Wednesday, 2 December 2020

Bell 212A

What is Bell 212A?

The Bell 212A modulation scheme defined a standard method of transmitting full-duplex asynchronous serial data at 1200 bps over analogue transmission lines (telephone lines). The equivalent, but incompatible International Telecommunication Union (ITU-T) standard is V.22. The Bell 212 Dataset is a 1979-vintage modem use for communicating over telephone lines at 300 or 1200 bits per second. The 212A standard provides for the ability of a modem to auto-answer a ringing phone. The Bell_212A scheme was the most common standard use for 1200 bps transmission on US data networks such as CompuServe during the period that dial-up Internet access was the norm (the 1980s and 1990s). The CMX869A V.32bis modem IC from CML Microcircuits offers multiple modem protocol support in both auto modem and manual modes, a rich telephony feature set, and low power consumption.

Bell 212A Single-Chip Modem

Detailed information on the Bell_212A physical layer implementation is very difficult to obtain, but an obsolete AT&T document entitled “Bell System Technical Reference, Data Set 212A Interface Specification, January 1978” contains useful information about the standard. In addition to the “high-speed” 1200bps operating mode, Bell_212A also supports a “low-speed” operating mode of 300bps, which is commonly known as Bell 103. Both asynchronous and synchronous formats are supported by Bell_212A.

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