thumb wheels (how do they work)

We have a very old Omron plc that has thumb wheels to enter data. (timers counters etc)

I have replaced some in the past and when I looked inside it just seemed to be different resistors.

We are now going to upgrade this plc and the gaffer still wants to use the thumb wheels as the people who use the machine are used to it???

Each one is wired back to in input but I cant work out how a differing resistance will give a different number value in the plc

Each thumbwheel is wired to one analogue input ?

All the thumbwheels I've dealt with in the past were just switches. Perhaps they're diodes or the resistors are part of some surge protection to make the contacts last?

L D[AR2 said:

Each thumbwheel is wired to one analogue input ?

Click to expand...

Digital not analog.

Most thumb wheels are BCD switches. The switches have four outputs that give a binary representation of the selected thumb wheel digit. For more than one digit a BCD format is used. Each digit requires four discrete inputs.

For Ronnie Sullivan this can only be resolved by an analysis of the original wiring and program. Theoretically thumbwheel switches, if the have an individual output terminal for each thumbwheel position could have resistors in an R/2R format to provide an analog signal. Guessing won't help.

digital means for each thumbwheel one output
and for all thumbwheels 4 inputs
analog means for every thumbwheel one analog input.

or when using resistors one analog input.
just put your fluke on it and measure what happens when you turn a wheel.
If you are planning to use a display then i would consider to make them virtual. a panel with touchscreen costing about 750 euros complete with windows etc.