Digital Inputs - Keithley 7021 Instruction Manual

Digital inputs

Input channels use positive true logic but can be pulled up or
pulled down based on the configuration of the pull-up resis-
tor. Each channel uses a 10k Ω pull-up resistor (R
up resistors can be pulled up to 5V or pulled down to ground
depending on the positioning of the jumper on the input logic
bank. Refer to Section 4 for more information. Figure 3-3
shows the resistor being pulled up to 5V.
When the resistor is connected to 5V, the channel is pulled
high. Thus, with nothing connected to the channel, the input
is pulled high to 5V which displays the channel as on.
7021
Figure 3-3
Input configuration
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When the resistor is connected to ground, the channel is
pulled low. Thus, with nothing connected to the channel, the
input is pulled low to ground which displays the channel as
off.
). The pull-
1
The digital input is compatible with external TTL logic.
Each built-in pull-up resistor provides level shifting so de-
vices such as micro-switches can be monitored. Each input
has a protection network that clamps the input at 5.7V. This
allows logic levels up to 42V peak to be monitored.
5V
R
1
10kΩ
R
2
10kΩ
INPUT
GND
Digital I/O Configuration
R = Pull-up resistor
1
R = Input protection resistor
2
3-3