Mahmood:

Encoders with differential outputs are really really common in the industrial equipment world. The reason differential signals are used is to allow the encoder to be mounted on a machine shaft or motion pickup wheel within a machine mechanism that may be in the presence of large common mode ground noise and voltages and at the same time be a long distance away from where the decoder equipment is located. On some machines or mechanisms it may be possible that the encoders are mounted many meters away from the decoder equipment. The differential interface offers the ability to maintain encoder readout accuracy in these extreme conditions.

The tried and true chip to use as a receiver for a decoder circuit board is the veritable AM26LS32A. These chips, being bipolar technology, are known for ruggedness on long cable wired interfaces such as one would see coming from an encoder in an industrial or machine environment. The part offers four differential receivers in one package and permits it to be suitable for use with encoders that offer A,A',B,B',C,C',Z,Z' differential outputs. Also applicable of devices with outputs designated in other ways such as A,A#,B,B#,M,M#,I,I#. Here is the link to the TI web page for this device: http://focus.ti.com/docs/prod...ls32a.html

The device is an active part and should be orderable from most suppliers. Here in the US the DigiKey distribitor shows the part available for between about 1 to 2$US in single quantity. They also list the part at 0.20$US if you were to purchase a whole reel of TI parts. Those from other manufacturers may have similar prices. I am aware that National Semiconductor also supplies this part. From the AM prefix on the part number you may guess that many years past the part originated from AMD.

You may certainly chose to use the encoder outputs in a single ended manner. This would be suitable if you have the encoder and decoder close to each other and wiring is relatively short and you pay good attention to good cabling and grounding practice. In this case you may use one of the A outputs and one of the B outputs for example. If you connect in this manner you should consider the use of a Schmidt trigger input type receiver part.

Some encoders come equipped with differential outputs driven in a totem pole push pull output format. The common internal chip used inside these devices is the AM26LS31 or DS26LS31 chip. You will also see some encoders that have open collector type outputs with internal NPN transistor or 7438 type driver chips. This type of output would correspond to an encoder designed for low speed operation. It is less common but you may very well find encoders with the open collector outputs that still offer differential signalling.

When the AM26LS31 and AM26LS32 parts are studied you will note that these parts are commonly specified for use on RS422 communications network applications. For long distance wiring or when there is a possibility of a fair amount of common mode noise/voltage on the encoder to decoder equipment interface it is important to apply terminations to the interfaces. There is a goodly amount of information on the web that you can find with Google that provides guidance on termination techniques for RS422 interfaces. This same information is fully applicable to encoder applications as well.

One thing to study carefully is to fully comprehend the encoders "line count" and the maximum RPM that you may operate the device. These two parameters relate directly to the high end frequency that you will see on the encoder outputs and can be surprisingly high for a high resolution encoder operating at a higher RPM.

My comments here have hinted that the encoder is a rotary device. But all comments apply equally to linear slide type encoders such as you may find in use on X/Y tables, lead screw mechanisms or other slide type devices.


Michael Karas