Spirax Sarco RIM20, Installation And Maintenance Instructions Manual, Page: 84 / 132

IM-P198-05 MI Issue 2 84
5.5.12 Modbus protocol
The Modbus RTU protocol is supported in this implementation. Supported baud rates are
1200, 2400, 4800, 9600, 19200, 38400, 57600, and 115200. The default baud rate is 19200
baud. Depending upon the Modbus protocol selected, data are transmitted in 8-bit data
frames with even or odd parity and 1 stop bit, or no parity and 2 or 1 (non-standard) stop bits.
The current Modbus protocol specification does not define register usage, but there is an
informal register numbering convention derived from the original (now obsolete) Modicon
modbus protocol specification, and used by many vendors of Modbus capable products.
Registers Usage Valid function codes
00001-09999 Read/write bits ("coils")
01 (read coils)
05 (write single coil)
15 (write multiple coils)
10001-19999 Read-only bits ("discrete inputs") 02 (read discrete inputs)
30001-39999
Read-only 16 bit registers ("input
registers"), IEEE 754 floating point register
pairs, arbitrary length strings encoded as
two ASCII characters per 16- bit register
03 (read holding registers)
04 (read input registers)
40001-49999
Read/write 16-bit registers ("holding
registers"), IEEE 754 floating point register
pairs, arbitrary length strings encoded as
two ASCII characters per 16- bit register
03 (read holding registers)
06 (write single register)
16 (write multiple registers)
Each range of register numbers maps to a unique range of addresses that are determined by
the function code and the register number. The address is equal to the least significant four
digits of the register number minus one, as shown in the following table.
Registers Function codes Data type and address range
00001-09999
01, 05, 15 Read/write bits 0000-9998
10001-19999 02 Read-only bits 0000-9999
30001-39999 03, 04 Read-only 16-bit registers 0000-9998
40001-49999 03, 06, 16 Read/write 16-bit registers 0000-9998