APPENDIX B: MODBUS PROTOCOL B.2 MODBUS F650
GEK-106310-AF F650 DIGITAL BAY CONTROLLER B-3
The following table shows the generic string format, valid for transmission and reception. However, each function has 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
unit
s from responding at the same time to the same request. All relay ports use this
address, which can be programmed to any value between 1 and 254.
When the master transmits a string with the slave address 0, this indicates that it is a
Br
oadcast. Every slave in the communication bus performs the requested action, but
none of them responds to the master. Broadcast is only accepted for writing since no
unit responds.
FUNCTION CODE
[B]
1 byte This is one of the function codes supported by the equipment. The generic function
cod
es are 3 and 4 for reading and 16 for writing. Special read function 66 and write
function 67 are available only for firmware version 7.50 or above.
When the slave responds with an exception to any of these strings, it places to 1 the
m
ost important bit of the corresponding function. For example, an exception to
function 3 is indicated with an 83 as function code, and an exception to function 16
or 0x10 in hexadecimal, is indicated with a 0x90.
SETTING GROUP SELECTED
[B1]
1byte This part of string format only applies to read function 66 and write function 67 (fw
7.50 or
above).This byte indicates the setting group being read or written. Values
should are as follow:
0 [Decimal]
Setting Group 1
1 [Decimal]
Setting Group 2
…
5 [Decimal]
Setting Group 6
DATA
[C]
N bytes This section includes a variable number of bytes, depending on the function code. It
m
ay 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
det
ection. If the slave detects a string with errors, based on an incorrect CRC, it
neither performs an action, nor responds to the master. The CRC order is LSB-MSB.
TIME OUT Required time to
transmit 3.5 By
tes
A string is finished when nothing is received during a period of 3.5 bytes, with the
actual time varying depending on baud rate:
15 ms at 2400 bps
2 ms at 19200 bps
300 μs
at 115200 bps
etc.
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