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THEORY OF OPERATION
GENERAL
The Model 8901A is a GPIB to CAMAC interface which is
composed of two circuit boards. The first board plugs into the
control (rightmost) slot of the CAMAC mainframe and inter
faces to the CAMAC station number (N) lines and LAM lines.
This board also contains the GPIB interface circuitry. The sec
ond board provides the interface to the CAMAC Dataway. It
has registers to store the CAMAC commands sent by the GPIB
controller and drive the function (F), address (A), and write
data (W) lines of the Data way during a CAMAC cycle. It also
has the circuit to generate the CAMAC cycle timing and latch
the read data (R) lines.
The 8901A circuitry can be broken down into logical subsec
tions as shown in the block diagram. The operation of each of
these subsections is discussed below.
GPIB COMMAND
DECODER
At the heart of the 8901A is the GPIB command decoder.
When a command is put on the GPIB bus it must be interpreted
by the 8901A and a decision must be made as to how (if at all)
to respond to it. The 890lA will respond to any of the follow
ing GPIB commands: interface clear (IFC), serial poll enable
(SPE), serial poll disable (SPD), my Talk address (MTA), my
Listen address (MLA), Unlisten (UNL), and Untalk (UNT).
The 8901A has four basic states: Idle, Talk mode, Listen mode.
Serial poll mode. These states are entered and exited as a re
sult of one of the above GPIB commands. The following table
lists each of the commands and describes its effect on the
8901A's state.
IFC: Resets all registers in the 8901A and places it in the Idle
state regardless of its current state.
SPE: Causes the 8901A to enter Serial Poll mode.
SPD: If the 8901A is in Serial Poll mode, this command returns
it to the Idle state. It also clears a pending service request
(SRQ).
MTA: If in Serial Poll mode, the 8901A will output its SRQ
status upon receiving its Talk address. If not in Serial Poll
mode, receipt of MTA will cause a CAMAC cycle to be exe
cuted and Talk mode to be entered.
UNT: Takes the 8901A out of Talk mode if it is active MLA -
Causes the 8901A to enter Listen mode so that it may accept
CAMAC or setup commands.
UNL: Places the 8901A into the Idle state if currently in Listen
mode.
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