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 poll to investigate which devices made the request and what
kind of services they requested.