SCM9B-A1000 & SCM9B-A2000 USERS MANUAL 2
GENERAL DESCRIPTION
The SCM9B-A1000 and SCM9B-A2000 series converter boxes convert
RS-232 communications signal levels to the correct electrical signals
required by RS-485. The RS-485 communications standard is
recommended when many DATAFORTH modules, or other addressable
devices, must be connected to a host computer over long distances. The
SCM9B-A1000 and SCM9B-A2000 converters allow communications
bus lengths up to 4,000 feet and baud rates up to 115K baud using one
twisted pair of wires.
The RS-485 communications standard is half-duplex, therefore data can
be transmitted in only one direction at a time. The SCM9B-A1000 and
SCM9B-A2000 automatically control the bus direction without external
handshaking signals from the host. Therefore, host software written for
RS-232 may be used without modification. RS-485 bus control is
completely transparent to the user.
The SCM9B-A1000 and SCM9B-A2000 can also operate as RS-485
repeaters. Repeaters are required to extend communications bus
lengths or allow more than 32 RS-485 devices to be connected to a
communications bus. A repeater simply reamplifies, or boosts, existing
RS-485 signals transmitted over long distances.
The SCM9B-A1000 converter boxes are powered by either 115Vac or
230Vac and contain an in24-volt 1A power supply for powering
DATAFORTH modules. The SCM9B-A1000 power supply is protected
against overloads and short circuits. The internal power supply may be
used to power accessory circuits such as relays or 4-20mA transmitters.
The SCM9B-A2000 converter boxes operate on a wide range of power
supply input v10 to +30Vdc unregulated.
