ProSoft Technology MVI69L-MBTCP - Manual (Page 110)

Reference MVI69L-MBTCP ♦ CompactLogix Platform
User Manual Communication Module
Page 110 of 152 ProSoft Technology, Inc.
January 6, 2014
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 01 00 13 00 25 CRC
Response
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 follows. 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.
Node
Address
Func
Code
Byte
Count
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 (2
bytes)
0B 01 05 CD 6B B2 OE 1B CRC
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.
7.2.3 Read Input Status (Function Code 02)
Query
This function allows the user 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 will be interrogated to
obtain status data.