COMMUNICATIONS GUIDE

RS485 INTERFACE (MODBUS RTU)

Function Code 08H

Modbus Implementation: Loopback

Test MM300 Implementation: Loopback Test

This function is used to test the integrity of the communication link. The MM300 will echo the request.

For example, consider a loopback test from slave 17:

Table 7: Master/slave packet format for function code 08H

MASTER TRANSMISSION

BYTES

EXAMPLE

DESCRIPTION

 

 

 

 

SLAVE ADDRESS

1

11

message for slave

 

 

 

17

 

 

 

 

FUNCTION CODE

1

08

loopback test

 

 

 

 

DIAG CODE

2

00 00

must be 00 00

 

 

 

 

DATA

2

00 00

must be 00 00

 

 

 

 

CRC

2

E0 0B

CRC error code

 

 

 

 

SLAVE RESPONSE

BYTES

EXAMPLE

DESCRIPTION

 

 

 

 

SLAVE ADDRESS

1

11

message from

 

 

 

slave 17

 

 

 

 

FUNCTION CODE

1

08

loopback test

 

 

 

 

DIAG CODE

2

00 00

must be 00 00

 

 

 

 

DATA

2

00 00

must be 00 00

 

 

 

 

CRC

2

E0 0B

CRC error code

 

 

 

 

Function Code 10H

Modbus Implementation: Preset Multiple Registers

MM300 Implementation: Store Multiple Setpoints

This function code allows multiple Setpoints to be stored into the MM300 memory. Modbus “registers” are 16-bit (two byte) values transmitted high order byte first. Thus all MM300 setpoints are sent as two bytes. The maximum number of Setpoints that can be stored in one transmission is dependent on the slave device. Modbus allows up to a maximum of 60 holding registers to be stored. The MM300 response to this function code is to echo the slave address, function code, starting address, the number of Setpoints stored, and the CRC.

For example, consider a request for slave 17 to store the value 00 02 to setpoint address 04 5C and the value 01 F4 to setpoint address 04 5D. After the transmission in this example is complete, MM300 slave 17 will have the following setpoints information stored:

Address

Data

 

 

 

04

5C

00 02

 

 

 

04

5D

01 F4

 

 

 

The master/slave packets have the following format:

MM300 MOTOR MANAGEMENT SYSTEM – COMMUNICATIONS GUIDE

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GE MM300 manual Function Code 08H, Function Code 10H, Master/slave packet format for function code 08H, Diag Code

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