Sequential control - several questions from a newbie

[hjf]

Hi everyone. I'm new to all about PLCs. My only prior experience has been with Telemecanique's Zelio smart relays. I do have a solid knowledge in microcontrollers and computer programming so my point of view will certainly reflect this.

I'm starting out with Delta PLCs since they're available in my country and are inexpensive. While the PLC I ordered arrives I decided to start practicing.

I have tested many of the basic functions (latching outputs, etc) and I understand that. Now my question is more about sequential control.

Let's suppose I want to emulate Zelio's block named "CAM" which acts like a camshaft, given a data table like this:

0: 01001010
1: 10010100
2: 10011110
3: 11110001
4: 00000000

the block has an internal counter and pulse input. Then, as pulses come thru the pulse input, the output register varies according to the table, and rolls over.

How would I go about implementing something like this on a PLC?

I thought about having something like:
M0 to M7 wired to Y0-Y7
Preload (where?) the values to a table

And a routine that:
1. Reads the memory position of a given counter (indirect addressing?)
2. Writes this value to M0-M7
3. Increases the counter
4. Waits for a while
Goes back to 1

If you have any experience with computers or microcontrollers you'll notice this is the common way to do it in a CPU.

Thanks in advance.

 

[Eric Nelson]

I have zero experience with the Delta PLCs, but I had a quick look at one of the manuals, and found two 'drum sequencer' instructions, ABSD and INCD. These should operate similar to that CAM instruction.



-Eric

 

[hjf]

I have zero experience with the Delta PLCs, but I had a quick look at one of the manuals, and found two 'drum sequencer' instructions, ABSD and INCD. These should operate similar to that CAM instruction.

Yes, I've seen those instructions in the manual. The PDF I saw first had terrible Chinese to English translation and it didn't make sense at all. I found a newer version of the manual, with a new translation, which makes a lot more sense, but still a little difficult to understand.

I'm still trying to figure them out, but apparently those only allow one activation (or deactivation) of the "cam" for each "cycle". I need the outputs to activate and deactivate several times in a cycle.

Unfortunately, Delta doesn't seem to have a device emulator, and I don't have yet a PLC where to test.

 

[Eric Nelson]

You could OR multiple 'cams' together to fire the same output at different points in the cycle.

Looks like you can have up to 64 'cams' per instruction, and there's nothing saying you can't use the instruction multiple times (with different registers of course) to get more.



-Eric

 

[hjf]

Unfortunately, Delta doesn't seem to have a device emulator, and I don't have yet a PLC where to test.

Correcting myself: delta's WPLSoft does have an emulator - except, unlike microcontroller emulators where you just have to start it, Delta's seem to "fully" emulate a device: you have to download your code to it first, just like you'd do with a real PLC.

A short vid that explain how it's done:

https://www.youtube.com/watch?v=lGgtaIc4je0