GE GFK-0804B Message Fields, Station Address, Function Code, Information Field, Error Check Field

C

Message Fields

A typical message has the fields shown below:

aGGGGGGGGGGGGGGGGGGGGG FRAME GGGGGGGGGGGGGGGGGGGGG'

Station Address

Function Code

Information

Error Check

Station Address

The station address is the address of the slave selected for the 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. The station address for a Micro PLC is one of its software Setup parameters.

Function Code

The function code identifies the command being sent. This field is one byte in length and may have a value from 0 to 255:

Function

Code

0

1

2

3

4

5

6

7

Description

Illegal Function

Read Output Table

Read Input Table

Read Registers

Read Analog Input

Force Single Output

Preset Single Register

Read Exception Status

Function

Code

8

9±14

15

16

17

18±127

128±255

Description

Loopback Maintenance Unsupported Function Force multiple outputs Reset multiple registers Report device type Unsupported function

Reserved for Exception Responses

Information Field

The information field contains all of the other information required to further specify or respond to a requested function. Detailed specification of the contents of the information field for each function code is found in the Message Descriptions that start on page C-9.

Error Check Field

The error check field is two bytes in length. It contains a cyclic redundancy check (CRC±16) code. Its content depends on the station address, function code, and information field. For information about generating the CRC± 16 code, see Cyclic Redundancy Check (CRC) beginning on page C-5. Note that the information field is variable in length. To properly generate the CRC± 16 code, the length of frame must be determined. See RTU Message Length (page C-8) to calculate the length of a frame for each of the defined function codes.

GFK±0804B Appendix C RTU Protocol C-3