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

Bell 208 A, Bell 208 B Modem - 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.

We stock used UDS 208A/B modems 208B/D modem 208C/D 208AB/AS

check out the other used modems at the bottom of the page

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)

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jump to PDF file of 208 A/B manual (5.5 megs)

Check out the DC powered modems jump to DC page

jump to new Racal Vadic / Zypcom 208A/B AC or DC powered modem


  • Speed 4800 bps
  • Carrier Frequency  1800 Hz
  • 2-wire lease, 2 wire dial half-duplex PSTN  4-wire full-duplex operation point 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. 

208A/B large version                              62085037
208A/B card long card version               62082066  
208A/B small "mini" case (RM16M)       62085064
208A/B RM16M card                            62085084

208A/BDC DC power                            6262085143

208B/D small "mini" case (RM16M)       6208096 or 6208097
208B/D RM16M card                           6208096 or 6208097

check out the
Racal Vadic / Zypcom
4800 bps HALF-DUPLEX (AT&T 208) MODEMS


RACKMOUNT AT&T 208 A/B compatible, Sync only, Up to 8 cards in a VA1681C chassis

(2 in VA1604). Dial-up or 2/4-wire leased line. Includes: modem, telco cable for dial-up and manual.

Note: 2-/4-wire leased line applications, order optional cable 15044-004.



STANDALONE AT&T 208B and ITU-T V.27ter compatible, Sync only. Synchronous SADL

autodialer, front panel control w/LCD, Integral 801 ACU. Dial-up or 2-wire leased line. Includes: modem,

110V power supply, telco cable and manual, 801 ACU applications, order optional cable 91009-094.

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Related Pages

301-924-7400 EXT 25

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