7½ Digit Graphical Sampling Multimeter Reference Manual Section 3:
DMM7510-901-01 Rev. B / May 2015 3-51
Digital I/O lines
You can place each digital I/O line into one of the following modes:
• Digital open-drain, output, or input
• Trigger open-drain, output, or input
• Trigger synchronous master or acceptor
When you configure the digital I/O lines for triggering applications, configure the output lines before
the input lines. This prevents possible false input trigger detection in certain situations.
Digital control modes
If you are setting a line for digital control, you can set the line to be open-drain, output, or input, as
described in the following topics.
Open-drain
When you place a line in open-drain mode, the line is configured to be an open-drain signal with a
100 kΩ pull-up resistor. This makes the line compatible with other instruments that use open-drain
digital I/O lines, such as other Keithley Instruments products that only support open-drain for its digital
I/O. In this mode, the line can serve as an input, an output, or both. You can read from the line or
write to it. When a digital I/O line is used as an input in open-drain mode, you must write a 1 to the
line to enable it to detect logic levels that are generated from external sources.
Output
When you place a line in output mode, you can set the line as logic high (+5 V) or as logic low (0 V).
The default level is logic low (0 V). When the instrument is in output mode, the line is actively driven
high or low. Unlike the input or open-drain modes, it will not respond to externally generated logic
levels.
When you read the line, it shows the present output status and an event message is generated.
Input
The input mode is similar to the open-drain mode, except that a line in this mode is intended to be
used strictly as an input. When you place a line in input mode, the instrument automatically writes a 1
to the line to enable it to detect externally generated logic levels.
You can read an input line, but you cannot write to it. You also cannot change the logic level while the
line is in input mode. If you attempt to change the logic level of a line that is in input mode, an event
message is generated.
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