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Modbus Commands

2-3

SIMATIC TI505/TI500 MODNIM User Manual

able 2-1

Modbus

Functions Supported

Code Type Description

01 Read Coil Status Get current status of a group of coils.

02 Read Input Status Get current status of a group of discrete

inputs.

03 Read Holding Register Get current values from holding registers.

04 Read Input Register Get current values from input registers.

05 Force a Single Coil Change the state of a logic coil to On or Off,

forced On or forced Off, or unforced.

06 Write a Single Register Write a value into a holding register.

07 Read Exception Status Get the 8 internal status coil values.

08 Execute Diagnostics Send diagnostic tests to a slave.

11 Get Communications

Event Counter

Enable the success or failure of a query to

be determined.

12 Get Communications

Event Log

Get the communications log for Modbus

network transactions.

15 Write Multiple Coils Change a number of consecutive coils.

16 Write Multiple Registers Write values into a series of consecutive

holding registers.

17 Report Slave I.D. Get the slave type and the condition of its

run light.

Note:

The highlighted function codes (5, 6, 8, 15, and 16) are supported by Broadcast mode,

for which no response is returned.

The

Data field contains the information needed by the specified node to

perform the required operation, or the information returned by the node to

the host computer

. The data is in multiple bytes, with two ASCII character

bytes representing a hexadecimal value (1 . . . 9 and A . . . F or a . . . f).

The Longitudinal Redundancy Checking (LRC) is the method employed by

ASCII mode to ensure that the message transmitted by the host is the same

as the one that arrives at the secondary (and vice versa). The characters are

passed to a mathematical algorithm creating a checksum of 2 characters in

length that can be duplicated at either end for comparison. The calculation

of this checksum is described in the

Modbus Protocol Reference Manual

The End of File (EOF) marker

, CR (carriage return) indicates the end of the

data and is the point up to which the LRC is generated.

The Ready field, LF (line feed) indicates that the sender is now ready for

any reply

. This character can be changed using Function Code 08, with

Diagnostic code 03. Unless there is a compelling reason to change this

character

, it should be left as the default value (LF).

The Data Field

The LRC Field

The EOF Field

The Ready Field

1 2 ... 29 30 31 32 33 34 35 36 37 38 39 ... 90 91

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