question about serial port GND

[wejos]

hi guys,

just want to ask for small input about - because i read the GND (pin 5 of DB9) could go 12+ sometimes, so since in my circuit they all have common ground, should i put a diode (say IN4004) to stop the wrong polarity and or from short circuiting the circuit? OR best to leave out the GND?

which one among the two you think?

thanks a lot in advance,

joey

 

[ericgibbs]

hi guys,

just want to ask for small input about - because i read the GND (pin 5 of DB9) could go 12+ sometimes, so since in my circuit they all have common ground, should i put a diode (say IN4004) to stop the wrong polarity and or from short circuiting the circuit? OR best to leave out the GND?

which one among the two you think?

thanks a lot in advance,

joey

hi Joey,
I have never seen pin5 go to +12v.??
If you fit a series diode it will interfere with the RS232 signals, dont forget they swing ~~+/-12V with respect to 0V ground.

 

[wejos]

thanks eric i must have read that explanation wrong then.

i am referencing to this guy's work named miguel, the JDM programmer **broken link removed**, he says the ground of pin 5 should never be directly connected to the rest of the ground in the circuit because the polarity of pin 5 change every now and then.

so i just picked up the idea and thinking if it will apply in my 12c508 project.

eric, meaning to say pin 5 (GND) is okay if connected to common ground? it does not swing? just DTR, RTS, and etc... GND always 0v?

thank you,

joey

 

[JimB]

hi guys,

just want to ask for small input about - because i read the GND (pin 5 of DB9) could go 12+ sometimes, so since in my circuit they all have common ground, should i put a diode (say IN4004) to stop the wrong polarity and or from short circuiting the circuit? OR best to leave out the GND?

which one among the two you think?

thanks a lot in advance,

joey

I have only ever seen anything like that once, a short circuit to earth on a flowmeter out in the field was killing the serial interface to a printer.
Wierd or what?

The 0v line in the processor was floating, so the s/c in the field put the 0v line to -24v (from memory, this was 25 years ago).
The RS232 gnd in the processor was at -24v, the RS232 gnd in the printer was at 0v and bad things happened when they connected together.

Unless your RS232 is fully isolated (very unlikely), the GND will connect to 0v, which in turn will probably be connected to the case and earth etc.

I dont think that you will have a problem, but putting a diode in there WILL CAUSE PROBLEMS.

JimB

 

[JimB]

It would have been usefull if you had asked that question first!

This is a special application, it is not RS232 in its normal form.

If you read what the hombre says, it says not to commect the "internal ground" of the programmer to the "RS232 ground".
If you study the circuit you will see that it is doing some strange voltage doubling (I think) from one of the RS232 signals.

JimB

 

[wejos]

sorry about that jim

so there is no danger, i will connect pin 5 to ground.

your signature fits the predicament i am in since i did not want my comp blown into pieces or something, i was afraid something like that might happen so i asked for guidance here.

eric, jim, thank you, appreciate both your help. i think can sleep well tonight now lol