RS232 Low Voltage...

[Istvan49]

Hi!

I use a NI serial RS232 Card (PCI-8430/8) and I checked the signals in output because my device is not answering. And I have measured -5V to +5V signal? I have read that I should have -10V and +10V!!? So can we change the voltage, or is something wrong with my board? I don't understand, please help me!

Thank you for paying attention to my request!

[K C]

Hi,

A rs232 transmitter should be 5 - 15 V

A rs232 receiver should be happy with 3 - 25V

So -5 and +5 should work (and as i know it does) But it is nothing something you expect when you measure direct on the connector of a board or device.

Are the levels from your PCI board or your device that low ?

Did you measure it without a connection ? 

[Istvan49]

OK so I will explain all the problem (sorry for the language, but I am french...):

I am trying to communicate with temperature controller, JC Systems Fastrac 600. But I send a command and most of the time there is no reply. Just sometimes I receive a "?", and that means that the command is not correct. I tried to see with the JC Systems company if I had something to do, but I did everything and I still have the same problem. So I checked the signals:

When the Wires are disconnected:

-The Controller is delivering a signal from -10V to +10V.

-The computer from -5V to +5V.

When the wires are connected:

-Controller -8V to +8V (Is that normal not to have the same voltage?).

-PC -5V to +5V.

So it's possible that the voltage from the computer is not enough. The problem is that most of the time, I don't receive any answer (and when I check the signal, there is no signal from the device). So it is not a problem of configuration, because in that case, the device should answer everytime "?" because of wrong command... I am really lost...

[K C]

The 8 and 10V is OK

But 5V from your PC port is not OK.  It can work but it should be higher to avoid problem at the other end of the cable..

 At which pin number are you measuring this voltage. Specially between pin 2 and 5 an between 3 and 5.

[Istvan49]

Yes, that's -5V between pins 5 and 2 and between 5 and 3 also.

I should have more? What can I do? Is there a switch somewhere or a parameter to change?...

Thank you for your help K C!

[K C]

Well, if your are measuring on the connector of your COM port (9 poles) with nothing connected to it the voltages should not be the same.

Do you have more COM ports on your system ? Can you measure there ?

[Istvan49]

!!! Yes that's crazy! On the other COM on the computer I measure -11V!! But on every COM of the National Instrument board, I have this -5V!!?

Very strange... I will try the commands on the other COM of the computer, but that's crazy because we just bought the National Instrument board to control the devices of the machine and it seems to be impossible without amplifiers...

I try the other COMs to see what is happening and I come back...

[Istvan49]

OK. I tried with the others COM, but I still have the same problem... (no answer, or sometimes a "?"). Something is not working well inside this machine, I have to find what...

But about the NI card, is there something wrong with the voltage? Or is it normal? I have to change it?

[AnalogKid2DigitalMan]

Is your cabling correct?

Have you tried communicating with the device in Windows Hyperterminal?

Are you using the correct termination character?

Have you tried using a COM port that resides on the motherboard (not the NI serial card)?

[Istvan49]

Is your cabling correct?

I think so yes. On the device, there are only three wires "TXD out, RXD in, GND". I checked the signals going out and coming from the device and they are ok. BUT, I don't know if I have to plug something on the computer like a handshacking... I am using a Xon/Xoff communication so if I understood well, I don't need to do that. Only the three wires going to the device.

Have you tried communicating with the device in Windows Hyperterminal?

No, but ok, I will...

Are you using the correct termination character?

Yes, but as I said, most of the time, the machine does not answer. If I had a wrong termination character, I will receive a "?" everytime I think. And it does not happen. Only SOMETIMES I receive a "?", after trying manytimes to send my message...

Have you tried using a COM port that resides on the motherboard (not the NI serial card)?

Yes, that's the last think I did. But I always have the same results...