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MVI69E-MBTCP ♦ CompactLogix Platform Reference
Modbus TCP/IP Enhanced Communication Module User Manual
ProSoft Technology, Inc. Page 113 of 150
June 28, 2017
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.
Node
Address
Func
Code
Byte
Count
Data Discrete
Input 10197 to
10204
Data Discrete
Input 10205 to
10212
Data Discrete
Input 10213 to
10218
Error Check
Field
(2 bytes)
0B 02 03 AC DB 35 CRC
The status of inputs 10197 to 10204 is shown as AC (HEX) = 10101 1100
(binary). Reading left to right, this show that inputs 10204, 10202, and 10199 are
all on. The other input data bytes are decoded similar.
Due to the quantity of input statuses requested, the last data field which is shown
as 35 HEX = 0011 0101 (binary) contains the status of only 6 inputs (10213 to
102180) instead of 8 inputs. The two left-most bits are provided as zeros to fill
the 8-bit format.
7.2.6 Read Holding Registers (Function Code 03)
Query
This function allows you to retrieve the contents of holding registers 4xxxx
(Modbus 4x range) in the addressed server. The registers can store the
numerical values of associated timers and counters which can be driven to
external devices. The addressing allows retrieving up to 125 registers at each
request; however, the specific server device may have restrictions that lower this
maximum quantity. The registers are numbered form zero (40001 = zero, 40002
= one, and so on). The broadcast mode is not allowed.
The example below reads registers 40108 through 40110 (three registers) 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
Registers
High
Data Start
Registers Low Data Number
of Registers
High
Data Number
of Registers
Low
Error Check
Field
(2 bytes)
0B 03 00 6B 00 03 CRC