DC Powered Bell 202/202T and V.23 Modems

Applied Systems Engineering DM series products are Bell 202 /202T and V.23 modems designed for operation over leased line communication circuits as commonly used in the electrical power and pipeline SCADA industries.


These modems have been designed with many commonly requested features including 900Hz soft-carrier turnoff, LED indicators, digital control over most timing parameters (in 1 millisecond increments), transmit levels from 0 to -31db, a PTT (Push to Talk) circuit, and input power from 5 to 160 VDC. The primary modem circuitry is based on commercially available ICs with many years of field proven operation.

All Bell modems are available in a 19″ rack configuration with a 16-slot custom backplane, or packaged in a metal box. For IED/RTU manufacturers, a low-cost “board only” version can mount directly in the Remote unit.

  • Power
  • Timing
  • Levels
  • Isolation
  • LEDs
  • Other
  • Packaging
  • Testing your modem
  • Modem Part Numbers

Box modem: 5 to 160 VDC, draws less than 500 mw

Rack Modems: 120/240 VAC to power supply

“Hot Card Swap” on rack mount configuration (backplane supports dual power supplies)
Carrier detect on delay: 2 to 30 msec., 1 msec. increments

RTS/CTS delay: 0 to 254 msec., 2 msec. increments

Transmit Gain: 0 to –31 dB, 1dB increments

Receiver Gain: –35 or -45 dB

1,500 VAC Telco

1,800 VAC Power Supply

Transmit (TD) and receive (RD) data, RTS, CTS, carrier detect (CD), and power

Analog side via 4-wire RJ-11 (box modem) or terminal screws (rack mount modem)

Digital side via DB-25 RS-232

2-wire or 4-wire (2-wire with optional 156 msec. receiver squelch)

Constant or switched carrier

900Hz carrier turnoff (Bell 202 modem only), 1 – 15 msec, 1 msec. increments

19” rack is 5 1⁄4” high with slots for 16 modems plus redundant power supplies

Metal enclosure is 187x105x36 mm (7.35×4.14×1.42”) and includes flanges with 4 mounting holes

“Board only” dimensions are 100x160mm (3.94×6.30”)

Your Bell 202 modem comes fully tested from the factory. However if you need to perform some tests on an installed modem, please follow the steps below to verify modems operation:

TELCO Loop Back Switch (SW4)

SW4 is an external switch. If ON, data received on the analog TELCO side will be looped back to the TELCO transmitter. In this mode, the digital (RS-232) side is disabled.