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04. April 2025
In the early days of computing, it was common for companies to have a large central computer system, with which users connected via character-based terminals. Early home computers like the Altair 8800 and the IMSAI 8080 also used serial terminals for user interaction before separate graphics cards became popular. In the 1970s and early 1980s, Lear Siegler Incorporated (LSI) introduced the ADM series of serial RS232C terminals.
The ADM 5 was introduced in 1981 following the popularity of the ADM 3 and 3A. It had the same iconic, curvy design (similar to the first Apple iMacs), similar features, but with a numeric keypad. The ADM5 monochrome CRT screen had a diagonal of 12 inches and was available in white or green phosphor. It weighed 14.5 kg and was approximately 35 cm high, 40 cm wide, and 51 cm deep.
The ADM series was prominently and humorously marketed as a "Dumb Terminal," a term trademarked by Lear Siegler in 1976. The expression became common in the industry when comparing terminals to desktop computers. But even compared to other terminals on the market, the ADM 5 was dumb. The simplicity of the internal circuitry, mainly consisting of TTL logic chips from the 7400 series, differed from competing terminals at the time, which often had a CPU and ROM containing code for setup menus, configuration settings, and other functions. The ADM 5 terminal is fully configured via physical DIP switches on the mainboard.
On the back of the terminal, there are two serial DB25 ports, one for connecting to a host computer system and the second (expansion) port for connecting a local peripheral device, such as a printer. The ports can be configured for use with the RS232C standard or a 20mA current loop interface (used in older teletype machines). The maximum speed of the serial interfaces is 19200 baud. The terminal can be directly connected to a host system, typically via a building's internal communication wiring, or remotely using a modem to dial into the host system over analog phone lines. The display format is 24 lines × 80 characters. A 2K ROM chip contains the bitmaps of 128 ASCII characters, including uppercase and lowercase letters, as well as 32 control characters. The single-character dot matrix is 5 × 9 pixels in size and is located in a 7 × 10 character field. The content of the video screen is stored at all times in four 1K × 4Bit RAM chips. During a CRT refresh scan, the dot rows for each displayed character are read from the ROM character memory.
The keyboard has 83 keys, including alphanumeric and special characters, a numeric keypad, and function command keys such as cursor movement control, etc. When the keys are held down, they automatically repeat at a rate of 22 characters per second. Above the numeric keypad is a screen brightness control. Bill Joy used an ADM terminal to develop the vi text editor for UNIX and implemented the same HJKL keys used for cursor navigation. The ADM 5 is fanless and quiet, but like most terminals of the time, it had a loud keyboard compared to what we typically use today. The keys are directly soldered onto the mainboard. The board also contains several internal and external DIP switches for configuring the terminal. The configuration options are limited and mainly focus on RS232C settings. The terminal has a linear power supply and can be ordered with North American 115 VAC or European 230 VAC. The transformer is located under the main logic board, which contains the rectification and regulation of power for the logic circuit and the CRT. The CRT has a separate board for controlling the cathode ray tube.
The image below shows the ADM 5 connected to a Linux computer. Modern Linux systems are capable of providing logins from a real (non-emulated) physical terminal using systemd. When you connect the terminal with a USB-to-RS232C adapter cable, a kernel device similar to /dev/ttyUSB00 should appear. You can then start the Agetty process, which manages the serial connection, accepts a username, and passes it to the login program for authentication. A login prompt can be sent to the terminal as follows using the systemctl command: $ sudo systemctl start serial-getty@ttyUSB0.service
Older Unix and non-systemd Linux systems may require configuration in the /etc/inittab file. Two excellent sources of information on the ADM 5 and other terminals are the Terminals Wiki (https://terminals-wiki.org/) and the Bitsavers archive (https://bitsavers.org/). There you will find original manuals and old promotional material. The ADM 5 documentation includes a 51-page user reference manual and a 158-page maintenance manual with the technical theory of operation and complete schematics.
The English original version of this article can be found here: https://digitalforensics.ch/enter/adm5.pdf
Sources: HISTEC References
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