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Theory of Operation
ACCEPTOR HANDSHAKE.
The GPIB protocol provides a handshake for all data transfers.
When a GPIB command is sent or the 890lA is in Listen
mode, the 890lA must indicate that it is ready to receive a
transfer by asserting the RFD line. The GPIB controller must
then assert DAV when valid data is on the bus. The 8901A
can then read in the data. It then asserts the DAC line to say
the transfer is complete.
LISTEN MODE
Bits 6 and 7 of the first byte the 8901A receives from the GPIB
Talker after entering Listen mode is used to determine whether
the byte is a command for the 8901A itself, a "global" CAMAC
command or a CAMAC command for a specific module. Bits
1-5 are then latched in the appropriate registers. If it's a
CAMAC F command for a particular instrument, a sequencer is
used to latch additional bytes in the following order: A, N,
Wl-8, W9-16, WlO-24.
GLOBAL CAMAC
COMMANDS
There are three global CAMAC commands: C, Z, and 1. For
each of these commands there is a corresponding line on the
CAMAC Data way which is driven by the 8901A whenever a
CAMAC cycle is executed. The command is asserted if its
latched value is a 1. At the end of the CAMAC cycle the C
and Z latch is reset to zero.
F, A, N CAMAC
COMMANDS
As described above, the values for F, A, N, and the Write data
are latched when the information is sent from the GPIB control
ler. When a CAMAC cycle is executed, all of these lines are
driven for the entire length of the cycle (while Busy is asserted).
The latched values will not change until they are overwritten by
a new command from the GPIB controller.
TRANSFER MODE ^
The commands for the 8901A determine how many bytes are to
be read back, whether a new CAMAC cycle should be executed
when data is read out (block mode), and what conditions (if
any) should cause SRQ to be asserted by the 8901A. The
latches for this information are cleared if an IFC is sent. The
block transfer enable flip flop also is cleared when a Q = 0 is
read in.
SERIAL POLL MODE
Serial poll is used by the GPIB controller to determine which
device asserted SRQ. Upon decoding the serial poll enable
command, the SPAS flip flop is set true, putting the 8901A in
serial poll mode. When the controller then addresses the
890lA to Talk, the serial poll status bytes are sent instead of
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