM was moved from the LAN card to the main controller. In 1991, ISDN connectivity was provided to AT&T Bell Labs.

Meanwhile, AT&T research came out with a totally new motherboard controller card using the 630 monitor with totally new software (Plan 9) and called it a GNOT. It had a Motorola 68030 as the main processor.

Also, Teletype came up with several more layers-compatible terminals but none of them were programmable via downloads like the 5620/630/730/730+. Some of the numbers are 605, 615, 620, and 705.

Tony Hansen ( provides us with a little pre-history, in appropriately Genesis-esque prose:

Actually, in the beginning was the Jerq, and the Jerq was white with a green face, and Locanthi and Pike looked upon the Jerq and said the Jerq was good. But lo, upon the horizon loomed a mighty management-type person (known now only by the initials VP) who said, the mighty Jerq must stay alone, and could not go forth into the world. So Locanthi and Pike put the Jerq to sleep, cloned its parts, and the Blit was brought forth unto the world. And the Jerq lived the rest of its days in research, but never strayed from those paths.


In all seriousness, the Blit was originally known as the Jerq, but when it started to be shown outside of the halls of the Bell Labs Research organization, the management powers that be decided that the name could not remain. So it was renamed to be Blit. This was in late 1981.

The development tools for the Blit are still found under /usr/jerq or /u/jerq on some of the systems that still support them.

After the initial trial models, the first Jerqs/Blits were built in a small garage shop on Staten Island. (I believe the company's name was North Atlantic.) The mice were originally in very short supply, as the supplier in Switzerland could not keep up with the orders. I remember having to work without a real mouse for 6 months while waiting for the mice to swim the Atlantic. (While waiting, I designed and built an electronic version of the mouse that you would rock in the direction you wanted the mouse to move. A number of these were built within Bell Labs by other people who were also waiting for their mice to arrive.)

Subject: 2 What is the history of layers?

(By Steve Bellovin and David Dykstra at AT&T)

(All people mentioned are/were AT&T employees unless otherwise noted)

In the beginning was Rob Pike's mpx program for 8th Edition. It used pt's, which were stream devices similar to ptys. He later converted it to pipes, which are streams on Research UNIX systems. That had the disadvantage that windows became anonymous, but that hasn't been much of a problem in practice.

Pike's mpx was ported to 4.1bsd, using the mpx driver. (This was a driver that had been fixed.) It worked, but not that well. When 4.2bsd came out, Steve Bellovin started afresh from Pike's code, and used pty's. It worked a lot better, mostly because because of the full tty semantics. Steve added code that was, in effect, an ancestor of UCNTL. All this was for the BLIT (aka the Jerq), incidentally, not the 5620.

When the 5620 came out, some other folks took Steve's code and adapted it. Not very much needed changing in mpx; they had a lot more trouble porting the cross-compiler, since they had to port things like System V's ld to a machine that didn't grok coff. That code was released through Teletype, which wanted to be able to sell 5620s to universities.

In the System V world, someone (we don't know who) apparently decided that a pty-like solution was too inefficient. Thus, they created the xt driver. Or perhaps it was just because System V did not support ptys. We think this was a fundamental error, since the time lost due to system crashes far exceeded that saved by keeping the mpx process out of the way. Possibly in deference to the change in implementation, the name was changed as well, to "layers."

Pike went off and created mux, which moved the tty processing to the terminal (among other things). It ran on 5620s but did not use any of the firmware except for booting.

Layers became part of the System V release in SVR3. It was not yet streams-based, since there was no standard tty line discipline for streams in that release. (One was added later as part of the Starlan package.) A streams-based xt-driver and layers was developed by Bob Bolotin and Hari Pulijahl and it became part of the USL standard for SVR4. The streams- based XT driver also had the capability of larger packet sizes (up to 252 bytes) with regular XT on terminals that support it (the 730) and the capability of network XT (no checksums, packets up to 4K) on terminals that support that (the 730). However, USL removed layers from SVR4.2 and very few vendors of USL-based unixes ever included any layers in their releases.

Meanwhile, Keith Muller at UCSD developed a pseudo-tty based layers for BSD unixes, apparently pretty much from scratch. It relied on TIOCUCNTL for some of the communication between layers and other programs, and it also listened on a unix-domain socket for libwindows calls.

Keith Muller writes, "I did a complete port of all the 5620 and software environment (compilers, loader, applications) for ATT Teletype as part of a grant (in exchange for some 5620 and later 630 terminals). The layers software was written from scratch for the BSD environment. We supplied ATT with a binary and source distribution for several different hardware platforms for a number of years. We stopped support when we retired the terminal lab here. After the last 630 was turned off, I stopped doing up dates and Dave [Dykstra] picked it up." [From email of May 24, 1994.]

David Dykstra picked up Keith Muller's layers and made it (with contributions from others) portable to more systems including SVR4 and added more of the features from System V layers that were missing in Keith's version. Many operating systems don't support TIOCUCNTL so he gave it the capability to do all its support-program communication using unix-domain sockets or named pipes. He also added support for larger packets and network XT.

Subject: 3 Where can I get software for the 5620 and related terminals?

(By David Dykstra at AT&T)

First, you need "layers". Some vendors provide binaries (e.g. AT&T 3b2, AT&T 6386/Starserver, NCR 3000, HP). A pseudo-tty based layers which is portable to many operating systems is available in source code form by http download from

Next, if you have a 5620, 630, 730, or 730+ you'll probably want to have the cross compilation system and downloader in order to program the terminal and/or load programs that others have written.

- 5620:

The 5620 cross-compiler source code is available by http download from

NOTE: if you have version 1.1 5620 roms (id 8;7;3), you need to have 32ld and lsys.8;7;3 and set_enc.j download files to effect- ively bring it up to the level of version 2.0 roms (id 8;7;5). A stripped-down version of 32ld and these download files were included in the SVR3 and SVR4 driver-based layers binary packages, but they are not included with pseudo-tty layers. They are, however, in both the 5620dev and 630_pkg packages.

Source for the 5620 2.0 roms is also available by http download from The 5620 roms are designed with transfer vectors so that pieces of the code can be patched in ram. The rom source code will be very useful to someone wanting to write patches.

- 630/730/730+:

Unfortunately there is no official way to get source code for the complete cross compilation system at this time outside of AT&T. The software development package WITHOUT the 68000 compiler is available on, in the hopes that someone will pick it up and integrate it with the GNU 68000 compiler. If you are interested in working on it, let Dave Dykstra know at

If you've aleady got a license for source for an older version of the 630 software development package, send email to Dave Dykstra perhaps something he could send you the AT&T 68000 compiler.

If you have access to 630 ".m" files (pre-compiled download objects), you can use the dmdld downloader from the freesdp630 package. You may need to convert the byte order of the .m file if the host that it was generated on has the opposite byte order of the host that you now have. You can use 'm32conv' from the dev5620 package to convert it or you can use 'mc68conv' if you've got it on some other host (it's one of the pieces removed from freesdp630).

Binaries of the complete 630 software development package are available from some unix vendors, most notably AT&T 3b2 and NCR 3000.

If you have access to a cross compilation system, you may want to have access to previously written downloadable programs. There is a package on which contains source for many of those.

This is what it contains:

The following applications are for the 630 MTG only:


Host software to support the 730X terminal is available via http download from

See also Subject 11 below.

Subject: 4 Where can I get schematics for the 5620 and related terminals?

(By Eric Smith)

A PDF file of the schematics is available for download at

For other terminals, the schematics are generally not available. If you really need 630 schematics contact Steve.Crawford@OrlandoFL.NCR.COM and maybe something can be worked out.

Subject: 5 What are the most common hardware problems with the 5620 and related terminals?

(By David Dykstra and Jeff Light at AT&T).


Subject: 6 How do you fix an unresponsive mouse?

(by (brian.j.prendota) from comp.terminals.tty5620)

There could be 2 fixes.

  1. The ball could be dirty and not making contact with the contacts inside the mouse. There is a little screw by the ball. Unscrewing the screw will allow the ball to come out. Clean the ball and try it again.
  2. If that didn't work, then you must re-adjust the two analog pots inside your mouse. Remove the cover (there are two other screws). The mouse cover will then come off easily. There are two pots underneath. The trick now is to realine these two pots. There is a guy here at work that is a wizzard with them. I think the pot on the left is Horz and the one on the right is Vert. Now turn the mouse on a circular motion while adjusting both pots. Watch out though, it's tricky. Continue adjusting the pots until the arrow on the CRT is moving in a circle. Then re-apply the cover on the mouse. That's it.

(by (Lee Derbenwick) from comp.terminals.tty5620)

It's probably not the ball, but the bridge circuits. There are 4 screwdriver-adjusted potentiometers, 2 for each direction of movement. With the cover off, tweak them while turning the corresponding roller ("corresponding" can be determined by experiment) until it behaves well. (I know I'm being vague, but it's so easy to do that it's never felt worth the effort of memorizing which pots go with which axis.)

(by (warren.a.montgomery) from comp.terminals.tty5620)

I've fixed several of these things, and the problem I saw most often was that the metal wheel that is supposed to spin when you move the mouse and interrupt the optical connection gets loose on its shaft and move erratically or not at all. I fixed a couple of these by gluing the wheel on the shaft, but this is very tricky work and you have to get the wheel on absolutely straight, or it will bind in the optical interrupter. There are also several designs of the mouse that were used over time and not all of them are equally repairable.

This is the wiring configuration for the mouse plug, as posted in comp.sys.att by Phil Gunsul -

DE9 Connector for 3-button 5620 mouse
Pin Color
1 Black
2 Green
3 Beige (?)
4 Slate
5 Blue
6 White
7 Brown
8 Yellow
9 Red

Subject: 7 What kind of memory is used in the 5620?

(by dmturne@ptsfa.PacBell.COM (Dave Turner) from comp.terminals.tty5620)

The 5620 CPU Logic card contains 32 sockets for memory chips.

The 256KB version uses chips that have 65,536x1 bits. They can be replaced with 262,144x1 bit chips to upgrade to a 1MB version.

There is also a 74S161 counter that should be replaced by a 74F161 counter to complete the upgrade.

The counter is NOT mounted in a socket and must be removed by clipping its 16 leads so that the mounting holes can be cleaned before soldering the 74F161 into the logic card. [Editor's Note: It's not always necessary to replace the counter; try it without first.]

The memory chips must have a 150 usec cycle time.

A quick check of the latest [as of 12/92 -Ed.] Jameco catalog gives:

	Part No.	Price
	41256-150	$1.69

Jameco does not list the 74F161.

I upgraded several 5620's using AT&T's memory upgrade kit. I ran many of them for a few months before replacing the 74S161 with the 74F161. They seemed to work ok but I eventually replaced them anyway.

NB One upgrade did not work. Pin one of the 41256 is A8 (unused on the smaller memory chip.) The logic board on one 5620 did not toggle pin one on any of the 32 memory chips and was therefore limited to 256KB.

The reason for this was never determined.

Subject: 8 How to I open the case?

(by woods@robohack.UUCP (Greg A. Woods) from comp.terminals.tty5620)

The case is opened by first removing the back "cover" (to which the motherboard is attached), and then sharply sliding the top cover forward and lifting it carefully off over the monitor. If the back cover doesn't just fall out when the screws are removed, you'll have to gently pry it out. Be careful, as there are two plug connectors (video and power) near one bottom corner (left if you're facing the back), and they don't have very much wire to stretch out, so pry from the top

Subject: 9 What do I do if I'm having trouble with communications between the terminal and the host computer?

(by woods@robohack.UUCP (Greg A. Woods) from comp.terminals.tty5620)

Indeed the dmd terminal emulator (actually the code that bitblt's character glyphs and does scrolling) has trouble at over 4800 bps [although most people have success with 9600, and some report using 19200 with no problems when set up as outlined below -Ed.]. In order to maintain a reliable display update, you must use some form of flow control. Since the UART(s) in the terminal are not capable of doing hardware flow control (RTS/CTS), you must use XON/XOFF flow control.

With 8;7;5 ROM's, you do this by turning on the "Rcv Flow" option (under "Port Options"). You should probably also turn on "Gen Flow" and "Pass Flow" (under "Host Options").

With all of the above mentioned options on, and when running under layersys (using the layers(1) command), each "layer" will behave as if you have hardware flow control, thus permitting use of XON & XOFF characters for applications such as emacs (provided you also have "stty -ixon -ixany -ixoff" for each session).

Subject: 10 What kind of keyboard does a 5620 use?

The 5620 uses a Teletype code number 56K-341-AAN keyboard, the same as used on the AT&T 4410 and Teletype 5410 terminal (which are identical). Although these are good, rugged terminals, they are rather undesirable today because of their age and non-standard keyboard. Therefore, if you are searching for a replacement or spare keyboard for your 5620, it may be worth your while to buy an entire 4410 terminal for the sake of the keyboard - they are pretty inexpensive on the used market.

The key switches are of a design pretty common with AT&T/Teletype in the 1980s. A rubber membrane holds each plunger above a printed circuit card. The plunger has a padded, conductive surface, which when pressed down on against the card, completes the circuit. This construction makes the kayboard fairly immune to dust, but over time it may need to be cleaned. To disassemble the keyboard, remove the several screws on the bottom and lift the top cover away. Now use a keycap puller to remove the key caps, and lift off the plastic membrane. Under the membrane is a plastic panel which centers the individual key plungers. Remove the screws holding it in place and lift it away, allowing the key plungers to scatter all over your work surface. ;-) Simply clean the key contacts on the circuit card with contact cleaner, and wipe the surfaces of the key plungers with alcohol. All the key switches are identical, so reassembly is a snap if you can remember which key caps go where.

Subject: 11 Where can I get the games I've heard were written for the 5620?

Editor's note: as of August 2005, the FTP site mentioned below no longer exists.

Several of the games and other interesting programs for the tty5620 and other DMD terminals are available at the 3B2 anonymous ftp archive ( administered by Bob Martel at Levin College, Cleveland State University ( This is the same archive formerly located on . The 5620 games are located in the directory pub/att/dmd/5620 and the 630 games are in pub/att/dmd/630. also contains a wealth of 3B2 binaries, and since the 5620 and 3B2 frequently go hand-in-hand, any user of these machines is sure to fine something of value there.

Another good site is the "official" AT&T GIS 3B2 anonymous uucp archive (that's right, good ol' uucp! :-) ) administered by Bill Simeon ( Use a Systems entry of:

bbslisl Any ACU 9600 7088107273 "" \d\r\d in:-\K-in: uuanon

(2400 and 1200 baud are also supported.) To get an index of available files just "uucp bbslisl!~/3B2/Index /usr/spool/uucppublic/". Both sites accept contributions of software as well.

Subject: 12 Is there a replacement for the red mouse? (draft)

Later 630 terminals, and the 730 terminal. used a "generic" square Logitech mouse of the type seen on some older PCs and NCD X-terminals. Once I get my DMD "testbench" up and going again, I'll confirm the backwards-compatibility of the Logitech mouse.

Subject: Who still supports the 5620 and related terminals?

There is very little support left for these terminals, but the rights have transferred to Boundless Technologies. The last person known to be the contact person was Edgar Merke at the Schaumburg, IL, office.

If you have any DMD information that would make this FAQ a better resource for DMD terminal owners, or have a nagging question about this family of terminals which you think should be addressed in this FAQ, please send it along to for inclusion in the next release of the FAQ. Thanks!

Most recent changes:

August 2005 Changed FAQ contact person to Eric Smith.
Updated availability of schematics (download PDF).
Updated URLs for software.
HTML Reformatting and cleanup.
December 1998 Changed email address of Eric Smith.
June 1998 Changed FAQ contact person from David Breneman to Dave Dykstra.
June 1998 Added support info of Boundless Technologies.

Last updated August 25, 2005

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