Controlling Digital Circuitry - Keithley 7001 Instruction Manual

Front Panel Operation
The Model 7001's Digital I/O port can be used to con-
trol external circuitry. The port provides four output
lines and one input line. Each open-collector output
can be set high (+5V) or low (0V) and will sink up to
100mA. The four output lines can also operate external
supplies from +5V to +30V.
Use the DIGITAL I/O MENU to select the following
options (information about each menu item is con-
tained in paragraph 4.6.3):
• OUTPUT-STATUS
ON or OFF selectable for each output line (1
through 4). Use this option to check or change the
output state. For example, the following OUTPUT-
STATUS display is set to ON for digital output #1,
while outputs #2 through #4 are set to OFF:
DIGITAL OUTPUT LINES
1=ON 2=OFF 3=OFF 4=OFF
Changing the display to the following turns digital
output #1, #3, #4 to OFF and output #2 to ON.
DIGITAL OUTPUT LINES
1=OFF 2=ON 3=OFF 4=OFF
• OUTPUT-POLARITY
ACTIVE-HIGH or ACTIVE-LOW selectable for
each output line (TTL1 through TTL4). Use to
check or change the logic sense. Select the desired
output line (TTL1 through TTL4), and check or
change the output-polarity. For example, the fol-
lowing shows the display used to check or change
TTL3 (digital output #3):
SELECT OUTPUT LINE
TTL1 TTL2 TTL3 TTL4
Selecting TTL3 and pressing Enter changes the dis-
play to:
TTL3 LOGIC SENSE
ACTIVE-HIGH ACTIVE-LOW
ACTIVE-HIGH is the current setting for digital
output #3.
• INPUT
ON or OFF. Use this option to check or change the
status of the digital input line. Sense is fixed at ac-
tive-high (ON=5V).
4-44
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4.9.1

Controlling digital circuitry

Each of the four digital, open-collector outputs (con-
nector J1007, pins 4 through 7) includes a built-in pull
up resistor to +5V. The output transistor is capable of
sinking up to 100mA at voltages up to +30V.
Early versions of the Model 7001 have an additional
10kΩ resistor connected between the collector and the
internal built-in test circuitry. Under certain combina-
tions of output states, this resistor limits the output
high to 2.5V. For example:
Output #1 set high (+5V)
Outputs #2, #3, and #4 set low (0V)
Actual output of digital output #1 is restricted to +2.5V
If using the digital output to sink current from external
devices a Model 7001 containing the additional 10kΩ
resistors will function correctly. The transistor switch is
open restricting current flow through the external de-
vice in the high state (+5V). However, if the output is
directly used as an input to a typical logic input, the
2.5V high state may not be reliably detected as a high.
To correct this condition (if a 2.5V high is unaccept-
able), use one of the following solutions:
• Add external 1kΩ pull-up resistors to the logic cir-
cuit inputs.
• Remove the built-in test resistors (R606, R608,
R610, and R612),
• Use an external voltage supply.
External voltage supply
Each output can be operated from external voltage
supplies (voltage range from +5V to +30V applied
through the device being driven). Refer to Figure 4-9
for a simplified schematic of the digital I/O port. The
high current sink capacity of the output driver allows
direct control of relays, solenoids, and lamps (no addi-
tional circuitry needed). Each output channel contains
a fly-back diode for protection when switching induc-
tive loads (such as a low power solenoid or relay coils).
To use these fly-back diodes, connect the external sup-
ply voltage to pin 3 of J1007 (the digital I/O port).
Make sure the external supply voltage is between +5V
and +30V and the current required by the device does
not exceed 100mA.