TM 5-6675-323-14
Provides the communications interface
I n t e r n a l interface bus PCA card A2.
b.
by directing data flow.
P C A A2 performs five major functions:
Recognizes that the plotter is
Address recognition.
being addressed.
Decoding.
Decodes input instructions to mean (a) pre-
pare to receive data (PRD) or (b) send data.
Service request.
Requests service from the computer.
A n s w e r s polls f r o m the computer.
Polling.
Clear and self-test.
A d d r e s s r e c o g n i t i o n . I s a c c o m p l i s h e d through an address comparator
(1)
( U 2 0 ) . T h e a d d r e s s comparator (U20) c o n s t a n t l y compares bits 1-5 of the address
s w i t c h e s o f P C A A2 and data bits 1-5 r e c e i v e d a t t h e i n t e r f a c e b u s t r a n s c e i v e r s ( U 7
and U33).
W h e n two addresses are the same value, then the plotter is beinq addres-
T h e my address signal is used
s e d and-comparator U20 produces a my address signal.
to activate the talk and listen decoder circuits.
PCA A2 receives instructions to prepare for receiving data
(2) Decoding.
(PRD) . W h e n PCA A3 is available to accept data, it generates a ready signal which
is the cue for the computer to send the first data word.
(a) The first data word received by the plotter is decoded by the PCA
A2 circuits to determine if the plotter is the device actually being addressed.
This first data word also tells the PCA A2 circuits to enter the acceptor handshake
m o d e , which means data will be transmitted by the computer and "accepted" by the
plotter.
(b) After the plotter has entered the acceptor handshake mode, the
computer may send data directly to the processor via the HP-IB Interface until the
mode has been terminated (Table 3-4).
(c) The plotter can send data via the talk logic circuitry on PCA A2.
Talk logic circuitry is only enabled when the plotter is addressed by the computer.
When the computer requests the plotter to send data, the plotter enters the talk
handshake mode. T h e request is made via a data word. After entering the mode, the
plotter may continue to send data until terminated (Table 3-5).
( 3 ) S e r v i c e r e q u e s t . T h e service request circuits in PCA A2 allow the
plotter to request service from the computer. A request is initiated by a request
for service (RSV) signal from PCA A3. Computer responds by initiating a serial poll
on the data bus to determine which device is requesting service.
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