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About SDU and an external device (PC etc.)

ID:
835
Issued:
Dec 25th, 2009
Updated:
May 17th, 2013

Communication works with the SDU connected to an external device (PC), but stops working after a different PC is connected to the CPU tool port and then removed. Later, when a different PC is connected to the tool port, the SDU is able to communicate, but it becomes unable to communicate when the tool port and the different PC are disconnected.

ANSWER
The pin layout on the RS232 cable is wired incorrectly.
For example, a straight cable is being used.

SDU device (PC)
2(SD) 2(RD)
3(RD) 3(SD)
5(CS) 4(GND)
7(GND) 7(RS)

With this wiring, communication is not normally possible because the GND of the SDU and the GND of the external device (PC) are not connected. However, if both the SDU and the GND of the external device (PC) have a similar potential, communication may be possible.

By removing and inserting the cable of a separate device in one of the ports of the PLC, the potential on the PLC side will change and communication will become disabled.

The solution is to rewire the connections correctly.
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