Encoder output types

Orn Jonsson

Join Date
Apr 2002
Boise Idaho
Hi Dudes,

I have an application where I need to read encoder pulses using a high speed PLC input. The input is 24VDC sink or source.
The app. is for counting the number of rotations on a filling auger so you can stop the auger rotation within a fraction of a revolution.

My question has to do with the terminology of encoder output signals.

Can't seem to figure out what the difference is between;
1) Line driver,
2) Push-pull,
3) Totem pole...

Any help is appreciated, (plain English please)



Differential Line Driver Output

A differential output refers to the fact that each channel has a complement channel, i.e. Channel A and Channel A not. A differential line driver is used to help increase noise immunity. A differential line driver also allows you to sink or source more current then a Totem Pole output. A differential line driver will work both with a sinking or sourcing circuit. It can also help in increasing the distance in which a signal is transmitted.

Push Pull Output

A Push Pull output is an output that allows you to connect either a sinking or sourcing circuit. This type of an output allows you to sink more current than a Totem Pole output and follow the input voltage. A Push Pull output is chosen when an Open Collector output will not work with the controller that is connected to the encoder

Totem Pole Output

A Totem Pole output is essentially the same as a Push Pull output; however, it is the terminology commonly used when referring to a TTL device. The major difference between it and a Push Pull is the amount of current that it can sink or source. The Totem Pole output is going to sink/source less current then a Push Pull output is capable of sinking or sourcing. The other major difference is the output voltage between the two. The Totem Pole is a 5V DC signal only, where the Push Pull will follow the input voltage.

Open Collector Output

An Open Collector output is a NPN transistor. A NPN transistor allows the sinking of current to common. It can be thought of as a switch that allows the circuit, after the load, to be connected to common. This means that a source is required for the output to work. A supply through a load must be connected to the output, otherwise the NPN transistor is simply creating a path to common, i.e. a dry contact. Therefore, if you were to measure the voltage at the output of an open collector that is not hooked up to some supply you would not see a change in voltage. The voltage should be measured across the output load to determine if the open collector is working properly.

Quadrature output

Quadrature output refers to the fact that the signals A and B are separated by 90 degrees of phase shift with A leading B or B leading A depending on the direction of rotation. It does not mean that the output will be 4 times the amount of the Pulses Per Revolution of the encoder. The fact that the signals are 90 degrees out of phase enables the controller to determine the direction that the encoder is spinning. You must use both the A and B signal to have a quadrature output and to get X2 or X4 logic
Push Pull ...

??? Am I wrong in the beleif that since Push Pull deals with both signals they are safer because you cannot short it to ground when you connect them, hence when you chose this type, you are less "nervous"???
Pierre, I am not sure what you mean. Push Pull is a term that goes back to tube usage, it allows the use of current flow in either direction..ie sinking or sourcing devices can be used. Normally when refering to these devices (npn/pnp) we are talking 5 to 32 volts DC. With an NPN(sinking) output the wire to the device should be the same as ground so no it cant short to ground, but the opposite is true of with a PNP (sourcing) output. This of course assumes that the DC power supply common is tied to actual ground.

Review of Sinking (NPN...ie Not Pointing In) and Sourcing (PNP...ie Pointing IN).
Sinking and Sourcing inputs simply refer to the current flow in a transistor. This means that they require a voltage and a load to operate. A sinking input requires the voltage and load to be present before connecting it to the circuit. This means that it is "sinking" the current to ground for the circuit. A sourcing input must be before the load in the circuit. This means that it is "sourcing" the current to the circuit. Voltage and a load must be present in either case to detect a voltage change at the input. The same is true for sinking or sourcing outputs.


Terry made an excellent post recently covering this.
Thanks a bunch for the info

It looks like I will go with the Push-Pull output into a sinking 24VDC PLC Input.

Thanks rsdoran for a detailed answer. I appreciate the help from y'all.

Definitions for push pull and totem pole


Usually these term imply that there is a positive and negative DC supply.

Ron, what you are showing is not push pull, it is push or it is pull but not both. The push pull amplifiers are just a way to implement differential line drivers which I recommend for higer performance applications.

Orn, stay away from the open collector devices and input cards. It is very hard to make a signal swing many volts quickly like a open collector must. Use a differential signal where there is is A+ A- B+ B- Z+ Z- and the + and- are differential signals. A differential line drive only needs to change the voltage by a little so the difference between the + and - lines goes from postivie to negative. These small change can happen much more rapidly than an open collector changing the voltage by 10 volts or so. New buses are being designed arond LVDS ( low voltage differential signal ) because it allows for faster transmission rates. The key point is the differential is good, open collector is .... not so good.

Motion controllers differential receivers for the inputs and can handle 8-10 millions counts per second.
Peter Nachtwey said:
Motion controllers differential receivers for the inputs and can handle 8-10 millions counts per second.

As an aside, is there a protocol whether 10MHz (for instance) refers to per channel, or 10MHz is a combination of A&B quad?

Very good question

paraffin power said:
As an aside, is there a protocol whether 10MHz (for instance) refers to per channel, or 10MHz is a combination of A&B quad?

We use counts to indicate the number of counts the motion controller receives per revolution. A 4000 count encoder normally has 1000 lines per wheel and is usually called a 1000 line encoder. Not all encoder decoders can see the 4 phases like motion controllers do.

We use lines to indicate the lines on the A or B phase. We use counts to indicate the number of counts the motion conroller sees. I wish everyone would be consitent with these terms. I know I am sloppy at times too because I usuall specify counts not lines. I don't care how the counts get to the motion controller or how they are generated. All I care about is counts per revolution or counts per distance unit. I need to think twice about converting the counts back to lines when I talk about encoders.

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