Schweitzer Engineering Laboratories SEL-351-5, Instruction Manual

Page: 648 / 706

H.4
SEL-351-5, -6, -7 Relay Instruction Manual
Date Code 20070117
Distributed Network Protocol
EIA-232 Physical Layer Operation
EIA-232 Physical Layer Operation
The RTS signal may be used to control an external transceiver. The CTS
signal is used as a DCD input, indicating when the medium is in use.
Transmissions are only initiated if DCD is de-asserted. When DCD drops, the
next pending outgoing message may be sent once an idle time is satisfied. This
idle time is randomly selected between the minimum and maximum allowed
idle times (i.e., MAXDLY and MINDLY). In addition, the SEL-351 monitors
received data and treats receipt of data as a DCD indication. This allows RTS
to be looped back to CTS in cases where the external transceiver does not
support DCD. When the SEL-351 transmits a DNP message, it delays
transmitting after asserting RTS by at least the time in the PREDLY setting.
After transmitting the last byte of the message, the SEL-351 delays for at least
PSTDLY milliseconds before deasserting RTS. If the PSTDLY time delay is
in progress (RTS still high) following a transmission, and another
transmission is initiated, the SEL-351 transmits the message without
completing the PSTDLY delay and without any preceding PREDLY delay.
The RTS/CTS handshaking may be completely disabled by setting PREDLY
to OFF. In this case, RTS is forced high and CTS is ignored, with only
received characters acting as a DCD indication. The timing is the same as
above, but PREDLY functions as if it were set to 0, and RTS is not actually
deasserted after the PSTDLY time delay expires.