MSA General Monitors S4100C - 8 Modbus RTU Serial Interface; General; Modbus Message Characteristics; Modbus Exception Codes

Model S4100C
31
8.0 Modbus RTU Serial Interface
8.1 General
The Modbus communications interface is based on the RS485 standard. It is
implemented as a 2 wire, half-duplex, balanced differential interface which conforms
to the EIA-485 specification. Each slave device must have its unique address so that
more than one device can be connected to an independently addressed on the same
RS485-link.
The Smart Transmitter Interface implements the RTU protocol as described in the
“Modicon Protocol Reference Guide PI-MBUS-300 Rev. G. The Modbus RTU is an
asynchronous NRZ format. The RTU mode and serial format must be the same for all
devices on a Modbus network. The instrument acts as a Modbus communications
“Slave”.
Two Modbus connections (Modbus 1 and Modbus 2), are provided, sharing the node
address and all other Modbus parameters.
The device receives and transmits on both connections simultaneously, requiring the
host for Modbus 2 to be quiescent when Modbus 1 connection is active and vice
versa.
The Modbus interface factory defaults are set to Node Address 1, 19K2 baud, no
parity and 1 stop bit. When the instrument is powered up, the Modbus setup defaults
to the settings used before it was powered down. The interface supports a maximum
of 2 bits for stop bit and parity information. A selection of 2 stop bits causes no parity
to be implemented.
The Modbus Interface and Menu Interface can be used simultaneously for Modbus
read commands only. For write commands, the operation is mutually exclusive. Any
attempts to perform a Modbus write are inhibited while the Menu Interface is active.
This is indicated by returning the Slave Device Busy response (Exception Code 6).
8.2 Modbus Message Characteristics
Baud rate
2K4, 4K8, 9K6 or 19K2
Byte length (11 bits) max
11 / (Baud rate) ms
Inter message spacing or Modicon specification min
3.5 bytes
Inter byte spacings per Modicon specification min/max
0 bytes / 1.5 bytes
Number of Bytes per message min/max
7 / 15
8.3 Modbus Exception Codes
Code Name
Description
Hex value
Illegal function
Function code is not recognised by the slave
01
Illegal data address
Data address specified is not supported by the slave
02
Illegal data value
Data value specified is not supported by the slave
03
Slave device busy
The slave is engaged in completing a long duration
programme command
06