How To Switch From SMS, To A Communications Protocol, When Using A GSM Modem For Both SMS And Remote Connection


When using a GSM modem with an Omron PLC, the serial port can be configured to to allow AT commands to be sent to the GSM modem (‘No Protocol’ mode) to facilitate Incoming and outgoing SMS messaging. A GSM modem is also capable of providing a data connection to the PLC for remote monitoring or programming. However, the Omron PLC does not have the ability to monitor the all important CD (‘Carrier Detect’) and RI (‘Ring Indicator’) signals originating from the modem, within the PLC application code. This is important, in order to know when to change to a communications protocol like Hostlink or Toolbus, as and when a remote host attempts to connect to the PLC.

Using a special cable (like the IAG-RI-CD-Int-001 developed by IAG Technology together with Omron UK), this is now possible. The cable is a standard “modem to PLC” cable, along with optically isolated CD and RI signals. These are wired into standard 24 VDC PLC inputs, and can then be used to change protocols dynamically (in conjunction with the ladder STUP instruction). Indeed, with a PSTN modem (Land Line), the cable also provides indication of the presence of an incoming call and confirmation that a remote connection has been established.


A simple application for the CJ PLC is included with this article to demonstrate the possibility of switching from SMS (no protocol) to Hostlink, along with a datasheet for the cable itself.


How to Order the Cable

The example in this article uses a cable designed & manufactured by IAG Technology Ltd, who have worked with Omron's Engineers & End-Users for many years, they have vast experience in custom electronic design. For more information on ordering the cable please contact IAG Technology:

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