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MELSEC-Q
3 SPECIFICATIONS
3.3.3 Precautions when transferring data using RS-422/485 circuit
The following precautions must be observed when transferring data with an external
device through the Q series C24 RS-422/485 interface.
Take the following into account when the transferring data with the external device.
(1) Error receive data countermeasures at external device during RS-
422/485 connection
If the external device may receive erroneous data, install a pull-up or pull-down
resistor to the external device as follows.
Installing a pull-up or pull-down resistor (resistance value criteria: approx. 4.7 k
1/4 W) can prevent the reception of erroneous data.
Receive data
External device
4.7kΩ 1/4W
4.7k
Ω
1/4W
Terminating
resistor
RDA
RDB
+
-
POINT
When there is a pull-up or pull-down resistor at the external device, erroneous data
is not received.
REMARK
The following describes the case when a pull-up or pull-down resistor is not installed
to the external device.
When no station is sending, the send line becomes high impedance and noise, etc.
may cause the send line to change and the external device to receive erroneous
data.
In this case, there is probably a parity error or framing error.
Therefore, skip the erroneous data.
Since the first data during data reception is fixed in the following cases, also skip the
receive data until the fixed head data is received.
• When using an MC protocol to send data, the first data is fixed according to the
frame and format the user uses.
• When transferring data using user frames with non procedure protocol or
bidirectional protocol, the first data is selected according to the user frame that the
user registers to the Q series C24.