GSM modems enhance PLC applications

Edited by the Engineeringtalk editorial team Mar 30, 2006

There are many advantages to connecting a low-cost GSM modem to a programmable controller application, explains Mike Brierley.

The advantages of connecting a low-cost GSM modem to a programmable controller application are many: alarm messages can be sent to a nominated mobile or fax number; diagnostic information can be retrieved on ringing the GSM modem number; or for extra security a coded text message can be sent, which the PLC will check before releasing information.

Whatever the requirements the operator knows he or she can be contacted by the PLC when situations arise, which is ideal for critical unmanned locations.

Communications between the modem and the PLC are by serial RS232 interface and the protocol used is by AT commands - the standard way of communicating with a modem.

The PLC must be capable of transmitting ASCII codes in order to achieve this.

Two minutes after power up (to allow time for the GSM to register with its network) the PLC will send a ATZ reset to the modem.

This will be resent every 2 minutes until an OK prompt has been received from the modem.

The PLC then sets up the modem for text mode and for incoming SMS reports via the AT+CMGF and AT+CNMI commands.

An alarm situation will be arranged to trigger a transmit message to the modem which is sent in two parts.

The first part is an AT+CMGS command which includes the telephone number of the receiving mobile or, if a prefix code is used a nominated fax number.

The second part is triggered when a prompt is received from the modem signifying it is ready.

This second message contains the actual SMS text which could also contain live PLC register data.

These could be data signifying for example piece part counts or analogue temperature/pressure values etc.

If the modem's own telephone number is rung from an outside source a ring prompt is sent to the PLC.

If the PLC recognises this it will respond with an AT+CMGS command as above and send its data.

If security of this information is of concern then an SMS text message can be sent to the modem containing a code or password known only to the user.

These text messages are stored sequentially in the modem in a number of memory locations.

On receiving a text message a +CMTI prompt from the modem identifies which memory location the new message is stored in.

The PLC will then respond with a CMGR command which reads and stores the whole message in its data registers.

The PLC checks these data using its standard compare instructions to see if a valid password has been sent and if so, release its information using an AT+CMGS command as described above.

The last thing the PLC is required to do is to clear out the text message stored in the last memory location by send an AT+CMGD command to the modem with its memory location number.

By putting the required programme in the PLC, any action can be triggered by a coded incoming text message.

The PLC and therefore the machine it is controlling could even be turned off if the user has not paid his bill.