Measurement Computing Extender 488HS Rev.2.0 - Manual (Page 23)

Section 3 IEEE 488 Primer
3.5
3.4.3 Remote Enable (REN)
When the System Controller sends the REN command, bus
devices will respond to remote operation. Generally, the REN command
should be issued before any bus programming is attempted. Only the
System Controller has control of the Remote Enable line.
3.4.4 End or Identify (EOI)
The EOI line is used to signal the last byte of a multibyte data
transfer. The device that is sending the data asserts EOI during the transfer
of the last data byte. The EOI signal is not always necessary as the end of
the data may be indicated by some special character such as carriage return.
The Active Controller also uses EOI to perform a Parallel Poll
by simultaneously asserting EOI and ATN.
3.4.5 Service Request (SRQ)
When a device desires the immediate attention of the Active
Controller it asserts SRQ. It is then the Controller's responsibility to
determine which device requested service. This is accomplished with a
Serial Poll or a Parallel Poll.
3.5 HANDSHAKE LINES
The GPIB uses three handshake lines in an "I'm ready - Here's the data - I've
got it" sequence. This handshake protocol assures reliable data transfer, at the rate
determined by the slowest Listener. One line is controlled by the Talker, while the
other two are shared by all Active Listeners. The handshake lines, like the other
IEEE 488 lines, are active low.
3.5.1 Data Valid (DAV)
The DAV line is controlled by the Talker. The Talker verifies that
NDAC is asserted (active low) which indicates that all Listeners have accepted
the previous data byte transferred. The Talker then outputs data on the bus and