Service Request. One of the management lines for GPIB.
The SRQ signal is used by each controlled device to notify the
controller when a process ends or an error occurs.
It indicates whether there is service request from slave
device.
When a device requests a service, it pulls the SRQ line low.
When the controller becomes
aware that the SRQ line is low, it conducts a polling
to investigate which devices made the request and what kind of services
they requested.