Siemens SIMATIC TI500 Bedienungsanleitung PDF herunterladen (Seite 53)

Modbus Commands

C-4

TIWAY I Gateway User Manual

Modbus Protocol Overview (continued)

The

Gateway ignores messages that contain invalid characters. Messages

containing the following errors will also be ignored.

•

Incorrect checksum

•

Parity error

•

Framing errors

•

Incomplete transmission

This section outlines some of the differences between Modbus and TIW

AY I

network commands.

•

Whereas Modbus uses

Functions Codes, TIW

Y I uses

Primitives.

Primitives differ from Function codes in one important way: primitives

use TT

-types to allow one primitive to address different types of data.

ith Modbus a different function code is required for each type of data.

•

For all SIMA

TIC TI Series 505 PLCs, the first address for each type of

memory is always 1. Modbus PLCs by comparison allow 0 as the first

legal address.

•

If each system were to return one byte (8 bits) representing eight

discrete coils, they would each be positioned as shown in Figure C-2.

I000000

7654321 87654321

Coil

No.

Coil No.

Modbus PLCs SIMATIC TI PLCs

MSB LSB<

< <

MSB LSB

< < <

Figur

e C-2

Bit

Orientation

Invalid Characters

and Messages

Modbus Functions

1 2 ... 48 49 50 51 52 53 54 55 56 57 58 ... 93 94

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