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Modbus Commands

2-12

SIMATIC TI505/TI500 MODNIM User Manual

Modbus Function Descriptions (continued)

Code

03 allows you to read the contents of holding registers in the attached

secondary device. 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 01 (V

ariable Memory)].

Example:

Request: 07 03 0064 0003 XXXX

Checksum (XXXX)

Number of registers (0003)

Address of first register (0064)

Function code (03)

Slave address (07)

Figure 2-10 Read

Output Register Example — Request

This request is for secondary address 7 to read the contents of registers

0064, 0065, and 0066 (100, 101, and 102 decimal).

Response: 07 03 06 032C 0001 0030 XXXX

Checksum (XXXX)

3rd register value (0030)

2nd register value (0001)

1st register value (032C)

Number of bytes (06)

Function code (03)

Slave address (07)

Figure 2-11 Read

Output Register Example — Response

The response has returned the number of bytes of data (06) and the values

for the requested registers; these are 032C, 0001, and 0030 respectively

(812, 1, and 48 decimal).

Code 03 — Read

Output Registers

1 2 ... 38 39 40 41 42 43 44 45 46 47 48 ... 90 91

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Siemens SIMATIC TI505 Manual de usuario Pagina 43