Measurement Computing Parallel488 - Manual (Page 26)

Section 4 IEEE to Parallel Operation
4.2
type of control might be appropriate in a single user environment.
To illustrate how this would appear, let's assume the Parallel488 is
connected to a parallel printer which will accept data at 120 characters per
second. The IEEE bus controller is capable of sending data to the Parallel488 at a
rate of 5000 bytes per second. The data would be transferred on the bus at 5000
characters per second for slightly over five seconds, filling over 24,000 locations.
At that time, the IEEE input would hold off additional data transfers until
memory becomes available to buffer more data. The parallel devices 120 cps
would then become the average IEEE bus data acceptance rate.
If the controller is set to detect a data time-out error, then it will do so if
the Parallel488 holds off IEEE input data transfers for too long. The 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.
4.2.2 Controlled Bus Data Transfers
If the controller must avoid waiting for the parallel device, it can 'serial
poll' the Parallel488. 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 Parallel488 provides eight bits of status
information to the controller. One of the bits [DIO1] of the Parallel488's serial
poll byte is set to a logic
"1" when the IEEE input buffer is EMPTY. The term
EMPTY is used to signify that all of the previous data sent to the interface has
been transmitted to the parallel device. If it is NOT EMPTY, the controller may
avoid sending any more data to the Parallel488. If this bit is a logic "1", then the
parallel device has accepted all previous data and the IEEE controller may send
more.
Another bit [DIO2] of the Serial Poll byte is used to indicate additional
information concerning the IEEE input buffer. This bit is set to