Read Coil Status (Function Code 01) - ProSoft Technology MVI56E-MNETC User Manual

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User Manual

5.4.2 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 Function
Address Code
0B 01
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 Func
Address Code
0B 01
Page 136 of 172
Modbus TCP/IP Client Enhanced Communication Module - Client/Server
Data Start Data Start
Point High Point Low
00 13
Byte Data Coil Data Coil
Count Status 20 Status 28
to 27 to 35
05 CD 6B
MVI56E-MNETC/MNETCXT ♦ ControlLogix Platform
Number of Number of
Points High Points Low
00 25
Data Coil Data Coil Data Coil
Status 36 Status 44 Status 52
to 43 to 51 to 56
B2 OE 1B
Error Check
Field (2 bytes)
CRC
Error Check
Field
(2 bytes)
CRC
ProSoft Technology, Inc.
September 20, 2018