GE Multilin G650, Manual

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B-2 G650 Generator Protection & Control System GEK-113285A
B.2 MODBUS G650 APPENDIX B
B
B.2MODBUS G650 B.2.1 FUNCTIONS USED
The protocol used is standard ModBus/RTU, so any program or PLC will be able to easily communicate with G650 units.
G650 always works as slave, which means that it never starts the communications. It is always the master who initiates
communication.
Only one ModBus/RTU functions subgroup are implemented:
• Reading function 3 (or 4).
• Writing function 16.
B.2.2 PHYSICAL LAYER
ModBus/RTU protocol is independent from the hardware. This way, the physical layer may be in different hardware
configurations: RS232, RS485, fiber optic or Ethernet.
G650 units incorporate a front RS232 port, two rear RS485 or fiber optic ports, and a 10/100Base T port, and in some
configurations two 100BaseFX ports. The data flow in any of the configurations is “half-duplex”.
Each data byte is transmitted in an asynchronous way and it is formed by: 1 start bit, 8 data bits, 1 stop bit and 1 parity bit if
programmed. Thus you have a 10 or 11-bit data, depending on whether it has parity or not.
The port baud rate and the parity are independent and programmable for each communication port. Any port may be
programmed to baud rates of: 1200, 2400, 4800, 9600, 19200, 38400, 57600 or 115200. Parity may be pair, impair or
without parity.
The master must know the client address with which it is going to communicate. No unit will operate after a master request
if the message address is not its own, except it the address is 0, which is the broadcast address. In this case the relay will
operate, but won’t send any reply.