GE Multilin G650, Manual

Page: 430 / 639

GEK-113285A G650 Generator Protection & Control System B-3
APPENDIX B B.2 MODBUS G650
B
B.2.3 DATA LINK LAYER
Communication is performed in strings, data groups sent in an asynchronous way. The master transmits a string to the
slave and then the slave responds with another string (except for the case of broadcast communication). A timeout or a
silence time in the communication marks the end of a string. The length of this time varies depending on the baud rate,
because it is equal to 3 characters.
The following table shows the generic string format, valid for transmission and reception. However, each function will have
its own particularities, as described later in this manual.
MODBUS FORMAT
CLIENT ADDRESS
[A]
1 byte Each device in a communications bus must have a unique address to prevent two
units from responding at the same time to the same request. All relay ports will
use this address, which can be programmed to a value between 1 and 254.
When the master transmits a string with the slave address 0, this indicates that it
is a Broadcast. Every slave in the communication bus will perform the requested
action, but none of them will respond to the master. Broadcast is only accepted for
writing because it is nonsense to perform a reading request in Broadcast, as no
unit will respond to it.
FUNCTION CODE
[B]
1 byte This is one of the function codes supported by the equipment. In this case, the
only supported function codes will be 3 and 4 for reading and 16 for writing. When
the slave has to respond with an exception to any of these strings, it will place to 1
the most important bit of the correspondent function. For example, an exception to
function 3 will be indicated with an 83 as function code, and an exception to
function 16 or 0x10 in hexadecimal, will be indicated with a 0x90.
DATA
[C]
N bytes This section includes a variable number of bytes, depending on the function code.
It may include: addresses, data length, settings, commands or exception codes
sent by the client.
CRC
[D]
2 bytes Two-byte control code. ModBus/RTU includes a 16-bit CRC in each string for error
detection. If the slave detects a string with errors, based on an incorrect CRC, it
will neither perform any action, nor respond to the master. The CRC order is LSB-
MSB.
TIME OUT Required time to
transmit 3,5 Bytes
A string is finished when nothing is received during a period of 3,5 bytes:
15 ms at 2400 bps
2 ms at 19200 bps
300
μ
s at 115200 bps
etc.