The digital open-collector is a simple open collector gate. The state seen
on the input line is reflected in the output. The state seen on the input
line determines the strength of the output. If the input to this device is a
1 then the output is a 1, with a HI_IMPEDANCE strength. If the input is a 0,
then the output is a STRONG 0, otherwise, the output strength is
UNDETERMINED. The falling (fall_delay) and rising (open_delay) delays may be
specified independently.