OMEGA Engineering, Inc. Fully Isolated Limited Distance Modem LDM485 Series

The LDM485 is a compact RS-232 to RS-485 converter which features a complete electrical isolation barrier and heavy duty electrical surge protectors. These devices feature a rugged aluminum enclosure small enough to mount the back panel of typical computer equipment, saving valuable desk and floor space. Isolation is provided by optical couplers and a DC-to-DC converter. The RS-232 connection is through male or female EIA 25-pin connectors. The RS-485 connections are made through convenient solderless screw terminals. The LDM485 series is designed for full duplex operation over two-wire pairs. Outputs are tri-state, allowing multidropping of up to 64 units. Hardware handshake is available over two separate wire pairs. Data rates are DC to 57.6k baud. Six diagnostic LED indicators are provided (see Figure 1) for installation guidance and system troubleshooting. The RS-232 interface supports Request To Send, Clear To Send, Data Set Ready, Received Line Signal Detect, and Data Terminal Ready. A convenient null modem switch is provided for the data lines. Also, a line termination switch connects a line termination resistor and line bias resistors to the RS-485 receive lines. The RS-485 interface supports Request To Send and Clear To Send on separate wire pairs. The LDM485 may be used to convert two sets of send and receive channels by using RTS and CTS circuits as the second data channels. Data rates are the same. The units use 12VAC from a wall-mounted transformer to screw terminals 1 and 2 on the RS-485 connector. Alternately, they can use 12VDC to pins 9 (+) and 10 of the RS-232 connector. The LDM485 conforms to EIA RS-232 and RS-485 specifications. Data Terminal Ready must be asserted by the host RS-232 port before the LDM485 can transmit data. When Data Terminal Ready is not asserted, all outputs of the LDM485 are high impedance, allowing up to 64 LDM485 units to be multidropped on a common communications cable. See Figures 1 and 3 for details. Request To Send and Clear To Send are carried through the RS-485 port as two separate wire pairs. These may be used for full duplex flow control. Data Terminal Ready, DTR, must be asserted before the LDM485 can transmit data. This is normally done by the host computer. For situations where the host equipment does not have the capability of supplying a DTR signal, RLSD may be used to automatically assert DTR. On the RS-232 connector P1 of each LDM485, simply connect RLSD pin 8 to DTR pin 20. This connection is not appropriate for multi-drop installations.