Question About Linear Encoder Signal Conditioning

[derekj308]

Hi Guys

I just scored a 1um resolution linear encoder off ebay and I am unsure if I need some signal conditioning to interface it to my PPIC. My PPIC has PC817 opto-isolators on the inputs and I have the datasheet but I cant work out if the 817's will work with a RS422 signal voltage which is the output of the encoder. Basically I am having a hard time interpreting the datasheet of the PC817.

I am running steppers and I am just playing around with bringing an encoder signal into Mach so I can use it like a DRO. The ultimate plan is to use steppers with encoders and a G-Rex with firmware that supports closed loop.

The encoder is a RSF Electronik MSA 670.63 and its still 4 weeks away hence my research now.

Any help would be appreciated.

TIA

[Al_The_Man]

I would think that it would be best to condition the signal using a RS422 differential reciever first, most of the IC manuf. make them.
What PIC are you using? There are types that take Quad encoder inputs direct.
Al.

[derekj308]

Hi Al

I'm using an older version of a cnc4pc PPIC before LEDs were introduced (see attachment). The inputs of the PC817 are set up on the PPIC as 'connect by ground'. There is a 330 ohm resistor in series with the input circuit which is tied to 5V. When the input is bridged the circuit is complete and the optoisolator fires.

[Al_The_Man]

Derek, It is possible the line drivers in the scale may not be able to sink the current required by the Opto inputs, although it is only 15ma, You may have to condition the scale with a RS422 Line reciever.
The optos are set up for sink input so an open collector device or one capable of the 15ma drive will do.
It also depends on if you are intending to use both quad A&B signals or just set up as single channel.
Also with optos, there is a restriction on the operating speed of the device which limits high speed reading of an encoder etc.
Al.

[derekj308]

Hi Al

Can I use a 26LS32 and connect the TTL output of the reciever directly to the input of the PPIC? The datasheet says it can sink 50mA and I only need it to sink 15mA. It is this simple? For my application do I need to use the 3-state control pins?

The chips are cheap also which is a bonus. Cheap as chips LOL.

[Al_The_Man]

Should work, strap the enables 4=H 12=L.
Al.

[derekj308]

Hi Al

I will run into frequency issues with this encoder as I am currently limited to 22.5kHz (1/2 of 45kHz Mach speed). I think the input frequency limitation of Mach will be my upper limit for input frequency before the opto-isolators. The best rapid I could achieve with a 1um encoder based on 22.5kHz is 1350mm/min which is well under what I can achieve mechanically. Its good enough to have some fun with and when I get a G-Rex I wont have any input frequency issues.

Thanks for the help.