Encoder vs Decoder

I am comparing and contrasting the differences between an encoder and decoder and am slightly confused.

I understand the general purpose for an encoder used to put a sequence of characters (letters, numbers, etc) into a specialized digital format for efficient transmission or transfer. And that the general purpose of a decoder is for the conversion of a digital signal into a sequence of characters.

What I have only seen (in my very limited knowledge and experience with PLCs) for encoders so far is that encoders are used to determine the position, velocity and direction of motor shaft speed and other mechanical motion. For this instance, what would be the decoder? The textbook required for my class doesnt even contain the word decoder and I want to make sure I am heading down the correct path. Unfortunately, taking the course through a military distance learning program doesnt grant me an instructor for my course and no one to direct questions to.

There are generally two types of encoders.

First are those that indicate that one increment has passed, thus named 'incremental'. They do not indicate where in a turn (or full range of measurement) the monitored item is - just that an increment has passed. A 'quadrature' type signal, by use of two channels (typically named 'A' and 'B'), also indicates in which direction the increment occurred. Some encoders provide a single pulse indicator of one full revolution. This is typically labeled the 'Z' signal.

The 'decoder' for this type signal would be some type of 'counting' function. A specialized 'high speed' input may be utilized for this function. This may be also coupled with a signal (probably external to the encoder) which indicates a known position. This is known as the 'home' input. The 'decoding' function may provide an automated method to utilize this 'home' signal and then, utilizing the 'A' and 'B' signals, provide an accurate position number.

The second type of encodes are 'absolute'. These provide a full indication of the position. This indication may be transmitted by multiple parallel digital signal channels. To provide protection against misreading when changing from one position to another the Gray Code (named after the inventor - Frank Gray) method may be used. Another method of transmitting the absolute information may be by a predefined serial message. 'SSI' is one of these methods.

The 'decoder' for these absolute encoders is the proper receiving device, a multi-line digital input for the first type. This may be used with a 'Gray - to - Binary' decoding operation to convert the Gray Encoded bit pattern to a usable number. Typically a dedicated hardware solution is used for the serial message type of encoder.

Edit - As you have indicated this is only a discussion of one type of 'encoding' - that is - 'position encoders'.

bernie_carlton is talking about the encoder/Decoder that you find in industry typically mounted on the back of a motor or other rotating equipment
For PLC Programming that is all you need to know.

Hodak04 said:

I understand the general purpose for an encoder used to put a sequence of characters (letters, numbers, etc) into a specialized digital format for efficient transmission or transfer. And that the general purpose of a decoder is for the conversion of a digital signal into a sequence of characters.

Click to expand...

That is correct for transmission of data via a communication protocol
Typically in an industrial situation you do not need to know about it - if it is used in a communications protocol you are using it is hidden from you.
one example of this is a compressed file (eg ZIP file) where large quantities of the data to be transmitted are very similar
- here the encoder is the putting the file into a Zip file the Decoder is extracting it from the Zip File