Inter face jitters

[Top Dawg]

I have a serial pic interface for a Jr radio and it is being used on windows Xp. I have calibrated it and everything works fine but for some reason I get inputs of the controls when I have not touched them. This gives everything a jittery motion. I have tried the interface on another computer with windows Xp and everything worked fine. Has anyone else had this problem and if so how did you rectify it. Any help would be greatly appreciated.

[Eric Brouwer]

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.

[Top Dawg]

Thanks Eric

[Alfred]

How do I make a power supply for the interface?

[Slowhand_47]

Look at the picture:



The "power supply" consists of D1, C1 and R3. The RS232 voltage is taken from pin RTS of the port and stabilized to about 5.6V by the Zener diode D1. R3 limits the current through D1 and when charging the filter capacitor C1.

All you have to do to supply external power is to disconect R3 from RTS and then connect a 9V battery or voltage supply (7-12 V) to the "open" pin of R3 (where it was connected to the RS232 plug).

HTH
Slowhand

[jeremy]

I dont see the picture of the power supply, can u pls repost/email/put on webpage?
TIA Jeremy

[Eric Brouwer]

Have a look on my . The Serial PIC interface circuit diagrams shouws where the external power supply can be connected.