Reference MVI56E-MCM ♦ ControlLogix Platform
User Manual Modbus Communication Module
Page 170 of 209 ProSoft Technology, Inc.
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.
6.8.3 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 are 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.
Note: This is the structure of the message being sent out to the Modbus network. The byte values
below are in hexadecimal display.
Response
An example response to Read Input Status is as shown in the table below. The
data is packed one bit for each input. The response includes the Slave address,
function code, quantity of data characters, the data characters, and error
checking. Data is packed with one bit for each input (1=ON, 0=OFF). The lower
order bit of the first character contains the addressed input, and the remainder
follow. For input quantities that are not even multiples of eight, the last characters
is filled in with zeros at high order end. The quantity of data characters is always
specified as a 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 reflects input status at the end of the scan. Some Slaves limit the quantity of
inputs provided each scan; thus, for large coil quantities, multiple PC transactions
must be made using coil status for sequential scans.
To Next Page
Loading...
Need help?
General
