Alright, so let me tell you about this whole “SCL map” thing I wrestled with not too long ago. It wasn’t pretty, let me tell ya, but sometimes you just gotta make things work, right?
I was plugging away on this automation project. You know the type – starts off simple, then the scope just balloons. Anyway, I found myself needing a way to quickly look up some parameters based on a text identifier. Think of it like, you have a machine part ID, and you need to grab its specific speed setting or a calibration offset. In most half-decent programming languages, you’d just reach for a dictionary or a hash map. Bing, bang, boom, done in two lines.
But SCL, in the Siemens PLC world? Oh, it had other ideas. My first thought was, “Okay, surely there’s a built-in for this. It’s 20-something, not 19-something.” So, I did the usual: dove into the manuals, poked around online forums, asked a couple of old-timers. Nothing. No straightforward, out-of-the-box map or dictionary type. I was a bit stumped, to be honest.
My first couple of attempts… or how I wasted a Tuesday
So, necessity being the mother of invention, or in my case, stubborn persistence, I started cooking up my own solutions.
- Attempt 1: The good ol’ array of structs. My first “brilliant” idea. I defined a struct with two fields: a STRING for the key and whatever data type I needed for the value. Then, I made an array of these structs. To find something, I’d just loop through the array, comparing keys. Simple, right? Yeah, simple and slow as molasses once you got more than a handful of entries. The PLC scan time started creeping up, and you know that’s never a good sign.
- Attempt 2: Trying to be clever. Then I thought, “Okay, arrays are slow for lookups. What if I get a bit fancier?” I vaguely remembered something about hashing from way back. Spent a good chunk of time trying to whip up some basic hashing function that would convert my string key into an array index. Let’s just say debugging that on a PLC, with its limited tools, was an exercise in pure frustration. Most of my “hashed” keys ended up colliding or pointing to the wrong place. Total mess.
I remember this one time, on a completely different project, we had a similar data lookup problem. The lead guy, super smart but sometimes overly ambitious with PLCs, was convinced we could implement some kind of advanced B-tree search directly in ladder logic. We spent days on it. The final “solution” could barely handle ten items and occasionally, just for kicks, it would return a random value. Good times, huh?
What I eventually settled for
After banging my head against the wall, I circled back to the array-of-structs idea but with a few… refinements. For some parts of the system where the keys were fairly static and didn’t change often, I’d sort the array once at startup (manually, or with a clunky sort function I wrote) and then implement a binary search. Still not as slick as a real map, but way faster than a linear scan for larger sets.
For other parts, where the data was more dynamic or the sets were smaller, I just stuck with the linear scan through the array of structs but kept the arrays really small, maybe breaking down bigger logical maps into several smaller physical arrays. It felt crude, like using a stone axe when you know chainsaws exist, but it worked. The machine did its job, the client was happy (mostly because they didn’t see the code!), and I could finally move on to the next fire.
It’s just one of those things in this field. You have these powerful controllers, capable of amazing things, but then you hit these weird limitations on basic software constructs. You end up spending more time wrestling the tool than solving the actual problem. But hey, I guess that’s what keeps us on our toes, right? Making things work with a bit of know-how and digital duct tape. That’s my SCL map story, anyway. Hope it helps someone out there feel a little less alone in the struggle!
