Digital output to a distance of 10mts from the DIO card

[SreedharT]

Hi,

I want to run 20 digital outputs for a distance of around 10 meters from my PCI 6509 DIO card, to drive some relays. How can I effectively do that. Are there any other alternatives. Can some electronic circuit be implemented in between?

The problem is, there is no isolation provided. Since this is a PCI card, I don't want to use this on Ethernet either.

Now, since the distance is so high, there will definitely be a problem with the analog input and output as well, how should this be taken care.

Any suggestion is welcome.

Thanks
Sreedhar T

[Buechsenschuetz]

Driving 10m cable with a digital output should not pose too many problems. You should keep in mind that 10m of cables can have a capacitance up to several nF (nano-Farads). Some digital circuits will not work stable with such a capacitive load. Since you are driving relays your switching frequency will not be too high, and line reflections etc will pose no significant problems.

It is different with transferring analog signals over such a distance. Again, the cable capacitance matters. Some analog circuits will not work correctly (i.e. oscillate) when there is a significant capacitance load.

Also, the input impedance of A/D converters typically is very high, and together with the cable capacitance you will get a first order lowpass, i.e. high frequencies will be cut off. You should use an analog driver with very low output impedance, and reduce the input impedance of the ADC as far as possible by connecting a resistor (in the kilo-ohm range) in parallel with the input. For long analog lines you should use differential signals if possible to avoid noise pickup problems. There are op-amps available providing a differential output from a single-ended signal, and most ADC's will accept differential signals.

As usual, proper grounding is essential to avoid noise and mains hum pickup. When you use shielded cables for the analog lines (recommended) the shield should be connected to the metal case(s) at one point only. Avoid wiring shemes which might cause any current flow over the shield. If both devices (signal source and ADC) have separate mains supply, and if their cases are grounded, make sure that both cases are grounded via the same branch of the protection earth line.