© National Instruments Corp.
3-1
GPIB-PRL User Manual
Chapter 3
Technical Information
The GPIB-PRL is actually a powerful 8-bit microcomputer tailored for use
as an IEEE-488-to-parallel protocol converter. The unit's operating system
is contained in Read Only Memory (ROM) and can address up to 256 KB
of dynamic RAM. The GPIB-PRL microprocessor contains an integrated
Direct Memory Access (DMA) controller for high-speed data transfers from
the GPIB circuitry. The processor also contains an integrated memory
management unit, a dynamic RAM controller, a clock generator, a
programmable timer unit, and an interrupt controller.
The parallel port on the GPIB-PRL has been designed to source data onto
the parallel bus or to receive data from the parallel port (switch selectable).
All GPIB functionality is provided by an LSI GPIB controller chip which is
programmed and maintained by the operating system. Both the parallel port
and GPIB port are interrupt driven and feature extremely fast response
times to external data transfers. For more information on how the
individual modes of operation work, refer to the following sections.
G Mode Operation
When operating in G mode, the GPIB-PRL accepts data from the GPIB port
using the onboard DMA controller. The DMA controller transfers the data
directly to the memory buffer without processor intervention. This allows
for very high-speed GPIB transfer rates – as high as 900 kbytes/s. This
means that the GPIB system performance will not be degraded by
interfacing a "slow" printer or plotter to the interface.
The buffered data is then dumped to the device on the parallel interface at
its own rate. These two asynchronous events take place concurrently and
without user interaction. All operations are completely transparent to the
user.
If very large amounts of data are sent to the GPIB-PRL (greater than the
amount of RAM in the system) and the parallel device is a very slow data
acceptor, the GPIB-PRL internal data buffer might become full. In the
unlikely event that this does occur, the FULL indicator on the front panel
will illuminate and the GPIB transfer rate will slow to approximately the
same rate as the parallel device accepting the data. Since the maximum
data transfer rate on the GPIB is only as fast as the slowest listener, the
