When a request becomes active, it is passed to the scanner of a channel to perform the request. The scanner also generates requests internally, for example to poll an outstation. Each request consists of a sequence of commands that are sent by the scanner to an outstation. The scanner periodically checks that at least one command has been sent (since the previous check). The frequency of this check is defined by the Scanner Command Timeout setting. If a command has not been sent since the previous check, the driver detects the request has failed and then aborts the request.

A new request is allowed double the Scanner Command Timeout for the first command to be sent. For example, if the Scanner Command Timeout is 30s, the first command for a new request has 60s to be sent.

An active request that is passed to a scanner may be suspended within the scanner. Typically, this occurs when an outstation indicates that it will take some time to process a particular command. Suspending the active request allows the scanner to continue sending commands to other outstations, while waiting for a response.

If there is no response within a specified amount of time, the driver detects that the request has failed and then aborts the request. The amount of time permitted for a response is defined by the Scanner Request Timeout setting.

Typically, the default settings of 30 seconds and 3 minutes are adequate. However, the timeouts may need to be increased if there are slow devices or communication links. If a request is aborted because of a timeout, ClearSCADA logs an event in the event journal.