by Eric Brouwer » Thu May 22, 2003 3:58 pm
I have noticed this behaviour on some of the newer PC's, and it is more prone to affect users of notebooks. The problem is that the serial ports no longer complies to the RS232 standards.
The signal levels as defined in the RS232 standard are something as follow:
- Zero represented by a voltage between +8 to +15V
- One represented by a voltage between -8 to -15V
However, on the newer PC's, these voltages can be as low as 3.3V, and for notebooks even as low as 2.2V, thus resulting in several serial connected devices not functioning correct.
As most of the PIC interface designs were done taking the RS232 protocol as standard, there was no problem taking the interface's supply from the serial port. Normal designs will work with serial port voltages as low as 4V, but lower than this, one needs to make some modifications to the circuit.
However, as the serial port voltages decreases, this eventually results in too low a voltage for the PIC to function correct. Trying to run the PIC at below 3V at 4MHz, results in inaccurate clock frequencies, thus the possiblity that the interface might behave as if faulty.
One possible solution is to make use of an external power supply to drive the interface.