Rotary Encoder and pull down resistors

Hi experts,

I have a Rotary encoder I bought from automation direct, it is 24vdc and is 500ppr..the problem is my plc only has NPN inputs and this heavy duty encoder I already purchased has PNP outputs A, B and Z. I have done some research and decided I want to to creat a small pull down resistor circuit! the encoder is going to be connected to the motor gear shaft(a worm gear) and then connected to the pull down circuit thence to the plc inputs. the shaft speed is 140 RPM and I have 500 ppr thats 1.2 Khz/s. I have off the shelve 2 watts resistors 300 ohm which also been calculated to be fine (24/300)squared * 300 = 1.92 watts. the help I need for you guys is this (sorry for talking too much):

1- is this value good for accuracy since this encoder will be reading length and the tolerance of machine is +- 1mm. will I lose pulses..

2- the encoder is 10 meters away from machine; what is the best cable for this purpose ?

3- I will connect A output to negative through a 300 ohms resistor and B as well...to invert the outputs from PNP to NPN what is you opinion..I know the logic will be reversed; but hey so what ??? I just want it to turn on and off...but ofcourse I want to know what you think ?


lastly...thank you for you help . cheers

You should specify your PLC model.
Also, be aware that the encoder outputs must be capable for 80mA current (24V/300ohm).
Answering your questions:
1. No comments on accuracy, as you never specified the length per encoder turn.
High-speed counter of any PLC I know should handle 1.2kHz with no lost pulses.
However, maximum total frequency on ALL high-speed inputs may be applicable for most PLCs.
2. Any cable, as these will be digital signals of high enough current. No noise problems expected.
3. Count will be OK, but the reset pulse may be a problem on some PLCs.

Are you sure the encoder only has PNP open collector outputs? Most common 24Vdc encoders do have push-pull outputs, so you would not need any pull down resistors.

Had a similar problem a few years ago (existing, impossible-to-replace Omron encoder with NPN open collector outputs vs Siemens S7-200 PNP inputs) and got it working using 1k5/0.5Watts pull up resistors. Just check your PLC specs to find out what's the input current and the min (or in your case max) voltage for a logical 1, but also check the encoder's max output current.

Any shielded cable will do the job.

BTW, Sergei already mentioned, your PLC would still need to be equipped with HSC inputs, since regular inputs tend to have input delays of several ms.

Sparkz said:

...1k5/0.5Watts pull up resistors...

Click to expand...

US terminology is 1500 Ohms, half-watt. Just make sure first 3 color bands are Brown, Green, Red and you'll be fine.