Service
each having its own numbering sequence. Table 8-1
summarizes this numbering s!ructure. Note that the 600
series numbers are used for calibration (Cal) com
ponents and that the numbering scheme deviates in this
instance due to the different Cat components being used
in four of the A20 circuits. Locating these components
together, behind a removabte front panel section per
mits a complete instrument calibration white the instru
ment is rack mounted.
Table 8·1. A20 Board Component Numbering Structura.
Componenl lII u mben
Series
Input Switching
1 0 0
Series
Ohm's Current Source
200
Series
Input Amplifier
300
Series
AID Converter
400
Series
Reference
500
Calibration
Series
600
Series
Range and Funclion Control Logic
700
8-30.
Input Switching and "Range and Function Con
trol Logic".
All of the reed relay and Fet switching con
trol signals for the Input Switching circuits, except for
HAF, LMES, and LGND, come from the Range and
Function Control Logic (see Figure 8-4 Range and
Function Control Switching). Logic Signals HAF,
LMES, and LGND come directly from the Inguard
microprocessor A30-U J3. A30-U J 3 also provides the
logic input for the Range and Function Control Logic.
8-31. 100
1000
V and
V Ranges.
WI
1000
switch QI09
'11 b
e ena
bl d b th
e
100 V
and
V ranges (see Figure 8-8 Simplified
ror
Switching).
Schema
tic
Inpul
(9.9 M and 99.975 k fine-line resistors) in this
divider
path at
tenuates the input
--
otection for these ranges is provided by the 2.55
limit pr
k ohm
resistor (RIOt) in series with the
(see Schematic 1 , Inpul Switching).
divider
I
U I '
8,4
e
l .a
ir
i
C
u
me
Relay KlO2 and Fet
TO
y
e con
t
I 1
ogle
f
or
The
100:1
voltage
10
the desired levels. Current-
-
10 M ohm
1
,
:
RANGE
AND
F U N C T I O N
:
CONTROL
:
L O G I C
,
,
:
L I N E S
Figure 8-4. Ranltl and Function Control SWltchmg.
8-32. 100
t
mY,
low thermal relay) and Fet switch Q l 16 are enabled for
the 100 mV to l O V ranges. Current-limit protection for
these ranges is provided by four 27 k ohm resistors in
series with K 103.
8-33.
Fet Switching Technique.
bias the Fet switches on or off have open collector tran
sistor outputs. These outputs are connected via 100 k
ohm pull-up resistors to the output levels of either Gate
Bias Amplifier #1 or #2, except for UI04b and U lO4c
(see Figure 8-5 Comparator/Fet-Switch circuit). With
the + 2 volt reference applied to its negative input ter
minal, a low logic input signal will cause the com
parator's output to be driven to
switch will be off (refer to Table 8-2). With a high logic
input signal, the comparator's output transistor is turn
ed off and its output will therefore rise to the pull-up
voltage level (usually the input signal level). The
N-channel Fet switch will be closed with this pull-up
voltage applied to its gate.
8-34, The pull-up resistors for U I04b and c are con
nected to ground, thus their outputs will rise to ground
when they are turned off. Note that in some cases the
+ 2 volt reference is applied to the positive terminal of
the comparator rather than the negative terminal. Com
parator operation will then be exactly opposite of that
just described. Table 8-2 lists all of the normal com
parator input levels, comparator outputs, and resultant
Fet switch state. The l O k ohm resistors connected
gates of Ihe Fet switches along with the stray
capacitance act as low pass filters to prevent pick up and
rectification of stray AC signals.
Table 8-2 Comparator Operation
th
e
." . rtlm.
Com.,.rltor:, Input
Pot. Term.
O V
+ 2 V
+ 2 v
+ 5 V
+ 2 V
+ 2 v
O V
+ 5 V
- -
HA'
L M E S
L G N D
[RELAYS AND F E T S l
: 0 :
: 0 :
: 0 :
:
. _ ,
2 D
D E C O D E D L I N E S
Model 3456A
10
V, and
V Ranges.
Relay KlO3 (a
'
The co
mparators that
-
1 8 V and the Fet
COmPlrltor'. Outjlul
- 1 8 V
--
-
Pull-Up Voltage
Pull-Up Voltage
- 1 8 V
-
I N P U T
S\. J J T C H J N G
--
HSi-�-.
10
o
the
Fit Switch
Open
Closed
Closed
Open
o