15
SHORT HAUL MODEM (SHM-FSK)
4. Operation
Point-to-point, full- or half-duplex operation is usually conducted with the
both the Master and Remote SHM-FSK carriers on all the time. This is
achieved by placing the P4 jumper in the B-C position and either not
connecting the RTS line (DB25 pin 4) to the SHM-FSKs or making sure that
the RTS line supplied by the devices connected to the SHM-FSK is either
floating or logic high.
In half duplex, multi-drop operating mode, with the P4 jumper in the
A-B position, the RTS line of the intelligent device that a Remote SHM-FSK
is connected to (PC, PLC, or RTU) is used to control the the SHM-FSK’s
transmitted carrier. When logic is low, the carrier is turned off; when logic is
high, the carrier is turned on. If the RS-232 cable is disconnected, the RTS
connection on the SHM-FSK is biased low and the carrier remains off. (If the
P4 jumper is in the B-C position, the RTS connection is biased high and the
carrier will stay on.)
When properly configured, the SHM-FSK is capable of half- or full-duplex
data transmission at rates up to 9600 baud. The SHM-FSKs require no settings
for data rate and will operate transparently as data is fed to them. They will
begin communicating data as soon as connections are made and they are
powered up.
