RS232 Low Voltage...

[K C]

Hi Jason,

>>> The transceivers used on this serial board are designed to output at +/- 5V. This meets the requirement of the RS-232 specification <<<

I think its very strange that NI sell boards for RS232 interface that just meets the spec. If you want to use this board for a long lines you will get into trouble.

Unless NI specifies this somewhere in the datasheet of this board that this board is only for a short distance interface.

[Istvan49]

The problem is that it is not written anywhere (or I didn't find...), and I was very surprised when I checked to see those +/-5V. But thank you for your reply, now I got all the answers.

Very good forum!

[JasonSmith]

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@K C wrote:

I think its very strange that NI sell boards for RS232 interface that just meets the spec. If you want to use this board for a long lines you will get into trouble.

Unless NI specifies this somewhere in the datasheet of this board that this board is only for a short distance interface.

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Hi KC,

You are correct, you will run into problems if you use too long of a cable, but the same is true for any RS232 device, or any communication cable of any type.  RS232 is just not designed for long distance applications.  For longer distances, RS485 or other communication mechanisms are more appropriate.

As far as the maximum cable length for this board, it is defined as a 2500pF load, as specified by the RS232 specification.  This is generally accepted as a 50 foot cable, but is really dependant on the performance of the cable used. 

Each cable has a total capacitance per unit length of cable.  The maximum cable length can be found be dividing this value into 2500.  It is important to note that the total capacitance must include capacitance between conductors and shielding (Or the environment if unshielded).  Some manufacturers provide several specifications for different sources of capacitance, and care must be taken that the TOTAL capacitance is used.  The lower the cable's total capacitance, the longer the RS232 transceiver will be able to drive the signal.

As an example, the standard bulk RS232 cable on Belden's website has a total capacitance of 65 pF/ft (Including capacitance to all conductors and shield), giving a maximum cable length of just over 38 ft.  Belden also sells low capacitance bulk RS232 cable with a total capacitance of 32pF/ft, giving a maximum cable length of just over 80 ft.  This demonstrates the role the cable plays in RS232 communication.

I hope that helps,

Jason S.
National Instruments

[K C]

Hi Jason,

Like I said the board is according the RS232 specification, but at the very low end !!!

What you forgot to tell in your good explanation is that the RS232 specification is applicable for speeds up to 20K