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12.4 RS485/422-interface – BL20-1RS485/422
The module BL20-1RS485/422 allows the transfer of serial data streams via the RS485/422 interface
and therefore enables various devices to be connected, such as printers, scanners or bar code read-
ers that use the RS485/422 interface for communication. The interface transfers the data received
from the device to the PLC or transfers data to be sent from the PLC to the device.
12.4.1 Data transfer method
The RS485/422-module enables flexible serial data transfer. The RS422 connection mode supports
two wire simplex or four wire full-duplex transmission. The RS485 connection supports two wire
half-duplex transmission.
An operational data transfer method can be set by the module’s parameters.
The data transfer can be parameterized as follows:
Data rate: 300…115200 Bit/s.
Data bits: 7 or 8 data bits in one data frame
Parity: none, odd or even
Stop bits: 1 or 2 bit.
The data flow control can be implemented in RS422 operation with a software handshake (XON/
XOFF) routine.
12.4.2 Data exchange
For the data exchange with a field device, the RS485/422-module provides a 64-byte transmit-buf-
fer and a 128-byte receive-buffer. This is a hardware-restriction. The data telegrams which have to
be sent or received can be larger.
The data transfer from the PLC into the transmit-buffer of the RS485/422-module or from the
receive-buffer of the module to the PLC is realized via a 8-byte transmission channel in the Process
input data (page 411) or Process output data (page 413).
To ensure the error-free data transmission, 2 byte of each data package are used to display status-,
control- and diagnosis information. The amount of user data is therefore reduced to 6 byte within a
data package
Fig. 281: BL20-1RS485/422
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