Emerson PACSystems RX3i - Page 170

PACSystems™ RX3i and RSTi-EP CPU Reference Manual Section 6
GFK-2222AK October 2019
Serial I/O, SNP & RTU Protocols 162
6.3.1.5 Message Fields
The message fields for a typical message are shown in the figure below, and are explained
in the following sections.
FRAME
Station Address
Function Code
Information
Error Check
6.3.1.5.1 Station Address
The Station Address is the address of the slave station selected for this data transfer. It is
one byte in length and has a value from 0 to 247 inclusive. An address of 0 selects all slave
stations, and indicates that this is a broadcast message. An address from 1 to 247 selects
a slave station with that station address.
6.3.1.5.2 Function Code
The Function Code identifies the command being issued to the station. It is one byte in
length and is defined for the values 0 to 255 as follows:
Function Code
Description
0
Illegal Function
1
Read Output Table
2
Read Input Table
3
Read Registers
4
Read Analog Input
5
Force Single Output
6
Preset Single Register
7
Read Exception Status
8
Loopback Maintenance
9-14
Unsupported Function
15
Force Multiple Outputs
16
Preset Multiple Registers
17
Report Device Type
18–21
Unsupported Function
22
Mask Write 4x Register