Last Call, CLIP: Identifying Callers

The Last Call Received function is located on the COM> COM Port> menu. This function enables a PLC to identify the caller number of an incoming call. Last Call uses the CLIP (Calling Line Identification and Presentation) feature offered by GSM modems.

CLIP is supported by:

Enhanced Vision: from O/S 42
Standard Vision: from O/S 500-19
VisiLogic versions 5.15 and later, which contain the CLIP command: AT + CLIP=1.

You can use CLIP to identify callers to a PLC modem by:

Using the Last Call function to write the number of an incoming call to a register
Creating a list of authorized numbers in a Data Table
Using the Find Row function to check if that number exists in the Data Table

To see a working example, check the sample application GSM - Prevent Illegal Calls (CLIP).vlp.

The process

The PLC must be connected to a GSM modem.

Device modem calls a PLC modem.
PLC Modem sends the CLIP command to the PLC, containing the Caller Number and Call Type (local or international) to the PLC.
PLC retains the data, and turns ON SB89 CLIP - Caller Number is here.
Note that SB89 must be reset by the user.
The Last Call function stores the caller number into a vector. In the Ladder application, a Data Tables Find Row function searches for a matching number in the Data Table. If no such number exists, the application can break the connection by activating COM Init.

The application

Notes ♦

Last Call can store the number when:

Enhanced Vision: the modem begins to ring
Standard Vision: the call is actually connected.

♦

If the call number is unidentifiable (meaning set by the caller as private or anonymous), no number will be stored in the Last Call vector. In most cases, Last Call will identify such a call as Type =128.

The ladder net that is shown below causes the following events when the PLC's modem receives a call from a remote modem:

SB89 turns ON when caller number is present
The Last Call Received element copies the caller number into a vector of MIs starting at MI10
Find Data Table Row looks up that number in the Data Table.

If no such number exists, Find Row returns a value of -1.

Reset SB 89 in the application. If SB 89 is not reset, you will not be able to write new CLIP data

Preventing unauthorized calls

The nets shown below guard against unauthorized callers:

If the caller number is not in the list, the Find Row output, MI21, will contain -1,

If the call number is unidentifiable (meaning set by the caller as private or anonymous), 0 will be stored in the Last Call vector starting at MI 10

In either case, MB1 will turn OFF, breaking the connection by activating COM Init.