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ProSoft Technology PLX31-EIP-SIE User Manual

Page 112

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MBTCP Protocol

PLX3x Series

User Manual

Ethernet and Serial Gateways

Page 112 of 218

ProSoft Technology, Inc.

September 17, 2014

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

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 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 3009 in server 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 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 3009 contains the decimal value 0.

Adr

Func

Byte Count

Data Input Reg Hi

Data Input Reg Lo

Error Check Field

11

04

02

00

00

E9