Operating instructions

Reference PTQ-MCM ♦ Quantum / Unity Platform
Modbus Communication Module
ProSoft Technology, Inc. Page 117 of 139
April 29, 2008
Response
The addressed slave 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 two 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 slave interface device is normally serviced at the end of the
controller's scan, the data will reflect the register content at the end of the scan.
Some slaves will limit the quantity of register content provided each scan; thus
for large register quantities, multiple transmissions will be made using register
content from sequential scans.
In the example below, the registers 40108 to 40110 have the decimal contents
555, 0, and 100 respectively.
Adr Func ByteCnt Hi Data Lo Data Hi Data Lo Data Hi Data Lo Data Error Check
Field
11 03 06 02 2B 00 00 00 64 CRC
8.6.4 Read Input Registers (Function Code 04)
Query
Function code 04 obtains the contents of the controller's input registers at
addresses 3xxxx. 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 slave device may have restrictions that lower this
maximum quantity. The registers are numbered for zero (30001 = zero, 30002 =
one, etc.). Broadcast mode is not allowed.
The example below requests the contents of register 3009 in slave number 11.
Adr Func Data Start Reg Hi Data Start Reg Lo Data #of Regs Hi Data #of Regs Lo Error Check Field
11 04 00 08 00 01 CRC
Response
The addressed slave 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 slave 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.