TM 5-6675-318-14
NOTE
T h e computer, u p o n d e t e c t i n g a s e r v i c e r e q u e s t , c o n d u c t s a p o l l o f a l l
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.
U29 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
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
requested service.
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
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.
U 3 2 receives E0I, 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.
This sets a data line equivalent to address
U21 is enabled by PPE.
A B C onto data bus to the computer.
Clear and self-test circuitry of PCA A2 performs
(5) Clear and self-test.
three
operations:
D e v i c e Clear
Interface Clear
Self-Test
(a) Device clear. Sets the plotter to a preset condition. The compu-
ter
i n i t i a t e s the device clear during a receive handshake mode by making the reset
signal t r u e . This causes the processor to position the plotter in a predefine
s t a t e i n d i c a t e d by the data byte read into PCA A3 during the receive handshake mode
operation.
3-16
