Chapter 3 Theory of Operation
© National Instruments Corporation 3-3 GPIB-120A User Manual
If a GPIB device on Bus A asserts DAV, the Bus A Source Handshake state becomes true and
the Bus B Source Handshake state becomes false.
If a GPIB device on Bus B asserts DAV, the Bus B Source Handshake state becomes true and
the Bus A Source Handshake state becomes false.
Bus A and Bus B Source Handshake states also become false when a parallel poll begins or
when the ATN signal changes states.
Data Direction Control
Bus B sends the data lines to Bus A if the Bus B Source Handshake state is true or if a Controller
on Bus A is conducting a parallel poll.
Bus A sends the data lines to Bus B if the Bus A Source Handshake state is true or if a Controller
on Bus B is conducting a parallel poll.
Parallel Poll Detection
Controllers can conduct parallel polls on Bus A or Bus B with no modification of user software.
Devices on both Bus A and Bus B can respond to parallel polls.
If a Controller on Bus A conducts a parallel poll, the parallel poll detection circuitry on side B
conducts a parallel poll on Bus B. The result of the parallel poll is driven on the data lines of
Bus A.
If a Controller on Bus B conducts a parallel poll, the parallel poll detection circuitry on side A
conducts a parallel poll on Bus A. The result of the parallel poll is driven on the data lines of
Bus B.