C. Digital I/O ASIC System Setup Considerations
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cabling is added, the capacitance goes up, resulting in the use of a smaller pullup
resistor until the maximum sink current of the output is achieved.
If the 16C50A Digital I/O ASIC device is driving the output, its maximum sink current at
a Vol of .4 V is 12 mA. This gives a lower limit of 420 ohms for the pullup resistor,
allowing a maximum cabling capacitance of 110 pF.
Note that while the input feature of the Digital I/O ASIC may not be used by your
application (Digital I/O ASIC used as an output only), the input circuitry remains in
parallel; therefore, the output rise time is still a critical parameter that the input still sees.
The output rise time must not exceed 50 ns. Be wary of using low pass filters to remove
electrical noise. The resulting capacitance is typically too large to meet the 50 ns rise
time requirement.
Typically, optical isolators are used to help remove electrical noise while providing for
different grounds. Separate grounds are achieved through the use of an additional
power supply for the optocoupler rather than using the computer's power supply. If the
computer's power supply powers the optocouplers, electrical isolation is defeated. An
example of one such circuit is illustrated in
. The circuit can be altered to allow
for design considerations.
Assuming a Vil of 1 V maximum for the 16C50A Digital I/O ASIC, the Hewlett-Packard
®
Dual Optocoupler must have a Vol of less than or equal to 1 V over the operating
temperature. Using a TTL-compatible optocoupler gives a Vol of .6 V maximum with rise
and fall times (50 ns and 10 ns, respectively) that are easily compatible with the Digital
I/O ASIC, given a 1 k ohm pullup.
Figure 8. Digital I/O ASIC-to-Optocoupler Interface Example
+
+
2.2K
+
24V
-
10mA
1K
HCPL-2630
ZT8907
Digital
16C50A
I/O
ASIC
Inductive Coupling
Inductive coupling on I/O lines can cause noise to be coupled into the chip, resulting in
intermittent operation. This situation occurs when the Digital I/O ASIC I/O signals are
routed with other signals within a wire bundle. One way to filter inductively coupled
noise, or any noise for that matter, within a system with the same ground (not using
optocouplers) is illustrated in
