1.1.2 Procedures

Driver procedures actually perform the communication with the device to control. They operate the device as commanded and poll the equipment settings and state.

There are two types of procedures a driver may define.

• A PUT procedure sends one or more parameters to the physical device after coding them into the format the physical device expects.
• A GET procedure sends a request to the device, reads the reply and decodes one or more variable values from the data received.

A procedure never can be both, PUT and GET at the same time.

The device driver executes the defined procedures in an endless loop, called the polling cycle. But a procedure is not necessarily called in every cycle. If there is nothing to do for a particular procedure, it is skipped. To determine which procedure must be executed in a cycle, procedures are bound to variables. A procedure may be bound to one or more variables, however, there may me at most one PUT and one GET procedure referencing a variable.

A PUT procedure is executed, if at least one of the variables it is bound to has received a new setting, e.g. from the user interface. For a GET procedure the following conditions cause the procedure to be executed:

1. The device driver has established communication to the device after power-on or after a communication interruption.
2. The (individually defined) polling interval for at least one of the variables bound to this procedure has elapsed.
3. At least one of the variables bound to the procedure has been commanded to the device. The parameter must be read back for verification.