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Does output logic "low" represent 0V?
Does output logic "low" represent 0V? That depends.

Many 'logical' people do not understand what the actual meaning of 0 and 1 when it comes to outputs.

If you output a 0 that means that the port pin will do the best it can to pull that pin to ground, when connected illegally, it may even try so hard that it destroys itself. It does NOT mean that it will pull the pin to ground regardless of circumstances. In many cases (such as driving a CMOS chip) output of a 0 will result in a voltage very close to ground.

now for the nitty-gritty:

Taking a Philips Rx2 as an example, you find the following in the datasheet

ABSOLUTE MAXIMUM RATINGS
Maximum IOL per I/O pin 15 mA

DC ELECTRICAL CHARACTERISTICS
VOL Output low voltage, ports 1, 2, 3 IOL = 1.6 mA Vol max 0.4 V
VOL1 Output low voltage, port 0, IOL = 3.2 mA Vol max 0.45 V

Vol is the voltage out when low, Iol is the current pulled by the port when low

so, what does that mean?
well it means that if you connect the port in such a way that more than 15mA flow into the pin when low, the chip may be destroyed.

It means that if you connect a port 1 pin in such a way that 1.6 mA flow into it when low you will see a voltage between zero and 0.4V on the pin.

If you go for the maximum 15mA I would not be surprised if you saw 1V as a low.

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