Theory
of Operation
POWER
SUPPLY
Secondary T601-14,15,16 supplies the vacuum fluorescent
RAM.
For hmher description of the
28
PC, refer tb the
display filament with 4.5V ac. The center tap is connected
heading "InGuard Microcomputer" above.
,
I
to the in-guard +5V supply in order to correctly bias the
display.
An
isolated secondary supplies 16V
ac
to the
5-39.
GuaH
Crossing
,
I
power
supply
on the IEEE-488 Interface.
The guard crossing circuit permits
communication between U901 and U202
Zener diode CR615
and
SCR
Q601
comprise
a
protective
the two electrically. One-half of the guard
crowbar
circuit. if the line voltage exceeds the nominal
is contained on the Main PCA; the other
ducts, turning on Q601, shorting out the power transformer
circuit is dwbed in
an
earlier heading.
secondary and blowing the line fuse. In normal operation,
value by approximately 30 percent or more, CR615 con-
IEEE-488 I~lterface
PCA.
Operation of the
these components have no effect.
5-40.
Bus
Interface
Circuitqt
,
The IEEE-488
bus
protocol is handled by the
p~r)h210
5-37.
IEEE-488 INTERFACE (OPTION
-05)
IEEE-488 talkerbistener IC (U911). It is control1
U901
as
a
inemory mapped peripheral through
The IEEE-488 Interface
has
five major parts,
as
shown in
d.ta
bus.
5-38.
Out-Guard
Microcomputer
output drive capability
and
receiver impedance.
I
The Out-Guard Microcomputer
(pC)
(U901) communi-
5-41.
Sigrlsll
Conditioning
cares with the IEEE-488 talkernisterner IC (U911) and the
The
SAMPE
COMPLm and
EXI.
TRIG signals
In-Guard PC (U202).
and 3904) are conditioned by
U909.
Diodes
CR904, CR905, and CR906 and resistors R917
The OutGuard PC is similar to the 1nGuard
28
PC
provide protection from excessive voltages.
except that it contains
8K
bytes of ROM and 236 butes of
and E903 allow selection of the polarity of the
EXT
TRIG
I
I
IN-GUARD
.
I
PC
I
GUARD
+
U202
1
CROSSING
4
I
I
TRANSFORM? T601
14-q
POWER
PC
SUPPLY U901
SAMPLE
SIGNAL
EXTERNAL CONDITIONING
TRIGGER
IEEE-488
TALKEW
LISTENER
IC U911
AND
BUS
TRANS-
CEIVERS
U912
AND
u913
Fgure
5-16.
IEEE-488
Interface Block
Diagram
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