Keithley 7020 Instruction Manual - Digital I/o Configuration
1
Digital I/O Configuration
1-1
1.1
Introduction
This section covers the basic digital input and output config
urations for the Model 7020. Connection information for
these configurations is provided in Section 2 of this manual,
while operation (front panel and IEEE-488 bus) is explained
in Section 3.
1.2
Digital outputs
Output channels use negative true logic. That is, the output is
pulled low when the channel is turned on (closed).
Conversely, the output goes high when the channel is turned
off (open).
1.3
Controlling pull-up devices
Typically, the digital outputs are used to provide drive for
relatively high current devices such as solenoids, relays and
small motors. The configurations for these applications are
shown in Figure 1-1. Figure 1-1A uses the internal voltage
source (5.3V), while Figure 1-1B allows you to use an
external voltage source (V) for devices that require a higher
voltage (25V maximum). Internal jumpers are used to select
the voltage source. At the factory, the internal 5.3V source is
selected.
As shown in Figure 1-1, each output channel has a resettable
fuse (F) that is used to protect the output driver from exces-
sive current. During normal operation, the resettable fuse is
in a low resistance state like a regular fuse. When output cur-
rent reaches the trip level (around 180mA at 23°C, 130mA at
50°C), the fuse goes to a high resistance state that interrupts
the damaging high current. The fuse will automatically reset
to the low resistance state when the fault condition is re-
moved.
Each output channel uses a fly-back diode for protection
when switching an inductive device, such as a solenoid coil.
This diode diverts the potentially damaging fly-back voltage
away from the driver.
1.4
Controlling devices using pull-up
resistors
When interfacing outputs to high-impedance devices (i.e.
logic devices), pull-up resistors are used to achieve the ap-
propriate logic level. Figure 1-2 shows the output configura
tion using the 10k
Ω
pull-up resistor (R
P
).
The configuration in Figure 1-2 uses the internal 5.3V source
as the high logic level. Notice that the 5.3V source is also
used to power the device. If you need a higher logic level,
you can place the jumper in the alternate position and apply
an external voltage (via V
EXT1
or V
EXT2
).
1.5
Digital inputs
Input channels use positive true logic. A channel is on
(closed) when the input is high (up to 42V peak). Conversely,
a channel is off (open) when the input is pulled low. As
shown in Figure 1-3, each channel uses a 10k
Ω
pull-up resis-
tor (R
1
) that is connected to 5V. Thus, with nothing connect-
ed to the channel, the input is pulled high to 5V which turns
the channel on (closed).
The digital input is compatible with external TTL logic.
Each built-in pull-up resistor provides level shifting so that
devices 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.