ProSoft Technology MVI56E-MNETCR - Manual (Page 119)

MVI56E-MNETCR ♦ ControlLogix Platform Reference
Modbus TCP/IP Multi Client Enhanced Communications Module for Remote Chassis User Manual
ProSoft Technology, Inc. Page 119 of 159
June 14, 2011
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.
Adr Func 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
11 01 05 CD 6B B2 OE 1B CRC
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.