Why 5V encoder is used instead of 24V if pulse freq is high

Hi;


I have noticed that at our machines, when pulse frequency of encoder is high, OEM uses 5V encoders instead of 24V. What could be the reason?

Improved noise immunity. 5vdc TTL vs 24vdc HTL design. With TTL you have A and B pulse along with the inverse signal (negated, A' and B'), and you can evaluate them against each other. With HTL you only have A and B.

CommissioningMan said:

With HTL you only have A and B.

Click to expand...

If you check this manual:
https://literature.rockwellautomation.com/idc/groups/literature/documents/in/844d-in001_-en-p.pdf

You'll see that with the 7272 line driver, you also have the negated A,B,Z as well at 24VDC.

CommissioningMan said:

Improved noise immunity. 5vdc TTL vs 24vdc HTL design. With TTL you have A and B pulse along with the inverse signal (negated, A' and B'), and you can evaluate them against each other. With HTL you only have A and B.

Click to expand...

Can't you have A' and B' on 24V encoders as well? What prevents that with HTL vs TTL.

A few things that come to my mind when looking at 5Vdc vs. 24Vdc.
First is transition time. The time it takes to go from 0 to 5Vdc is much shorter than the time it takes to go from 0 to 24Vdc. Also at 24Vdc you need a system that can swing 48V (0 to 24 / 0 to -24).

The other thing involves noise. Whenever you go from driving current to off you are collapsing a magnetic field which generates its own current which will be induced on any nearby wires and look like noise. The bigger the swing the larger the effect. This also has an impact on PCB size. The lower the voltage the closer you can have PCB traces to each other and the smaller you can make the PCB without getting cross talk or inducing noise onto the PCB's ground plane.

There are a lot of "little details" that add up most of which favor lower voltages.

My best guess would be TTL logic draws more current, therefore would be more noise immune.

And, yes, HTL or TTL can have compliments. TTL is 5v line drivers, and HTL is totem pole transistors. Same, only different!