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Chapter 8 Tutorial
Multifunction Module
Using an External Pull-Up
In general, an external pull-up is required only when you want to set
the output "high" value greater than TTL levels. For example, to use a
+12V external power supply, the value of the external pull-up resistor is
calculated as follows:
V
= 12 Vdc
cc
I
= I
x safety factor = 1 mA x 0.5 = 0.5 mA
max
out low
Vcc
12
R =
= 24 k
0.0005
I
max
The value of the logic "high" level with the external 24 k pull-up
resistor is calculated as follows:
R
external
V
= V
x
high
cc
R
external
Driving External Switches
You can use two digital output channels to control an external switch.
For example, you can drive the HP 865X series of microwave switches
using an external power supply and two digital output channels.
The HP 865A switch provides an internal protection diode. The state
of the 2-to-1 multiplexer is changed by setting the appropriate output
bit low (0).
Digital Output Channel
394
24 k
= 12 x
R
24 k
internal
HP 865X Microwave Switch
24 Volt
Power
Supply
–
+
= 8.47 Vdc
10
k
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