GE VMIVME-4514A Gas turbine card

To be sure that your device will drive the 1771-VHSC, you must know the electrical characteristics of the output driver component used in your signal source device. The output voltage differential Vdiff = (Voh - Vol) is critical, because this is the drive voltage across the 1771-VHSC input terminals 1 and 2, and the photodiode current is a function of Vdiff - Vdrop. The manufacturer of your shaft encoder or other pulse-producing device can provide information on the specific output device used.Important: Neither output of the differential line driver can be connected to ground. Damage could occur to your driving device.

Any signal source which uses a standard 

TTL output device driver rated to source 400µA or less in the high logic state is not compatible with the 1771-VHSC module. Many popular differential line drivers, such as the 75114, 75ALS192, and the DM8830 have similar characteristics and can source or sink up to 40mA. In general, the output voltage Voh will be higher both as the supply voltage and the ambient temperature increase. For example, vendor data for the 75114 shows Voh will be about 3.35V at Vcc = 5 V, Ioh = 10mA and 25oC. Vol will be about 0.075V under the same conditions.

Assuming that you could supply 

5mA to the 1771-VHSC input terminals, how much voltage across the field wiring arm terminals would be required? Vdrop would be about 1.9V as previously noted. And 5mA through 150 ohms gives an additional 0.75V drop. Thus, you would have to apply about (1.9V + 0.75V) = 2.65V across the terminals to cause a current of 4mA to flow through the photodiode. The 75114 will give about 3.3V at Vcc = 5V and 25oC. Thus you know that this driver will cause more current to flow than the minimum required at 5mA.

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