Control Functions
PSI BOX PLUS+ Reference
PPOLL.
Perform A Parallel Poll.
________
_____________________
Purpose.
To perform the Parallel Poll function, this allows the
__________
controller to simultaneously determine the status of several
devices.
Unlike the Serial Poll which is conducted after a DEVICE
Requests Service, the Parallel Poll routine is initiated by the
controller. It allows a whole group of devices to simultaneously
return a status bit on the data lines. The data line, and the sense
of each devices response is determined by the controller when it
send CONFIGURE or PPC, parallel poll configure. Devices
power up in the PPU state and cannot respond to the Parallel
Poll until after they have been configured.
The Parallel Poll makes good use of the controller’s time
since it can quickly check the state of 8 or more devices with
one command.
The PSI BOX knows which device is responsible for
which data line because it has previously assigned those lines to
each device using the PPC with PPE commands. Depending on
the Parallel Poll Response the controller may take a variety of
actions, chosen by the user’s program.
The PSI BOX asserts EOI and ATN, so becoming the
active controller, the PSI BOX then waits minimum of 25 micro
seconds for the configured IEEE devices to put their parallel poll
status bit on the IEEE data lines. The PSI BOX reads the data
line and store the response in bytes 36-38 of the IEEE control
string. The PSI BOX then releases the EOI line so ending the
parallel poll. The ATN remains true, PSI BOX is the active
controller.
The 3 bytes 36-38 in the IEEE control status string contains the
parallel poll response read during the last PPOLL . These bytes
hold a value between 0 and 255. Knowing what parallel poll
configure information he has sent the user can interpret the data
accordingly.
Format.
PPOLL
_________
Params.
None.
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Chapter 8
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