Any help with this would be greatly appreciated. We currently have an Omron C28k PLC with a 4 digit BCD multiplexed thumbwheel. What I mean by this is that there are 4 digits (16 bits) of data from the thumbwheels multiplexed internally to 4 bits of data (4 inputs) going to the PLC. We are changing the User interface (multiple buttons, knobs, thumbwheels, lights, etc.) to a touch screen. Since it will not be cost effective to replace the PLC (cannot have the production line down that long) we were planning to use a touchscreen interfaced to a Direct Logic PLC then have that PLC output the button functions to the OMRON. My only problem is the thumbwheels. I can put a BCD number to the PLC via the HMI then have the PLC output that via a 16 bit digital output card. However the Omron is only recieving 4 inputs from the Thumbwheels. How do I multiplex these signals internally in the PLC code to send this data out on 4 bits. Maybe I am over engineering this so any help would be grateful. Thanks.

 

Write to a register. Tie each of the last 4 bits to an output. For example a rung with a NO contact for the 0 bit of the register and a coil for the 0 bit of the thumbwheel inputs. Done.

Regards

cww

 

There should also be a couple (or possibly 4, depending upon the thumbwheel type) of outputs from the C28K PLC going to the existing thumbwheel switch to select the digit (to multiplex it). You'll need to duplicate the response of the thumbwheel in the DL205.

Since the DL205 program won't react as fast as the thumbwheel hardware, you may have to play with (or add) timers in the C28K to make sure the data from the correct digit is on the lines and stable before reading it. This isn't easy to get working properly. Alternatively (and more securely), you may have to come up with a more robust handshake scheme (where each PLC signals to the other side when it is ready).

I have a feeling that when this is all over, all parties concerned are going to wish they just went ahead and replaced the C28K with a suitable new PLC. The key to projects like that though is to figure out how to do the retro-fit in small (and reverseable) stages.

 

Actually, Michael, if he wants to spend the money, he could make the DL205 EXACTLY like the thumbwheels, with four outputs per digit, and feed the IO common for the outputs for that digit from the C28 multiplex selector line. Then the DL205 doesn't have to react in software as the outputs are already set, and the C28 will see his inputs instantaneously the instant he sets his multiplex line. PLC Direct outputs are like $5 a point and you'd avoid having to add timers in the C28.