Appendix D. ModBus Protocol
The Serinus supports a limited Modbus implementation. The only function codes supported are 3 (read
holding register) and 16 (write multiple registers). The Serial ID is assigned in the Main Menu →
Communications Menu → Serial Communication Menu.
D.1 Command Format
<Slave address><Function code><Start register (MSB)><Start register <LSB><Register count
(MSB)><Register count (LSB)><Write byte count><Write data><CRC (MSB)><CRC (LSB)>
Where:
Slave address The instrument Serial ID. If the request is being made via TCP, this field
is omitted.
Function code 3 (read) or 16 (write).
Start register Specifies an Advanced Protocol IEEE index (refer to Table 25 to see what
values are available and what index to specify for them). The ModBus
index is calculated from the Advanced Protocol index via the following
formula:
Mobus Index = Advanced Protocol Parameter List number x 2 + 256
Register count A single read command may request from 2 - 124 registers, which is to
say from 1 - 62 values. The first index is specified by Start register; all
following indexes are in sequential order. To read values that are not
sequential requires using another read command. Note that the
number of registers must be even, as each value is returned as a floating
point value (4 bytes) and each register is a word (2 bytes).
A write command can only write a single IEEE value at a time. Thus for
write commands this value must be 2.
Write byte count This field is only supplied for a write request; it indicates the amount of
bytes of data that will follow, and must be set to 4 (since only one value
can be written at a time).
Write data This field is only supplied for a write request. It is the value to be written,
expressed in IEEE format. The “Endian” structure can be selected on the
Modbus Serial Communications menu. Big Endian means that the MSB
byte of the IEEE value is at the right end of the four bytes; Little Endian
means it is at the left.
CRC Calculated by the standard Modbus CRC method. If the request is being
made via TCP, this field is omitted.
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