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MELSEC-Q
3 SPECIFICATIONS
3.3.3 Cautions during data communications with a RS-422/485 line
Caution is necessary in the following points when carrying out data communications
with the opposite device via the QD51 (-R24)'s RS-422/485 interface.
As for the opposite device, please send and receive data with consideration of the
following points.
(1) Countermeasure for data error reception on the external device
side when connected to the RS-422/485
When data containing errors are received by the external device, install a pull-up
or pull-down resistor in the external device.
By installing a pull-up or pull-down resistor (the criterion for the resistance value
is approximately 4.7 k
Ω
, 1/4 W) it is possible to prevent reception of data
containing errors.
Receiving Data
Opposite Device
4.7k
Ω
1/4W
4.7k
Ω
1/4W
Terminal
Resistor
RDA
RDB
+
-
POINT
If there is a pull-up or pull-down resistor in the opposite device, there will be no
errors in the data which are received.
REMARK
Here, we will explain concerning cases where a pull-up or pull-down resistor is not
installed in the opposite device.
When no station is sending data, the send line is in the high impedance state.
In such a case, transmission may be unstable due to noise, etc. and thereby
erroneous data may be received on the opposite device.
At such a time, it is possible that parity errors or framing errors, etc. could occur, so
when an error occurs, skip over the data that are sent at such a time.
Содержание QD51
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