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The Tixi Modem is mostly configured as the master for communication to the PLC. It scans the PLC variables cyclically (e.g.: every second, every minute, every day) and stores a copy of the actual PLC values required.
The Tixi Modem only requires an alarm flag in the PLC to be set from 0 to 1 in order for the sending a fault alarm from the Tixi Modem to be triggered. During the next poll cycle Tixi will detect the change of the monitored flag and will store this in an internal process variable with which other calculations (such as comparisons with other ports or a schedule) can produce an event state.
The event handler then starts the job generator that then generates the message with the current variable values using a text processor. The job system then handles the sending of the message(s) to the recipient via the communication channel used (SMS, fax, e-mail) in use.
 See also signal processing diagram
PLC - Tixi Alarm Modem Comunication
A PLC can communicate with a Tixi Modem in three different ways:
- PLC protocol is integrated in the Tixi Modem (see PLC protocols)
- Tixi driver loaded in the PLC
- Tixi and the PLC use a standard such as MODBUS, M-Bus or similar.
Tixi Alarm Modems can communicate with the PLCs using their internal protocols without having to load a program, driver or function block into the PLC concerned. Not one bit has to be changed in the PLC.
The Tixi Alarm Modems then have access to all variables, markers and I/O on the PLC. PLCs in the network are usually also supported.
You can write a Tixi driver for the PLC your PLC protocol is not yet available for Tixi (and the PLC does not support the MODBUS standard).
There are two options available for communication with a TAM:
- TiXML commands for powerful PLCs
- Tixi bus protocol for small PLCs that have little memory
TiXML is the XML text protocol with which all the functions of the Tixi Alarm Modem can be configured.
In order to trigger a cascade of operations (e.g. send 100 SMS messages, e-mails or faxes), the PLC just has to send a simple TiXML command with the name of the event concerned, such as
[<DoOn _=“TemperatureAlarm“>
<House _=“12“/> (User-Variable: Hausnummer)
<Room _=“153“/>
<Temperature _=“35“/>
</DoOn>]
The three user variables are transferred to the TemperatureAlarm event and the defined operations are triggered. The variable values can be transferred to the message texts.
Tixi bus is a simple, character-based ASCII protocol for communication between several devices that is particularly suitable for CPUs with minimum resources (e.g. 8 bit). Tixi bus allows read and write operations for variables and I/Os. The scan command “Has anything changed?” is of special importance.
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