Measurement Computing Serial488A - Manual (Page 52)

4.3
error can be used to alert the operator to the problem, such as a printer out
of paper, so that it can be corrected. If the controller then restarts
transmission exactly where it left off, no data will be lost.
If data is requested by the controller and no serial input data is
available in the Serial488A, the bus will hang until serial data is received.
If no serial data is received it will hang forever or until the controller times
out.
4.3.2 Controlled Bus Data Transfers
If the controller must avoid waiting for the serial device, it can 'serial
poll' the Serial488A. Serial poll is a method by which the controller can
inquire the internal status of the interface without disturbing any data being
transferred, slowing data transfers or locking up the bus. You should refer
to the programming manual of your controller to determine the method of
performing serial polls.
When serial polled, the Serial488A provides eight bits of status
information to the controller. The most significant bit [DIO8] of the
Serial488A's serial poll byte is set to a logic "1" when the IEEE input
buffer is NOT EMPTY. The term NOT EMPTY is used to signify that not
all of the previous data sent to the interface has been transmitted to the
serial device. If it is NOT EMPTY, the controller may avoid sending any
more data to the Serial488A. If this bit is a logic "0", then the serial device
has accepted all previous data and the IEEE controller may send more.
Another bit [DIO4] of the Serial Poll byte is used to indicate
additional information concerning the IEEE input buffer. This bit is set to
a logic "1" when there is 1280 or less locations in the buffer for data. It is
cleared, set to a logic "0", when there is greater than 1280 locations
available. This bit is referred to as the IEEE input buffer FULL bit.