An M-Series PLC can place an outgoing modem call yet remain unreachable from a host PC if the remote analog modem is not configured to answer incoming calls. Configure the modem for Auto Answer, then verify that it answers the telephone line and establishes the modem-to-modem connection.
Identify the One-Way Communication Symptom
The confirmed symptom is directional: the PLC and its analog modem can initiate a telephone call, but dialing the PLC site from a host PC produces no response from the PLC. Successful outbound dialing confirms only the outbound path; it does not prove that the remote modem will answer an inbound call.
| Test | Observed result | Decision |
|---|---|---|
| PLC initiates a call | Telephone connection succeeds | Outbound dialing operates |
| Host PC dials the PLC site | No response from the PLC | Check the remote modem's incoming-call behavior |
Configure the Remote Modem for Auto Answer
Set the modem connected to the PLC to Auto Answer mode. This setting allows the remote modem to answer when the host PC modem calls and then negotiate a connection. The evidence does not identify a modem model or command string, so use the modem manufacturer's documented configuration method rather than assuming a specific command.
For the PLC-side setup details, consult Appendix 3: PLC to Modem Communication Setup in the Internet TRiLOGI reference manual. Preserve the Auto Answer setting using the modem's supported method if the connection must survive a power cycle; the required method is modem-specific and is not established by the available evidence.
Test the Incoming Connection
- Enable Auto Answer on the analog modem connected to the M-Series PLC.
- From the host PC, dial the telephone number assigned to the remote modem line.
- Confirm that the remote modem answers and that the two modems establish a connection.
- After the carrier connection is established, verify that the host software can communicate with the PLC.
If the line rings but the remote modem does not answer, recheck Auto Answer configuration at the remote modem. If the modem answers and connects but the PLC still does not communicate, separate the fault domains: the incoming telephone and modem path has passed, so continue with the PLC-to-modem serial configuration using the TRiLOGI reference procedure.
Keep Modem and PLC Diagnostics Separate
The PLC does not answer the telephone line directly; the attached modem must first detect and answer the call. Diagnose the sequence in order: incoming call, modem answer, modem-to-modem connection, then host-to-PLC communication. This prevents a missing Auto Answer setting from being misdiagnosed as a PLC communication failure.
FAQ
Why can my M-Series PLC modem dial out but not receive calls?
Outbound dialing and inbound answering are separate functions. Configure the remote analog modem for Auto Answer so it responds when the host PC calls.
Where is the TRiLOGI PLC-to-modem setup procedure?
Use Appendix 3: PLC to Modem Communication Setup in the Internet TRiLOGI reference manual.
What should I check after the remote modem answers?
Confirm that the modems establish a connection, then test host-to-PLC communication. If the carrier connection succeeds but PLC communication fails, check the PLC-to-modem serial configuration.