Siemens SIMATIC TI505 Manual de usuario descargar pdf (Pagina 44)

Modbus Commands

2-13

SIMATIC TI505/TI500 MODNIM User Manual

Code

04 allows you to read the contents of input registers in the attached

PLC. These registers hold the values returned by the I/O devices. The data

field of this command includes the start address of the registers and the

number of registers to be read.

Addressing is sequential up to the maximum memory size for a particular

PLC. The maximum number of registers that can be addressed with one

command is 125. If more are requested, the whole command will be rejected

with an Exception code 03.

[This command is equivalent to issuing a TIW

Y I Primitive 20 request with

a TT

-type 09 (W

ord input WX)].

Example:

Request: 07 04 0008 0001 XXXX

Checksum (XXXX)

Number of registers (0001)

Address of first register (0008)

Function code (04)

Slave address (07)

Figure 2-12 Read

Input Register Example — Request

This request is for secondary address 7 to read and return the value stored

in input register 0008.

Response: 07 04 02 0020 XXXX

Checksum (XXXX)

Input register value (0020)

Number of bytes (02)

Function code (04)

Slave address (07)

Figure 2-13 Read

Input Register Example — Response

The response has returned the number of bytes of data (02) and the value

0020 (32 decimal) from the requested input register (0008).

Code 04 — Read

Input Registers

1 2 ... 39 40 41 42 43 44 45 46 47 48 49 ... 90 91

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Siemens SIMATIC TI505 Manual de usuario Pagina 44