Can I connect a 15V output Encoder directly to the UMI-7764(PXI-7344)

[Aussie]

Is it safe to attempt the above.?
Would additional circuitry be necessary ?

[algames]

The UMI only has isolation up to 7v, if you connect anything higher than this you could damage it. You can always use a voltage divider circuit to reduce the voltage sent.

[Aussie]

You've answered my question.
Thanks

[Buechsenschuetz]

See page 5-9 of the 7344 hardware manual for details on encoder inputs. Anything in excess of the +5V supply voltage will be fed into the supply rail via a protection diode.

A voltage divider will work but will encrease line impedance when arranged close to the encoder and thus the connection between encoder and 7344 will be more prone to glitches. Usually it is better to use a level converter with a true TTL output.

[Buechsenschuetz]

In case you use a simple voltage divider you should clamp the line which goes into the 7344 encoder inputs with 4.7v zener diodes. In case the 'lower' resistor of the divider gets open the input circuitry of the 7344 might be damaged due to excess voltage.

Also, when using any level converters (with voltage dividers or active circuitry) it is essential to provide a fail safe overvoltage protection to protect the 7344 inputs - and also your computer. The best method is galvanic isolation via optocouplers, but usually these need extra circutry to drive their inputs (they require several mA to drive the LED) and to re-shape their output signals since most optocouplers have rather long rise and fall times.