ProSoft Technology MVI56E-MNETC - Manual (Page 146)

Guide to the MVI56E-MNETC User Manual MVI56E-MNETC ♦ ControlLogix Platform
User Manual Modbus TCP/IP Client Enhanced Communication Module
Page 146 of 183 ProSoft Technology, Inc.
February 3, 2013
5.5.2 Read Coil Status (Function Code 01)
Query
This function allows the user to obtain the ON/OFF status of logic coils 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 will be
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 from server device number 11.
Adr Func Data Start Pt
Hi
Data Start Pt
Lo Data # Of Pts
Ho Data # Of Pts
Lo Error Check
Field
11 01 00 13 00 25 CRC
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 server 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 server interface device is serviced at the end of a controller's scan,
data will reflect coil status at the end of the scan. Some servers 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.
Ad
r Func Byte
Coun
t
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
11 01 05 CD 6B B2 OE 1B CRC