inject noise onto a serial line

[James_R]

I am trying to inject noise onto the TX line of a serial port.  when i tie the analog output from the noise to the TX line i get a 1.75v drop in amplitude even when the vi is not running.  when i run the vi the noise appears on the signal.  Should i place a capacitor inline ?

Thanks

James

[ThomasD]

Hello James,

are there any errors on the signals you recive?

In the scope from -3V to +3V the signal from the RS-232 is not defined. The value defined signal is logic high in the scope from -3V to -15V and logic low in the scope from +3V to +15V. Are the signals ot in these limits?

Regards ThomasD 

[Trager]

I am not sure that an inline cap would do what you want.  Sounds to me like what you have is an electrical problem and that you have a voltage divider situation going on.  Without knowing more about your setup, it's hard to say, but my generic recommendation would be to put a relay in between your AO and the TX and trip that relay using a DO when you start up your VI.

[Ben]

"Should i place a capacitor inline ?"

That would be my thought as well. The size of the cap is another Q. Probably the larger the better.

If a quick experiment using the cap still shows the serial signal amplitude is degraded, then you probably have a situation were the output impeadance of the AO is loading the serial drivers. This may require additional circuitry to present a high impeadance output. But first just try a cap.

Ben

[Trager]

I would think that a blocking cap would also reduce the effectiveness of your noise injection.  I was assuming that the problem is the apparent output impedance of the AO.  If this is the case, you probably just need to use an op-amp to boost the impedance that you present to the serial line.

Although, now that I think about it, I know that RS422/485 are supposed to have termination resistances in the 0-200 ohm range.  I can't imagine an AO output loading that significantly.  I don't know what the electrical specs for RS232 (other than the voltages, of course) are offhand, though.

[Ben]

I agree with everything siad by Trager!

RE:rs-232 spec

That was inveted before standards were invented!

I believe it was high impeadance becuase you could wire more than one recieve line in parallel and get away with it most of the time. It was also limited to 50 feet max cable run.

Ben

[James_R]

I am using an E series DAQ can i use a unity gain opamp with rails at +-10v ?

-James