Designs, materials, weights and performance ratings are approximate and subject to change without notice. Visit
armstrong
international.com for up-to-date information.
81
Serial Communications – continued
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)
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
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.
