ProSoft Technology MVI69E-MBTCP User Manual

Page 111

MVI69E-MBTCP ♦ CompactLogix Platform

Reference

Modbus TCP/IP Enhanced Communication Module

User Manual

ProSoft Technology, Inc.

Page 111 of 150

May 20, 2015

7.2.4 Read Coil Status (Function Code 01)

Query
This function allows you to obtain the ON/OFF status of logic coils (Modbus 0x
range) used to control discrete outputs from the addressed server only.
Broadcast mode is not supported with this function code. In addition to the server
address and function fields, the message requires that the information field
contain the initial coil address to be read (Starting Address) and the number of
locations that are interrogated to obtain status data.

The addressing allows up to 2000 coils to be obtained at each request; however,
the specific server device may have restrictions that lower the maximum quantity.
The coils are numbered from zero; (coil number 1 = zero, coil number 2 = one,
coil number 3 = two, and so on).

The following table is a sample read output status request to read coils 0020 to
0056 (37 coils) from server device number 11.

Note: This is the structure of the message being sent out to the Modbus network. The byte values

below are in hexadecimal display

Node
Address

Function
Code

Data Start
Point High

Data Start
Point Low

Number of
Points High

Number of
Points Low

Error Check
Field (2 bytes)

CRC

Response
An example response to Read Coil Status is as shown in the table below. The
data is packed one bit for each coil. The response includes the server address,
function code, quantity of data characters, the data characters, and error
checking. Data is 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
follows. For coil quantities that are not even multiples of eight, the last characters
are 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 server interface device is serviced at the end of a controller's scan,
data reflects coil status at the end of the scan. Some servers 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.

Node
Address

Func
Code

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

(2 bytes)

CRC