- to-find Bell 202 leased-line modem standard
The Modem 202T (MD845A-R2) functions in the same way as the 202T Modem Standalone. However, the Modem 202T supports speeds up to 1200 bps over unconditioned lines, while the 202T Modem Standalone supports speeds up to 1800 bps over conditioned lines.
Widely installed and a prerequisite for many leased-line applications, both the Modem 202T and the 202T Modem Standalone support voice-band frequency shift keyed (FSK) modulation within asynchronous point-to-point or multipoint systems that use voicegrade 2-wire, half-duplex or 4-wire, full-duplex dedicated or leased lines. These can be loaded or unloaded circuits, equal to or better than the ITU (formerly CCITT) M.1020/1025 or Bell 3002 leased lines.
A Modem 202T application typically involves using it with remote terminal units (RTUs) and programmable logic controllers (PLCs) to control and monitor a large number of remote devices— traffic control lights, for instance. In addition to SCADA applications, the modem is quite useful for temperature and pressure controllers in numerous industries, particularly those in which one expects the number of automated devices to only grow.
The Modem 202T is preferred by the water-treatment industry because it’s compatible with battery systems from 11 to 18 VDC.
The Modem 202T is also relied upon heavily by the U.S. Government for daily commerce.
Noisy lines aren’t a problem either. The Modem 202T or 202T
Modem Standalone circuitry prevents data errors that occur on such lines, particularly those within multidrop systems where carrier turn-offs and other modems on the same line can cause noise spikes and other harmful high frequencies. The modems combine soft-carrier generation and softcarrier detection as well as a signal-noise-ratio (SNR) detector to eliminate such line-noise errors.
The modems also have antistreaming features, which prevent them from remaining in Transmit Mode when they’re not actively transmitting data. Great for multipoint applications, antistreaming prevents a malfunctioning slave modem from occupying the line indefinitely. When anti-streaming is active, the Modem 202T or 202T Modem
Standalone can only transmit data for up to 56 seconds. Thereafter, it automatically turns off its transmitter and looks for an ON-to-OFF transition of Request To Send (RTS) before proceeding with normal operation.
Compatible with the Bell 202T standard.
Operates over 2- or 4-wire voice-grade leased or private lines.
For point-to-point or point-to-multipoint polling networks.
Prevents data and carrier-detect errors over noisy lines.
LEDs for power, RS-232 control, data status, and testing.