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About SDU and an external device (PC etc.) Continuation

ID:
834
Issued:
Dec 25th, 2009
Updated:
May 17th, 2013

When I connect a Serial data unit (SDU here after) to an external device (PC) and attempt to communicate, the SDU can communicate when the PC is connected to the CPU tool port, but when the PC is disconnected, the SDU is not able to communicate.

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

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

With this wiring, communication is not possible because the GND of the SDU and the GND of the external device (PC) are not connected. However, by connecting a different PC to the CPU tool port, the GND in the PLC and FG become shared through the PC COM port connected to the tool port. Furthermore, since the SG of the PC connected to the SDU is connected to the FG in the PLC through that PC's FG, the FG of the communication cable and the FG of the SDU, it becomes shared with the SG in the PLC. As a result, communication becomes possible. The solution is to rewire the connections correctly.
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