Conrad 12 79 49 Bedienungsanleitung Seite 15

from 2 Ohm to 1 KOhm. The input resistance set at any time is indi-
cated at the top left of the display.
To carry out frequency measurement (Hz) or pulse/interval measurement
(Duty) or pulse width measurement (delta t) or period measurement
(delta T), press the "Hz" button (see under 4.4.5).
If the graphical display is preferred, press the "MODE" button once (see
under 4.4.3).
The "RANGE" button (manual range selection) has no function during
current measurement.
Attention!
When a "-" appears in front of the DC current measured value, the mea-
sured current is negative (or the test leads are reversed).
Do not measure any currents in circuits in which voltages greater than
250 VDC or VAC rms can occur, so that the measuring unit is not dama-
ged and your life is not endangered as a result. Never measure currents
over 20 A. Measure only in circuits themselves fused at 16 A and in
which no power greater than 4000 VA can occur.
Measurements of currents at 20 A may only be of 30 seconds duration
max. and may only be carried out at intervals of 15 minutes (shunt coo-
ling phase).
5.8 LOGIC
This measurement function is used to establish the logic levels in digital
circuits (TTL, 3V or 5V logic and others). With this logic function the fol-
lowing conditions can be determined:
• if the test point shows a high level greater than 70 % of Vcc (or V++ or
Vc or V+ or similar) (to 40 VDC max.), then "^" appears in the left of
the display.
• if the test point shows a low level of less than 30 % of Vcc, then "^"
appears in the left of the display.
• if the test point shows a level between 30% and 70% of Vcc, then "---"
appears in the left of the display
To establish the logic level or to set the level ranges, proceed as follows:
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a) Switch the measuring unit on.
b) Set the measurement function switch to "LOGIC". In the display
"00.00" VDC appears with "v" to its left (with measurement inputs
open) for a low level. Beneath, above button F1, the symbol "TTL"
flashes
c) Connect the black test lead to the measuring instrument "COM"
socket and the red test lead to the "-I(-•-I<-•-mA•SIG•AUX•LOGIC".
d) Now connect the other end of the black test lead (test probe) to the
"earth" of the digital circuit, normally "-".
e) While the black test lead remains connected to "earth", touch the
points to be tested with the red test probe.
basic menu
TTL Press button F1 to set a TTL level as measurement level.
3VCM Press button F2 for 3 V CMOS logic, i.e. 3 V = 100 %, 30%= 1V, 70
% = approx. 2.1 V.
5VCM Press button F3 for 5 V CMOS logic, i.e. 5 V = 100 %, 30%= 1.5V, 70
% = approx. 3.5 V.
5.9 Use of the GDM-703 as TTL signal generator
The Graphical Display Meter GDM-703 contains a type of "function
generator" delivering a stepped-variable frequency of from 1 Hz to 20
(19.99) kHz at the centre measurement socket at a fixed voltage (ampli-
tude) of 5V.
To output a TTL level at a frequency variable within these limits, proceed
as follows:
a) Set the rotary switch to "SIG OUT"
b) Connect the black test lead to the measuring instrument "COM"
socket and the red test lead to the "-I(-•-I>-•-mA•SIG•AUX•LOGIC"
socket.
TTL 3VCM 5VCM
F1 F2 F3 F4
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