Ken Shirriff Unlocks the Secret of an Unusual IBM SMS Board — Reverse Engineering an Early Modem
Noted computer historian and reverse engineer Ken Shirriff has turned his attention to an unusual IBM device: A communications modem, or part thereof, with a gigantic metal block at its heart.
"It turns out that the board is part of a modem," Shirriff explains of his findings, having delved into what the extremely unusual Standardized Modular System (SMS) add-in card could be for, "and the large metal box is a transformer. In the late 1950s, IBM introduced the Standardized Modular System card, small circuit boards that held a simple circuit, and used these boards to build computers and peripherals into the mid-1960s."
"The idea was to design a small number of standardized boards that implemented logic functions and other basic circuits."
The SMS standard would bring forth thousands of designs, of which Shirriff has documented over 1,400 in an online database, but this particular card stood out from the rest. Ideas for its potential purpose included its use as a temperature-compensated crystal oven (TCXO) for high-precision timing, until measurements proved the box was an hefty transformer — most likely filled with oil.
"I found a document that gave the board's part number as a transmitter board for an IBM modem," Shirriff writes, "transmitting data across phone lines. The large transformer would have been used to connect the modem to the phone lines while maintaining the necessary isolation. The modem used frequency-shift keying (FSK), using one frequency for a 1 bit and a second frequency for a 0 bit."
"I reverse engineered the board by closely studying it, and discovered that the board generates these two frequencies, controlled by a data input line. This confirmed that the board was a modem transmitter board."
Shirriff's full write-up, which includes a reverse engineered schematic for the board, is available on his website now.