70
•
Fieldbus controller 750-842
Data exchange
Modular I/O System
ETHERNET TCP/IP
3.2.5.3
Data exchange between master and I/O modules
The data exchange between the MODBUS master and the I/O modules is
made via the MODBUS functions implemented in the controller by reading
and writing in bits or bytes.
The controller handles four different types of process data:
•
Input words
•
Output words
•
Input bits
•
Output bits
The word for word access to the digital input and output modules is made in
accordance with the following table:
Digital inputs/
outputs
16.
15.
14.
13.
12.
11.
10.
9.
8.
7.
6.
5.
4.
3.
2.
1.
Prozess data
word
Bit
15
Bit
14
Bit
13
Bit
12
Bit
11
Bit
10
Bit
9
Bit
8
Bit
7
Bit
6
Bit
5
Bit
4
Bit
3
Bit
2
Bit
1
Bit
0
High-Byte
Low-Byte
Byte
D1
D0
Table 3.10: Allocation of digital inputs/outputs to process data word acc. Intel format
Adding 0x0200 to the MODBUS address permits to read back the outputs.
The register functions addressing can be by the means of the implemented
MODBUS function codes (read/write). The individual register address is refer-
enced instead of the address of a module channel.
0x000
0x0FF
0x000
(0x200)
0x0FF
(0x2FF)
PII = Process Input
Image
PIO = Process Output
Image
MODBUS master
PII
PIO
I/O modules
Inputs
Outputs
Programmable Fieldbus Controller
Fig. 3-27: Data exchange between MODBUS master and I/O modules
g012929e