TM 5-6675-320-14
b.
Internal interface bus PCA card A2.
Provides the communications interface
by directing data flow.
PCA A2 performs five major functions:
Address recognition .
Recognizes that the plotter is
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.
P o l l i n g .
Answers polls from the computer.
C l e a r a n d s e l f - t e s t .
( 1 ) A d d r e s s r e c o g n i t i o n.
Is accomplished through an address comparator
(U20).
The address comparator (U20) constantly compares bits 1-5 of the address
switches of PCA A2 and data bits 1-5 received at the interface bus transceivers (U7
and U33).
When two addresses are the same value, then the plotter is being addres-
sed and comparator U20 produces a my address signal. The my address signal is used
t o a c t i v a t e t h e t a l k a n d l i s t e n d e c o d e r c i r c u i t s.
( 2 ) D e c o d i n g.
PCA A2 receives instructions to prepare for receiving data
(PRD).
When 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
mode, which means data will be transmitted by the computer and “accepted” by the
p l o t t e r .
(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 4-4).
(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.
The request is made via a data word. After entering the mode, the
plotter may continue to send data until terminated (Table 4-5).
( 3 ) S e r v i c e r e q u e s t .
The 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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