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My goal is to use a PC side Enfore modem (GSM-KIT-41J UNITRONICS ENFORE GSM1208 QUAD BAND) to communicate with a PLC side Enfore modem (same exact type) so that I can go online and troubleshoot the program remotely.

I have successfully initialized the modem and I can send / receive SMS messages just fine. My problem is that I can not successfully dial the PC modem to make a remote call to the PLC side modem. After I click on the the dial button, I see the dial command go to the modem and the modem replies OK. Then two seconds later, I get a no carrier error and the communication fails.

I have read all the help documents I have made sure I am using a serial cable with all the pins wired straight through. I have tried both a desktop PC and laptop PC running windows XP. Both of these PCs have an actual serial port (I am not using a USB to serial converter).

The help file mentions an initialization code of S7=30 to give the modem more time, but it doesn't seem to have an effect or I am not entering this command in the inititaliztion string properly.

How do I fix my problem?

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There are few points to check there:

If the SIM is open for Data calls?

If thhe provider supports Data calls at all?

If the number for data calls is the same as the number for voice?

This SIM is on a prepaid gophone AT&T account. The account is active and has a credit balance.

This SIM is open for voice, text and data. There is no separate phone number for data.

Do I need to modify the initialization string?

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Yes both SIM cards are from AT&T prepaid wireless accounts. Both accounts have the data plan activated.

I did some more testing using hyperterminal. If I issue the command "ATD14695001234;" the modem will dial the other modem in voice mode. If I issue the command "ATD14695001234" without the semicolon then I get the "no carrier" response.

Has anyone had success using the AT&T SIM cards? Is there another US carrier that is known to work such as TMobile?

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I have never seen a "pure" data call work on US carriers as of recent. The best method would be to use GPRS data to transfer your information. Please note this will not be lightning fast, but does work. It also helps if you obtain a static IP address from AT&T. Otherwise you are assigned a different dynamic IP each time the SIM card registers on the GSM network, and often useful ports are blocked.

We do have a webinar detailing GPRS communications as well as a sample project if you are interested.

http://forum.unitronics.com/index.php?/blog/15/entry-61-webinar-modems/

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I have never seen a "pure" data call work on US carriers as of recent. The best method would be to use GPRS data to transfer your information. Please note this will not be lightning fast, but does work. It also helps if you obtain a static IP address from AT&T. Otherwise you are assigned a different dynamic IP each time the SIM card registers on the GSM network, and often useful ports are blocked.

We do have a webinar detailing GPRS communications as well as a sample project if you are interested.

http://forum.unitron...webinar-modems/

Thanks for your help. Can I use GPRS to go online with the PLC from a remote location? I need the ability to troubleshoot remote PLCs.

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I did some research and found out why I'm having problems. I am writing this for everyone else to benefit and not make the same mistake I made. AT&T prepaid currently uses GPRS for data they do not support CSD (although special business accounts can be setup to use CSD for the right price). CSD is required to dial another GSM modem for data access. Instead you must use GPRS. See below for an explaination of the different data types available.

GSM Data Overview

GSM divides each radio channel into 8 time slots. Data speeds are determined by the speed of an individual time slot, and the number of time slots used:CSD (Circuit Switched Data)9.6 Kbps over a single time slot. Most GSM phones support CSD. (2G) AT&T prepaid does not support this.HSCSD (High Speed Circuit Switched Data)Each time slot can be 9.6 or 14.4 Kbps, and multiple time slots can be combined for higher speeds, up to 38.4 and 57.6 Kbps respectively. Much less efficient than GPRS from a network standpoint, a significant drawback. (2.5G)GPRS (General Packet Radio Service)Each time slot can be 9.6 to 21.4 Kbps, and multiple time slots can be combined for higher data speeds, up to 171.2 Kbps (all 8 time slots), although current phones are limited to lower data speeds. In practice, GPRS data connections are usually limited to a maximum of 4 downstream time slots with 1 upstream time slot. Packet technology greatly enhances network efficiency. (2.5G)EDGE (Enhanced Data GSM Environment)Effectively an extension of GPRS that increases maximum time slot speed to 48 Kbps, pushing maximum combined data speed up to 384 Kbps (all 8 time slots). In trials, but not yet widely deployed. (3G)In each case, actual availability and speed is determined by the lowest common denominator of:

  1. what the carrier offers;
  2. how the subscriber is configured by the carrier;
  3. configuration of the actual access point in use; and
  4. the capabilities of the phone in use.

It's a common misconception that GSM phones can make analog (dial-up) modem calls directly, but they can't. GSM is digital, and analog modems will only work over analog (AMPS) cellular. The "modem" in a GSM phone actually makes a digital connection to an IWU (Inter Working Unit) located at a carrier's Mobile Switching Center (MSC). The IWU has an analog modem bank that makes the actual dial-up modem calls. If a carrier doesn't provide an IWU, then analog modem calls are not possible.

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