TM 5-6675-323-14
NOTE
The computer, upon detecting a service request, conducts a poll of all
t h e devices on the bus that may have requested service. The following
i s the sequence of events performed by the PCA A2 circuitry for service
request:
The microprocessor generates an RSV signal and a clock pulse on the write
interface bus.
U9 transmits an RSV signal to service request latches U28 and U29.
Z U9 generates an RSV true signal that is sent to the computer via IBT U8.
(4) Polling.
Is directed by the computer as a response to a service
There are two polling modes:
serial poll mode and parallel poll mode. In
request.
the serial poll mode, the computer initiates a serial poll enable (SPE) signal and
then sequentially commands each device on the interface bus to talk. The plotter
responds by sending a BIT 7 true signal indicating that it is the device that has
requested service.
NOTE
The followings the sequence of events for responding to a serial poll:
The computer detects a service request and initiates a universal serial
p o l l enable (SPE) by placing a data byte 1 on the data bus.
PCA A3 initiates a parallel poll enable (PPE) true signal.
U32 receives EOI, attention (ATN) and PPE signals via U9 and addresses
D and E via U5.
G e n e r a t e s PPE signal to U21.
U21 is enabled by PPE. T h i s s e t s a d a t a l i n e e q u i v a l e n t t o a d d r e s s
A B C onto data bus to the computer.
(5) Clear and self-test.
Clear and self-test circuitry of PCA A2 performs
three
operations:
D e v i c e Clear
Interface Clear
Self-Test
Sets the plotter to a preset condition. The compu-
(a) Device clear.
ter initiates the device clear during a receive handshake mode by making the reset
signal true. This causes the processor to position the plotter in a predefine
s t a t e indicated by the data byte read into PCA A3 during the receive handshake mode
operation.
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