Bell 208 A/B modem
multi-point, multi-drop

Bell 208 A, Bell 208 B - 
208 A: Synchronous data transmission, full-duplex operation over 4-wire leased lines, half-duplex operation over 2-wire leased lines; 4800-bps data rate. 208 B: same as 208 A, but over 2-wire dialup lines. Comparable to ITU V.27. The first standards to enable higher-speed data transmission (4800 bps) over leased lines for multipoint, multidrop networks. For example, these standards enable us connect IBM mainframes at a central office to terminals in branch office, with fewer modems.

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Reliable 4,800bps transmission over bad lines with 208A/B and 208 AB/AS (DPSK 8 phase) modem. Multi-Point / Multi-Drop, Dial, 2 or 4 wire lease line.

UDS 208 SERIES 4800 bps

The UDS 4800 series includes Bell 208 compatible modems which operate 4800 bps half duplex over 2-wire dial-up or leased lines. All models include automatic answer and line equalization, and all are compatible with the Universal Data Shelf".


? 208A/B Operates full duplex over 4-wire leased lines; Includes antistreaming to prevent network tie-ups on multipoint networks

? 208AB/AS Transmits asynchronous as well as synchronous data; Also operates full duplex over 4-wire leased lines; Includes antistreaming

? 208B/D Includes synchronous automatic dialer which utilizes bisync protocol for DTE dialing communications; Features call progress monitoring for dial tone, busy signal, ring back and answer back tones

UDS_208A-B_AB-AS_B-D.gif (12160 bytes)


  • Speed 4800 bps
  • Carrier Frequency  1800 Hz
  • 2-wire lease, 2 wire dial half-duplex PSTN  4-wire full-duplex operation piont to point,
    mutli-point, mutli-drop
  • Conforms to EIA Standards RS-232-C and CCITT V.24




The standalone 208A/B modem unit contains a linear type power supply. All the power supply components are contained on the modem board except for the ac step down transformer. 

The maximum ac power input to the unit at a line voltage of 110 Vac is 10 watts. The maximum dc power used by the unit is 3.5 watts. 


The UDS 208 A/B transmitter is a 4800 bits per second synchronous quadrature modulated transmitter with a Bell 208A/B compatible data scrambler. The transmitter provides the option of a strap selectable fixed pre-equalizer to improve performance on poor quality phone lines. 


The UDS 208A/B receiver is a 4800 bit per second synchronous quadrature demodulator with a Bell compatible 208A/B descrambler. The receiver front-end contains an improved adaptive equalizer that provides for dynamic telephone line equalization.