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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 modems 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 high-speed 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 12 Vac from a wall-mounted transformer, while the host-powered unit takes +DC power from pins 9 and 10 of the RS-232 connector. The modem 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 is available
Interface
LDM70 conforms to EIA RS-232 and CCITT V.24 specifications. Pins 4 and 5 are
internally connected. Pin 6, Data Set Ready (DSR), is internally tied to the
valid state through a resistor. Data Terminal Ready (DTR), pin 20, must be
asserted by the host equipment before LDM70 can transmit data. When DTR is
asserted, the LDM70 at the other end of the
communications cable will assert Receive Line Signal Detect (RLSD), pin 8, to
its host equipment. This is used by host equipment to coordinate data
exchanges. If a DTR signal is not available from the host equipment, an open
circuit on pin 20 will be equivalent to assertion, allowing operation without
handshake. Pin 7 is signal ground. Pins 2 and 3 are switch-reversible.
Isolation
The LDM70 is unique compared to other popular limited-distance modems in that
it completely isolates the host equipment from the field wiring through the use
of optical couplers and a
DC-to-DC converter. Units that use only optical couplers break ground loops but
circuit power is still taken from an unisolated source, and/or host circuit
ground is still connected to one field wire pair. Thus, even though the ground
loop between the two pieces of host equipment has been broken, induced
transient currents will still flow from the field wiring through the ground
circuits of the host equipment. This can cause faulty operation of the host
equipment, or even damage in extreme cases. To prevent this, the LDM70 models
have a transformer-coupled DC-to-DC converter which powers the field circuits,
with no path for induced transients to flow into host equipment. This isolation
barrier is rated to 1000 Vrms. Continuous operation at high voltages is not recommended for safety reasons.
View an Adobe Acrobat pdf file for more
product info (requires Acrobat Reader).
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