ProSoft Technology MVI69-MCM User Manual

Page 135

MVI69-MCM ♦ CompactLogix or MicroLogix Platform

Reference

Modbus Communication Module

User Manual

ProSoft Technology, Inc.

Page 135 of 167

March 22, 2011

Response
An example response to Read Coil Status is as shown in Figure C2. The data is

packed one bit for each coil. The response includes the Slave address, function

code, quantity of data characters, the data characters, and error checking. Data

will be packed with one bit for each coil (1 = ON, 0 = OFF). The low order bit of

the first character contains the addressed coil, and the remainder follow. For coil

quantities that are not even multiples of eight, the last characters will be filled in

with zeros at high order end. The quantity of data characters is always specified

as quantity of RTU characters, that is, the number is the same whether RTU or

ASCII is used.
Because the Slave interface device is serviced at the end of a controller's scan,

data will reflect coil status at the end of the scan. Some Slaves will limit the

quantity of coils provided each scan; thus, for large coil quantities, multiple PC

transactions must be made using coil status from sequential scans.

Adr Func

Byte
Count

Data Coil
Status 20 to
27

Data Coil
Status 28 to
35

Data Coil
Status 36 to
43

Data Coil
Status 44 to
51

Data Coil
Status 52 to
56

Error
Check
Field

CRC

The status of coils 20 to 27 is shown as CD(HEX) = 1100 1101 (Binary). Reading

left to right, this shows that coils 27, 26, 23, 22, and 20 are all on. The other coil

data bytes are decoded similarly. Due to the quantity of coil statuses requested,

the last data field, which is shown 1B (HEX) = 0001 1011 (Binary), contains the

status of only 5 coils (52 to 56) instead of 8 coils. The 3 left most bits are

provided as zeros to fill the 8-bit format.

5.6.3 Read Input Status (Function Code 02)

Query
This function allows the user to obtain the ON/OFF status of discrete inputs in the

addressed Slave PC Broadcast mode is not supported with this function code. In

addition to the Slave address and function fields, the message requires that the

information field contain the initial input address to be read (Starting Address)

and the number of locations that will be interrogated to obtain status data.
The addressing allows up to 2000 inputs to be obtained at each request;

however, the specific Slave device may have restrictions that lower the maximum

quantity. The inputs are numbered form zero; (input 10001 = zero, input 10002 =

one, input 10003 = two, and so on, for a 584).
The following table is a sample read input status request to read inputs 10197 to

10218 from Slave number 11.

Adr

Func

Data Start Pt Hi

Data Start Pt Lo

Data #of Pts Hi

Data #of Pts Lo Error Check Field

CRC