TM 5-6675-323-14
m. The microprocessor sends a new byte available (nba) which is gated through
U 2 2 0 B and U220C and with data enable (den) and ready for data (RFD) sets the data
available (DAV) latch U205 output true. The output of the serial poll flip-flop
U 2 1 3 B and (den) set the nand gate U204C to output a false serial poll active state
( s p a s ) s i g n a l . T h i s signal enables the data selectors U218 and U224 to select and
c o u p l e the output of the transmit latches U217 and U223 through the transmitters
U 2 2 8 and U232 to the HP-IB lines.
I f the LISTEN ONLY switch is set for listen only,
a low will be input to the clear and preset of the talk and serial poll flip-flops
d i s a b l i n g t h e m . T h i s prevents the (den) signal from being generated, and the bus
transmitters will not be enabled.
n.
S e r v i c e r e q u e s t . T h e service request (SRQ) allows the plotter to asynchron-
ously request service from its controller. A service request is initiated when the
plotter microprocessor generates a request for service (rsv) which causes the HP-IB
service request logic circuit to set the SRQ line true. This true SRQ state indi-
cates over the interface that the plotter is requesting service. The controller,
upon detecting a service request, conducts a serial poll of all devices on the bus
that may have requested service. T o i n i t i a t e t h e s e r i a l p o l l , t h e c o n t r o l l e r t r a n -
s m i t s the universal command SPE (serial poll enable), then sequentially commands
each device on the HP-IB to talk.
o. The serial poll enable (SPE) command sets the serial poll mode flip-flop when
the controller sets the plotter as a talker. The plotter indicates it has requested
service by sending the status byte. The controller has the option of determining
when and if the request will be granted.
p. Parallel poll mode.
Parallel polling permits the controller to simultaneous-
l y check the status of up to eight devices on the HP-IB. The operator assigns each
device a data line which the device will pull low during the parallel poll routine
if it is requesting service. The parallel poll function requires that the control-
ler periodically poll the instruments connected to the bus. The controller interro-
gates the instruments by sending EOI and ATN true. The plotter HP-IB circuitry
decodes these messages with the parallel poll enable (ppe) signal from the micropro-
cessor circuitry and activates the parallel poll driver circuit U236. This pulls
the selected DIO line low indicating that the plotter is requesting service. The
parallel poll address for the plotter is selected by the ADDRESS switch on the
i n t e r c o n n e c t PCA A1.
q. Device clear (DCL) and selected device clear (SDC) function. The controller
can set all devices on the HP-IB system to a predefined or initialized state by
sending the universal command device clear (DCL). The controller can also set
selected devices to this state by sending selected device clear (SDC). Upon re-
ceiving either command, the plotter decodes the data lines and sets the respective
gates.
T h e output of the enabled gate sets a DCL latch which requests that the
m i c r o p r o c e s s o r g o t o a n i n i t i a l i z e d s t a t e . The microprocessor receives the request
during its read interface (RDIB) cycle and determines when the request will be
granted.
T h e microprocessor next causes the HP-IB control gate to output a clear
knowledge (cla) pulse to reset the data clear latch when the next write interface
b u s (WRIB) cycle occurs.
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