Anybody know encoders?

I am trying to figure out what the effective difference between a simple "bidirectional" encoder and a "differential, bidirectional" encorder is.

Ok. I know that:

1) Bidirectional alone indicates that A & B signals are sent.
2) Differential, Bidirectional indicates that A, A-not, B, B-not, Z, and Z-not are sent.

But shouldn't I be able to replace a simple bidirectional encoder with a like encoder that is a differential, bidirectional encoder?

I ask because it appears that, in reality, it doesn't work when I connect it to my PLC.

Anybody out there know the theory?

Steve

The term "bidirectional" is only marginally relevant as all quadrature encoders are by definition can be decoded so that the direction information can be found.

What is more important to you is the distinction between "single ended" and "differential" signal lines.

Single ended typically uses a 24 vDC sink (or source) style transistor O/P to either pull down or up a matching source (or sink) 24 v input.

The advantage of this is that it is cheaper to implement. The downside is that to make the input fast enough to read the encoder pulses it becomes sensitive to noise.

The more desirable approach uses a TTL style 5vDC differential style driver that uses two lines per signal. This has numerous electrical advantages and performs better at high frequencies, plus being a differential signal, is inherently more immune to noise.

Your problem is that a system set up for single-ended encoder simply will not accept the signals from a differential encoder, and vice versa.

Usually most PLC encoder modules have jumpers on them that allow the module to be set up to accept signals from either kind of encoder.

Thanks PhilipW,

Your response is right on the money. We just found out that we could attach pull-up(?) resistors to each signal in the circuit to make the encoder work.

Unfortunately I don't believe the PLC module I am using (GE Fanuc IC693APU300 High Speed Counter Module) has the setup capability you are refering to.

You say that a system using the TTL style 5vDC differential style driver has numerous advantages. Would you care to clarify a little more on this? This is the type of info I could really use.

Thanks
Steve

I can think of two main advantages:

1. A TTL encoder output always has one leg of its output ON, ie a conducting transistor pulling up or down, so the output impedance of the driver is normally very low, and this reduces the induction of most noise in the cable.

2. Being a differential signal the input module is looking for a "polarity difference" to decide which state the input is in, and so any common mode noise that is induced will not affect the TTL input module's decision (within the specified common-mode range of the input.) ie the input module is able to reject most noise.

If you want a high count encoder ie > 1000 cpt to run at a decent speed say > 10kHz, then the differential style input is best practise, otherwise a single ended encoder will be acceptable for less demanding applications.

PLC people often trip up with encoder inputs because in order to function these inputs must have a wide open frequency response (>100kHz) which is quite different to normal PLC inputs which are filtered to 2-10 mS or so. For this reason you will far encounter more noise problems in an encoder circuit than all the other normal inputs in the same system.

To help understand encoders, these two technical bulletins from Encoder Products are worth a read...

"Specifying Output Types"

"Noise Suppression of Differential Signals"

beerchug

-Eric

It doesn't happen often, but...

Be a little careful with the term 'bidirectional' with encoders. While it is true that all A-quad-B encoders are bidirectional, not all bidirectional encoders are A-quad-B. Two other formats I have seen are pulse and direction output and one channel for clockwise motion and another channel for counterclockwise motion. These formats aren't overly popular these days but they are out there.

Keith

Other terms used for those described above are Line Driver and open collector.

Output types include open collector, open collector with pull-ups & line driver. Open collector types are low cost, have poor noise immunity & can have a maximum wire length of 10 meters. Line driver outputs have a high speed response, high noise immunity & can be transmitted over a length of 30 meters. A differential line driver has two signals for each of the outputs. Each signal has a compliment or inverse (mirror image) referred to as A not, B not & Z not. These can be transmitted 100 meters. Do not connect A not, B not or Z not together or to the + or- side of DC power.

Another place to get MORE THAN YOU WANTED TO KNOW ABOUT ENCODERS