4 ms·

Can a Pi match the sort of real-time performance of something like a Controllogix PLC?

by Causality1 5y ago

glouwbug 5y ago
It'll be infinitely easier to program than ladder logic, that's for sure
- Causality1 5y ago
  Seriously? I've hit "real" programming and bounced off but I spent a few months as a PLC tech and ladder logic was very easy.
cameron_b 5y ago
No, not on its own. What I've seen done for this sort of application is to stack the RPi on top of, say, an Arduino Mega or SAMD51 in team. The Pi handles database, network (NTP, HTML interface, update work) and the micro is sort of a real-time port expander. Think of an access control scenario ( doors not ports ) where you have n doors ( potentially 2n readers, either wiegand or directly RS485 ) you then have 2n contact closure sensors ( door position and request-to-exit ) and an output relay to control the lock hardware power. The real-time needs of this type of thing scale to super-wide while the database lookup scales separately. You keep a fraction of the user table on the micro controller, based on frequent use of the doors attached directly. Hang another micro controller when you've used up your I/O but connect it to RS485 doing the "same thing" with different labels/IDs. But the Pi just keeps the full list, the full log if wanted, and gives a target for your user update work and adding new doors. I think what people like about the Pi is the modularity and ecosystem. I can get a hat for relays and sensors and things (and a super-duper hat thing for door control if you dig deep enough) You have to engineer all of the reliability ( RTC, watch-dog timers, power state control, opto-isolation of inputs, debouncing of buttons, on and on...) but if you have more time than money, you can learn a fair deal about a particular field by building something rather than buying it. Then when you finally have horsepower at your disposal, you may actually appreciate the value of the readily-available devices built to perform such duties reliably.
- Causality1 5y ago
  I'd enjoy a good writeup on how and why simple processors make better real time controllers than vastly more powerful ones.

Can a Pi match the sort of real-time performance of something like a Controllogix PLC?

glouwbug 5y ago

It'll be infinitely easier to program than ladder logic, that's for sure

Causality1 5y ago
Seriously? I've hit "real" programming and bounced off but I spent a few months as a PLC tech and ladder logic was very easy.

cameron_b 5y ago

No, not on its own. What I've seen done for this sort of application is to stack the RPi on top of, say, an Arduino Mega or SAMD51 in team. The Pi handles database, network (NTP, HTML interface, update work) and the micro is sort of a real-time port expander. Think of an access control scenario ( doors not ports ) where you have n doors ( potentially 2n readers, either wiegand or directly RS485 ) you then have 2n contact closure sensors ( door position and request-to-exit ) and an output relay to control the lock hardware power. The real-time needs of this type of thing scale to super-wide while the database lookup scales separately. You keep a fraction of the user table on the micro controller, based on frequent use of the doors attached directly. Hang another micro controller when you've used up your I/O but connect it to RS485 doing the "same thing" with different labels/IDs. But the Pi just keeps the full list, the full log if wanted, and gives a target for your user update work and adding new doors. I think what people like about the Pi is the modularity and ecosystem. I can get a hat for relays and sensors and things (and a super-duper hat thing for door control if you dig deep enough) You have to engineer all of the reliability ( RTC, watch-dog timers, power state control, opto-isolation of inputs, debouncing of buttons, on and on...) but if you have more time than money, you can learn a fair deal about a particular field by building something rather than buying it. Then when you finally have horsepower at your disposal, you may actually appreciate the value of the readily-available devices built to perform such duties reliably.

Causality1 5y ago
I'd enjoy a good writeup on how and why simple processors make better real time controllers than vastly more powerful ones.