Configuring V130 Remote Access over GPRS and GSM Modems

V130 remote monitoring over an Enfora 1308 can use GPRS/TCP or a GSM data call, but the two paths require different network reachability and VisiLogic modes. For GPRS, choose which endpoint initiates the TCP connection before configuring anything: either the PLC calls a listening PC after receiving an SMS, or the PC calls a PLC that is listening at a reachable public IP.

Choose the connection path from the required readings

GPRS carries IP traffic; GSM circuit-switched data is a separate dial-up path. A SIM capable of sending SMS does not by itself prove that either inbound TCP access or GSM data calls are available. Decide what the modem and carrier can actually provide, then confirm the connection direction and endpoint before changing settings.

Use a GPRS modem at the PC only when it supplies the PC's Internet connection; an ordinary ADSL connection can serve the same role for the PLC-calls-PC arrangement. If the goal is monitoring and acknowledging alarms, slow access may be acceptable, but test the actual response and write behavior before relying on remote alarm clearing.

Check whether the PLC has a reachable GPRS address

Read the IP address assigned to the Enfora 1308 while its GPRS session is active, using the modem/PLC diagnostics or a value displayed by the application. Compare that address with an externally observed address and establish with the mobile operator whether the SIM has a public, inbound-reachable address. Cellular service can place devices behind operator NAT; seeing an address in the PLC does not alone prove that an unsolicited Internet connection can reach it.

1. If the address is public and inbound-reachable, select the PC-calls-PLC branch: set the V130 to TCP Listening and configure VisiLogic to make a TCP Call to the modem's current address.
2. If the address is dynamic but inbound-reachable, arrange a way to communicate the current address to the PC. The controller can send an email containing its current IP, as described for that branch; otherwise the PC cannot know which address to call.
3. If the address is private, translated by the operator, or inbound access is blocked, do not keep troubleshooting the VisiLogic call mode as though it were a PLC setting. Use the PLC-calls-PC branch, obtain an operator service that provides inbound reachability, or use a network solution that bridges the endpoints.

A fixed public address associated with the modem SIM was identified as one way to make direct calls predictable. Verify the address and inbound routing with the operator rather than assuming every GPRS subscription assigns a public address.

Check the PC-side listener and router path

For PLC-initiated TCP, configure VisiLogic for TCP Listening so the PC waits for the V130's connection. If the PC sits behind a home router, find the router's current public IP and forward TCP ports 20256 and 20257 to the PC's local address. Permit the same ports through the PC firewall. The router's public address must be reachable from the cellular network, and the forwarding destination must remain the same local PC.

1. Read the router's WAN/public IP and compare it with the address visible from outside the home network. Matching addresses indicate the router is the externally addressed endpoint; a mismatch means another network translation or upstream router is present and must also be addressed.
2. Read the router forwarding table. Both 20256 and 20257 must target the intended PC address using TCP.
3. Check the PC firewall and confirm VisiLogic is in TCP Listening mode. If a connection fails, test reachability from an external network; testing only inside the same LAN does not validate Internet routing.

The PLC then needs the PC's public IP as its call destination. Send that address to the V130 by SMS and have the controller issue its configured CALL. An SMS carrying the destination does not create a listening socket, open a firewall, or repair an incorrect NAT rule; each layer must already be ready.

Use SMS without confusing it with the GPRS data session

The Enfora 1308 is described as a GPRS modem, and the V130 can use it for GPRS connectivity while sending SMS. The described SMS mechanism sends the telephone number and message text in a Modbus frame from the V130. Thus SMS and remote TCP access are separate functions even when they use the same modem and SIM.

GPRS may disconnect when the controller sends an SMS and reconnect afterward. Treat that as a possible interruption to the TCP session, not as proof that the listener or port forwarding is misconfigured. Record whether the TCP link drops at the same time as an SMS transmission, then verify that the modem re-establishes GPRS and the PLC can make or accept a new connection. If alarms must be cleared remotely, confirm the PLC's application permits the required command after reconnection.

Check VisiLogic mode against the initiator

TCP Listening means the device waits for a remote connection; TCP Call means it initiates one to a specified destination. Configure exactly one intended initiator for each test, and confirm that the other endpoint is actually listening. Reversing these roles is a common cause of an apparently dead link even when the modem has Internet access.

Use the VisiLogic help example for the SMS-triggered call/listening arrangement and the modem documentation for the actual modem-specific configuration. A Siemens TC35 example was identified as having similar steps with the Enfora modem model changed; that is a starting point, not evidence that all settings or modem models are interchangeable.

Evaluate the GSM data-call alternative

A modem attached to the PC can call the modem attached to the V130 over GSM without using an Internet TCP route, but this is not the same as making a GPRS connection. The described GSM data path was stated to connect at 9600 bit/s. Treat that as the reported connection rate for the described method; verify the negotiated rate and the actual remote-access function with the selected modems, SIMs, and carrier.

Before choosing this route, confirm that the mobile operator still supports circuit-switched data calls on both subscriptions and that the modem/VisiLogic arrangement is configured for that call type. A USB GPRS modem's ability to access packet data or send SMS does not establish that it can place a GSM data call. Older phones were used as GSM modems in some setups, but phone model and operating-system support vary; test the exact PC modem interface and dialing behavior rather than relying on a phone's GPRS capability.

Procedure and connection verification

Perform a staged test with the PLC program running locally and an external PC connection. Keep GPRS/TCP and GSM call tests separate so that a failed bearer is not mistaken for a VisiLogic setting problem.

1. Record which path is under test, the initiating endpoint, each endpoint's current IP or dialed number, and whether SMS is expected during the test.
2. For PLC-to-PC GPRS, set VisiLogic to TCP Listening, verify the PC firewall, forward TCP 20256 and 20257 to that PC, and confirm the router's public address is externally reachable.
3. Send the PC public IP to the V130 by SMS and trigger the configured CALL. Expected result: VisiLogic reports an established connection to the waiting PC, not merely an active GPRS modem session.
4. For PC-to-PLC GPRS, verify the Enfora's active IP and inbound reachability, set the V130 to Listening, then make a TCP Call from VisiLogic to the current modem IP. Expected result: the TCP connection reaches the PLC without an uncommunicated address change.
5. Send a test SMS while observing the TCP state. Expected result may include a GPRS/TCP interruption; verify the modem reconnects and establish a new VisiLogic session afterward.
6. For GSM data, place the call from the PC modem to the modem at the PLC, verify that the data session answers and record the negotiated rate. Expected result: a modem data connection rather than an Internet TCP session.
7. With remote monitoring connected, read a known PLC status value, then test the authorized alarm-clear action and confirm the PLC value or alarm state changes as expected. End the test by confirming the connection remains usable after any SMS-triggered reconnect.

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