TM 5-6675-320-14
NOTE
The computer, upon detecting a service request, conducts a poll of all
the devices on the bus that may have requested service. The following
is the sequence of events performed by the PCA A2 circuitry for service
r e q u e s t:
• The microprocessor generates an RSV signal and a clock pulse on the write
i n t e r f a c e b u s .
• U9 transmits an RSV signal to service request latches U28 and U29.
• U29 generates an RSV true signal that is sent to the computer via IBT U8.
( 4 ) P o l l i n g .
Is directed by the computer as a response to a service
request.
There are two polling modes:
serial poll mode and parallel poll mode. In
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
r e q u e s t e d s e r v i c e.
NOTE
The following is the sequence of events for responding to a serial poll:
• The computer detects a service request and initiates a universal serial
poll enable (SPE) by placing a data byte ØØ1ØØØ on the data bus.
• P C A A 3 i n i t i a t e s a p a r a l l e l p o l l e n a b l e ( P P E ) t r u e s i g n a l.
• U32 receives EOI, attention (ATN) and PPE signals via U9 and addresses
D and E via U5.
Generates PPE signal to U21.
• U21 is enabled by PPE.
This sets a data line equivalent to address
ABC onto data bus to the computer.
( 5 ) C l e a r a n d s e l f - t e s t.
Clear and self-test circuitry of PCA A2 performs
three operations:
Device Clear
I n t e r f a c e C l e ar
S e l f - T e s t
4-16
( a ) D e v i c e c l e a r .
Sets the plotter to a preset condition. The compu-
ter tiates 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
state indicated by the data byte read into PCA A3 during the receive handshake mode
operation.
