GE Multilin 745 Communications Manual page 6

OVERVIEW
1–2
Each data byte is transmitted in an asynchronous format consisting of 1 start bit, 8 data
bits, 1 stop bit, and possibly 1 parity bit. This produces a 10 or 11 bit data frame. This is
important for transmission through modems at high bit rates (11 bit data frames are not
supported by many modems at baud rates greater than 300).
The baud rate and parity are independently programmable for each communications
port. Baud rates of 300, 1200, 2400, 4800, 9600, and 19200 are available. Even, odd, and
no parity are available. See the 745 instruction manual for further details.
The master device in any system must know the address of the slave device with which it
is to communicate. The 745 will not act on a request from a master if the address in the
request does not match the relay's slave address (unless the address is the broadcast
address -- see below).
A single setpoint selects the slave address used for all ports with the exception that for the
front panel port the relay will accept any address when the Modbus protocol is used. The
slave address is otherwise the same regardless of the protocol in use, but note that the
broadcast address is 0 for Modbus and 65535 for DNP. The relay recognizes and processes
a master request (under conditions that are protocol-specific) if the broadcast address is
used but never returns a response.
DNP may be used on, at most, one of the communications ports. Any port(s) not selected
to use DNP will communicate using Modbus. The
port will communicate using DNP.
The maximum time for a 745 relay to return a response to any (non-broadcast) master
request never exceeds 1 second.
DNP PORT
745 TRANSFORMER PROTECTION SYSTEM – COMMUNICATIONS GUIDE
CHAPTER 1: OVERVIEW
setpoint is used to select which