The LDM70 series of products is designed to allow video display terminals (VDTs) and other RS-232 devices to be connected over distances sufficient to cover any industrial or institutional complex of buildings. These line drivers feature a rugged aluminum enclosure small enough to mount on the back panel of VDT units, saving valuable desk and floor space.

The LDM70 series is designed for full duplex, asynchronous operation over two DC-continuity, non-loaded, twisted-wire pairs. Through special highspeed optically coupled circuits, they may communicate at data rates up to 57,600 baud. A handshake operation is implemented over the same two-wire pairs. A self-powered model and a host-powered model are available. The self-powered unit uses 12VAC from a wall-mounted transformer, while the host-powered unit takes ±DC power from pins 9 and 10 of the RS-232 connector. The line driver circuits - and, consequently, the host device - are protected from electrical transients due to lightning strikes or operation of heavy industrial equipment.

Each device features a convenient Data-Communication Equipment (DCE) to Data-Terminal Equipment (DTE) switch which reverses pins 2 and 3 of the RS-232 connector. For installation and troubleshooting, each unit has diagnostic Light-Emitting Diodes (LEDs) on the transmit and receive lines. In addition, LEDs indicate valid carrier detect and data terminal ready.

The RS-232 connector may be ordered as a male or female 25-pin connector. Field connection is made through a modern, solderless, screw-termination assembly. Alternatively, a convenient four-wire modular phone jack (RJ-11) is available.
  • DC to 57,600 Baud
  • Complete Isolation with Optical Couplers and Power DC-to-DC Converter
  • Data Terminal Ready, Carrier Detect Handshake without Extra Wires
  • Surge Protectors
  • Four LED Diagnostic Indicators
  • Operation to 3 Miles (5km) at 9600 Baud, 1 Mile (1.7km) at 19,200, 0.5 Miles (0.8km) at 57,600
  • Four-Wire Full Duplex, Two-Wire Simplex
  • Self-Powered or Host-Powered
  • Selection of Connectors
  • Wide Operating Temperature Range, 0 to +70°C
  • CE Compliant
Fully Isolated RS-232 Line Driver

LDM70 Devices


Part Number Configuration
Male, Host-powered, Screw termination
Male, European transformer, Screw termination
Male, U.S. transformer, Screw termination
Female, Host-powered, Screw termination
Female, European transformer, Screw termination
Female, U.S. transformer, Screw termination

Frequently Asked Questions


Can Dataforth provide calibration reports for modules I purchased?
Yes we can provide calibration reports for the modules that you purchased.

You can either
a) visit https://www.dataforth.com/TestDataReport.aspx to search for Test Report Datasheets by Serial Number or
b) you can send us a list of model numbers and their serial numbers to support@dataforth.com

Can I communicate between a two LDM Converters / Line Drivers without DTR (Data Terminal Ready) control signal?
Yes, two LDM Converters / Line Drivers can be setup to communicate between two remote RS-232 terminals with only TD (Transmit Data), RD (Receive Data) and GND (Ground) signals. The setup is possible only if the communication is Simplex (one-way over two-wires) or Full-Duplex (two-way over four-wires). For Half-Duplex (two-way over two-wires) communication DTR control signal is required.

Short DTR line to RLSD (Received Line Signal Detect) on both end of RS-232 terminals at the LDM connectors. The RS-232 terminals at the LDM is a DB-25 connector. For DB-25 connectors, DTR is Pin 20 and RLSD is Pin 8.

If a DB-9 to DB-25 converter is used at the RS-232 terminal then short Pin 4 (DTR) to Pin 1 (RLSD or DCD - Data Carrier Detect) on the 9-pin connector.

Is the LDM35 a suitable replacement for the LDM30?
Please observe the block diagram in the datasheets of the LDM35 and LDM30 modules.

In the block diagram, you can see the direction of space current and mark current through terminals 3 and 4 (TD+ and TD-). Since these currents flow in the opposite direction between the LDM30 and LDM35, the LDM35 cannot be used to replace the LDM30.

We recommend replacing the LDM30 with another LDM30, or you can use the LDM70 as a suitable replacement as well.