Serinus 44 User Manual 3.0
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).
Read Holding Register
The user must specify a slave address for Serial requests (but not for TCP requests). This value is the
Serinus Serial ID.
Read requests specify which Advanced Protocol IEEE value they want to read as the starting
reference. Refer to Table 12 – Advanced Protocol Parameter List 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
The user may read from 2 to 124 registers.
Note: The user must read an even number of registers because the return data is always 4
bytes (a float).
The Serinus expects eight data bits, one stop bit and no parity. The baud rate is specified by the
Serial Communications Menu.
The value will be returned as a big-endian 32-bit IEEE floating point value.
Write Multiple Registers
The user must specify a slave address for Serial requests (but not for TCP requests). This value is the
Serinus Serial ID.
The start reference is the same as for reading.
Only two registers may be written at a time; that is, a single IEEE value. Currently the only supported
value is 85, to put the instrument into span (3), zero (2), cycle (1) or measure (0) mode.
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