Support / Knowledge Base / DigiOne does not talk using Digi supplied NULL-Modem cable to a Modicon 984 PLC.

DigiOne does not talk using Digi supplied NULL-Modem cable to a Modicon 984 PLC.

Digi products with a 9-pin DTE port usually come with a 9-to-9 null modem cable. The 9-pin Female port on older Modicon PLC is still DTE, but pin #1 is assumed to be FRAME GROUND. You will need to use a male/male gender changer, but that is not enough. This misuse of pin #1 is incompatible with our null modem cable, which shorts pins #1, 4, and 6 together at the PLC. This causes the PLC to see DSR low and ignore the port.

So you have one of 2 solutions:
1) Create a custom cable that ignores pin #1 and has the correct M/F connectors for the PLC. Remember to loopback the DTR/DSR at the PLC - This is VERY important! You can also connect your cable shield to pin #1 of the PLC.
2) Another option is to disengage pin #1 with some method such as a needle-nose plier to pull out, shorten or similarly disengage pin #1 within the gender changer.
Last updated: Aug 23, 2017

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