C. Digital I/O ASIC System Setup Considerations
ÛZIATECH
130
Figure 4. Computer and External Power Supply with Common Switch
Correct Power Supply Sequence,
Potential Signal Level Mismatch
Power
Supply
ZT 8907
Interface Cable
External
Power
Supply
24-Position
or
Custom
Application
Vcc
24
ZT8907
Digital
16C50A
I/O
ASIC
Signal Level Mismatch
Power supplying the external signal in
is always relative to the Digital I/O ASIC
input circuitry power because power is provided over the interface cable. Signal level
mismatches will not occur and proper system operation will result. However, if separate
power supplies are used, there are two predominant causes of signal level mismatches.
The first (assuming no sequencing problems) occurs when the two supplies are not
referenced to each other, as illustrated in
, and
. This results
in signals that may be higher than Vcc or lower than ground, potentially causing SCR
latchup. All that is generally needed is to reference one supply to the other, typically by
connecting a common ground. The most convenient way of connecting a common
ground is to use the interface cable.
illustrate correct
ground connections.
The second cause of mismatch occurs when the two power supplies are referenced to
each other but the Vcc difference between the two power supplies exceeds 0.5 V. This
results in signals that could be greater than Vcc, causing SCR latchup. This is easily
remedied by adjusting the external power supply voltage to be within 0.5 V of the
computer power supply voltage.
Figure 5. I/O Rack Vcc Supplied Externally, Common Ground
Potential Power Supply Seq. Mismatch,
Correct Signal Level Match
Power
Supply
ZT 8907
Interface Cable
External
Power
Supply
24-Position
or
Custom
Application
Vcc
24
ZT8907
Digital
16C50A
I/O
ASIC