ProSoft Technology MVI69E-MBTCP - Manual (Page 112)

Reference MVI69E-MBTCP ♦ CompactLogix Platform
User Manual Modbus TCP/IP Enhanced Communication Module
Page 112 of 150 ProSoft Technology, Inc.
May 20, 2015
The status of coils 20 to 27 is shown as CD(HEX) = 1100 1101 (Binary). Reading
from left to right, this shows that coils 27, 26, 23, 22, and 20 are all on. The other
Data Coil Status 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.
7.2.5 Read Input Status (Function Code 02)
Query
This function allows you to obtain the ON/OFF status of discrete inputs (Modbus
1x range) in the addressed server. PC 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 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 server 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 (22 coils) from server 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)
0B 02 00 C4 00 16 CRC
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 server 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
follows. For input 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 a 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,
the data reflect input status at the end of the scan. Some servers 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.