ProSoft Technology MVI69L-MBTCP - Manual (Page 113)

MVI69L-MBTCP ♦ CompactLogix Platform Reference
Communication Module User Manual
ProSoft Technology, Inc. Page 113 of 152
January 6, 2014
7.2.5 Read Input Registers (Function Code 04)
Query
This Function Code obtains the contents of the controller's input registers from
the Modbus 3x range. These locations receive their values from devices
connected to the I/O structure and can only be referenced, not altered from within
the controller, The addressing allows up to 125 registers to be obtained at each
request; however, the specific Server device may have restrictions that lower this
maximum quantity. The registers are numbered for zero (30001 = zero, 30002 =
one, and so on). Broadcast mode is not allowed.
The example below requests the contents of register 30009 in Server number 11.
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 04 00 08 00 01 CRC
Response
The addressed Server responds with its address and the function code followed
by the information field. The information field contains 1 byte describing the
quantity of data bytes to be returned. The contents of the registers requested
(DATA) are 2 bytes each, with the binary content right justified within each pair of
characters. The first byte includes the high order bits and the second, the low
order bits.
Because the Server interface is normally serviced at the end of the controller's
scan, the data will reflect the register content at the end of the scan. Each PC will
limit the quantity of register contents provided each scan; thus for large register
quantities, multiple PC scans will be required, and the data provided will be form
sequential scans.
In the example below the register 30009 contains the decimal value 0.
Node
Address Function
Code Byte
Count Data Input
Register High Data Input
Register Low Error Check Field
(2 bytes)
0B 04 02 00 00 CRC
7.2.6 Force Single Coil (Function Code 05)
Query
This Function Code forces a single coil (Modbus 0x range) either ON or OFF.
Any coil that exists within the controller can be forced to either state (ON or
OFF). However, because the controller is actively scanning, unless the coil is
disabled, the controller can also alter the state of the coil. Coils are numbered
from zero (coil 0001 = zero, coil 0002 = one, and so on). The data value 65,280
(FF00 HEX) will set the coil ON and the value zero will turn it OFF; all other
values are illegal and will not affect that coil.