C. Digital I/O ASIC System Setup Considerations
ÛZIATECH
128
Power Supply Sequence Mismatch
A common application is to interface to a 24-position ZT 2226, Opto 22, or equivalent
I/O module rack. Vcc and ground are provided from the ZT 8907 through connector J5,
with Vcc protected by a 1 A fuse. This application is illustrated in
. In this
application, no power supply sequence mismatch exists because the power supplying
the input circuitry within the Digital I/O ASIC is applied before or at the same time as the
power supplying the external signals. Proper system operation will result.
However, if a power source other than that supplying the Digital I/O ASIC is used to
power the external signals, then a power sequence mismatch could occur, resulting in a
latchup condition. An external power source might be required if the external circuitry
requires more than the 500 mA supplied by the cable or if a custom interface is being
designed (see
).
One solution is to switch the external signals' power supply with an output that is
controlled by the computer. In this manner, if the computer is off, so is the external
power supply. This solution is illustrated in
A simpler solution is to power the relay controlling the external power supply directly
from Vcc and ground supplied by the interface cable.
Another solution is to utilize the same switch to control the computer's power supply and
the external signals' power supply (see
). This is an acceptable solution for
power supply sequence mismatches as long as the computer supply ramps up faster
than the external power supply. This ensures the Digital I/O ASIC input circuitry is
powered before the external signal circuitry.