Physical Layer - GE 750 Manual

PHYSICAL LAYER

Physical Layer
1 - 2
Both the Modbus and DNP protocols are hardware-independent so that the physical layer
can be any of a variety of standard hardware configurations including RS232, RS422,
RS485, fiber optics, etc. The 750/760 includes a front panel RS232 port and two rear
terminal RS485 ports, one of which can also be configured as RS422. Data flow is half
duplex in all configurations. Refer to the Serial Communications section in Chapter 3 of the
instruction manual for details on wiring.
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 Communications section in Chapter 5 of the 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 750/760 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
DNP DNP PORT setpoint is used to select which port will communicate using DNP.
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The maximum time for a 750/760 relay to return a response to any (non-broadcast)
master request never exceeds 1 second.
S1 RELAY SETUP
750/760 FEEDER MANAGEMENT RELAY – COMMUNICATIONS GUIDE
CHAPTER 1: OVERVIEW
COMMUNICATIONS
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