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MODBUS FUNCTIONS
MM200 supported functions
Modbus Functions
Function Code 03H
2-4
be reset (i.e. all slaves start listening for a new transmission from the master). Thus at 9600
baud a delay of greater than 3.5 x 1 / 9600 x 10 x = x 3.65 x ms will cause the
communication link to be reset.
The following functions are supported by the MM200:
•
FUNCTION CODE 03 - Read Setpoints and Actual Values
•
FUNCTION CODE 04 - Read Setpoints and Actual Values
•
FUNCTION CODE 05 - Execute Operation
•
FUNCTION CODE 06 - Store Single Setpoint
•
FUNCTION CODE 07 - Read Device Status
•
FUNCTION CODE 08 - Loopback Test
•
FUNCTION CODE 10 - Store Multiple Setpoints
Modbus implementation: Read Holding Registers
MM200 implementation: Read Setpoints
For the MM200 implementation of Modbus, this function code can be used to read any
setpoints ("holding registers"). Holding registers are 16 bit (two byte) values transmitted
high order byte first. Thus all MM200 Setpoints are sent as two bytes. The maximum
number of registers that can be read in one transmission is 125.
The slave response to this function code is the slave address, function code, a count of the
number of data bytes to follow, the data itself and the CRC. Each data item is sent as a two
byte number with the high order byte sent first.
For example, consider a request for slave 17 to respond with 3 registers starting at address
006B. For this example the register data in these addresses is as follows:
Address
006B
006C
006D
The master/slave packets have the following format:
Table 2-1: MASTER/SLAVE PACKET FORMAT FOR FUNCTION CODE 03H
MASTER TRANSMISSION
SLAVE ADDRESS
FUNCTION CODE
DATA STARTING ADDRESS
NUMBER OF SETPOINTS
CRC
CHAPTER 2: RS485 INTERFACE (MODBUS RTU)
Data
022B
0000
0064
BYTES
EXAMPLE
1
11
1
03
2
00 6B
2
00 03
2
76 87
MM200 MOTOR MANAGEMENT SYSTEM – COMMUNICATIONS GUIDE
DESCRIPTION
message for slave 17
read registers
data starting at 006B
3 registers = 6 bytes
total
CRC error code
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