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Modbus Functions489
Communications Guide
http://www.GEindustrial.com/multilin
8
GE Multilin
Function Code 16:
Performing Commands
Some PLCs may not support execution of commands using function code 5 but do
support storing multiple setpoints using function code 16. To perform this operation
using function code 16 (10h), a certain sequence of commands must be written at
the same time to the 489. The sequence consists of: Command Function register,
Command operation register and Command Data (if required). The Command
Function register must be written with the value of 5 indicating an execute operation
is requested. The Command Operation register must then be written with a valid
command operation number from the list of commands shown in the memory map.
The Command Data registers must be written with valid data if the command
operation requires data. The selected command will execute immediately upon
receipt of a valid transmission.
Message Format and Example:
Perform a 489 RESET (operation code 1).
Error Responses When a 489 detects an error other than a CRC error, a response will be sent to the
master. The MSbit of the Function Code byte will be set to 1 (i.e. the function code
sent from the slave will be equal to the function code sent from the master plus
128). The following byte will be an exception code indicating the type of error that
occurred.
Transmissions received from the master with CRC errors will be ignored by the 489.
The slave response to an error (other than CRC error) will be:
• SLAVE ADDRESS: 1 byte
• FUNCTION CODE: 1 byte (with MSbit set to 1)
•EXCEPTION CODE: 1 byte
•CRC: 2 bytes
The 489 implements the following exception response codes.
01: ILLEGAL FUNCTION
The function code transmitted is not one of the functions supported by the 489.
02: ILLEGAL DATA ADDRESS
The address referenced in the data field transmitted by the master is not an
allowable address for the 489.
03: ILLEGAL DATA VALUE
The value referenced in the data field transmitted by the master is not within range
for the selected data address.
Master Transmission Bytes Example Description
Slave Address 1 0B message for slave 11
Function Code 1 10 store setpoints
Data Starting Address 2 00 80 setpoint address 0080h
Number of Setpoints 2 00 02 2 setpoints = 4 bytes total
Byte Count 1 04 2 registers = 4 bytes
Command Function 2 00 05 data for address 0080h
Command Function 2 00 01 data for address 0081h
CRC (low, high) 2 0B D6 computed CRC error code
Slave Response Bytes Example Description
Slave Address 1 0B message from slave 11
Function Code 1 10 store multiple setpoints
Data Starting Address 2 00 80 setpoint address 0080h
Number of Setpoints 2 00 02 2 setpoints (4 bytes total)
CRC (low, high) 2 40 8A computed CRC error code