why do plc's have so little memory still?

I was a production worker in a tire plant for a couple of years before I got into their maintenance tech school. I had some experience programming in basic and assembly so PLC class came easy for me. I had been building graphics and custom character sets in binary and knew hexadecimal well.

In class when I was introduced to the SLC 500 and PLC-5 and found out we were controlling multi-million dollar machines with "graphical assembly" (my nickname for ladder logic) and 16 k of ram, I was stunned at first. Really? We control ARFs and extruders with 16K of memory for ladder and data? My Atari 400 had 16K in 1982!

I later came to appreciate ladder logic and the reliability of PLC hardware.

+1 OkiePC

Peter Nachtwey said:

Imagine a FPGA based PLC that does everything in parallel and has a scan time in the micro seconds.

I think Siemens has a product that is a crude version of this.

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If there were only something like this where you could Do more! That would be pretty awesome. Be even better if it had software and a simulator that you could play with free of charge.

icky812 said:

If there were only something like this where you could Do more! That would be pretty awesome. Be even better if it had software and a simulator that you could play with free of charge.

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I'm confused. What does your link have to do with the text you quoted? Are you being ironic?

mk42 said:

I'm confused. What does your link have to do with the text you quoted? Are you being ironic?

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He's just pushing his product, he works for AD

Peter Nachtwey said:

Imagine a FPGA based PLC that does everything in parallel and has a scan time in the micro seconds.

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+1

In the future, the perfect industrial device will consist of:

microcontroller running real-time
FPGAs
application processor to do the non-time-critical things

There are already some boards out there that have an MCU+CPU pairing. Lot of CPU+FPGA boards too.

Epy said:

He's just pushing his product, he works for AD

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Actually I was just being ironic. The Do-more! PLC's use FPGA's. Not really pushing the product, just saying the technology exists and it is already being used.

Right, great for AD (seriously, not sarcasm). Just saying that we should have the cutting edge of technology available to us.

In some of my classes I get asked about that topic.

I also get asked why a Siemens SD-like card costs an arm and a leg.
My answer is that this card has a lot of approvals and certifications.
The SD card from ebay has no certifications whatsoever. Imagine an automation system in a ship or an offshore oil and gas facility failing because of that ebay memory card.
I think some PLCs can operate with a conventional SD card but there is a big fat note in the manual that states that an industrial grade SD card should be used.

I think the schneider SD card for M340 has to be formatted with a schneider software tool that will only format schneider sold SD cards, so they retain their ability to charge customers who require more memory and therefore have larger systems, and are probably making more money, more than those of have smaller systems.

Rockwell, Siemens, Automation Direct, Mitsubishi, and the lower capability Schneider-Electric lines probably shape 90% of people's perception on what a PLC is, how much it costs (low end for AD, high end for AB), and what they can do.

However, even within Schneider-Electric's portfolio you'll find things like PacDrive (formerly ELAU) that blow ControlLogix out of the water in every possible way. B&R is probably the best platform out there by any metric, including longevity of existing systems (you may have noticed all the calls for help on ancient B&R hardware as it trickles into the middle east and asia as 3rd and 4th hand equipment). Beckhoff is another modern platform, but it rubs a lot of people the wrong way by being WindowsRT based instead of VxWorks based.

I mean, compare ControlLogix 1756-L85E with B&R's X20CP1586. Then remember ControlLogix is still rack mounted and the X20 line is only the mid to low range of B&R PLCs. Rockwell has so many artificial limitations, you can actually connect to more PointIO using a B&R PLC than a Rockwell one, but that would be silly because X20 IO is faster, less expensive, more varied, more compact, and doesn't label the LEDs in columns and the wire numbers in rows (what the heck is wrong with you Rockwell?).

Industrial Grade vs. Commercial Grade

https://www.cactus-tech.com/cactus-difference/industrial-grade-vs-commercial-grade

Industrial grade RAM, SRAM, and flash memory is maybe 3X as expensive as commercial grade. Industrial grade SD, CF, and CFast cards have been available in multiple gigabyte sizes for nearly a decade (the holdup was the added control circuitry and coatings cutting into the actual physical space available for memory in these standard form factor devices).

RAM and SRAM are advanced for industry every 5-ish years by taking a stable consumer product and hardening it, so whatever a consumer device had 5-6 years ago should be in the latest controllers. Some PLCs are in the 10 to 15 years ago timeframe and this is more or less a symptom of warehousing outsourced components for longevity and then just continuing to use the warehoused **** for years and years until they run out because they bought way too much.

crawler009 said:

you can get a plc with several gigs of memory, the ab CompactLogix 5480 Controller. Enough space to store lots of data.
But most of the space is for Windows and data, not for your Logix (because that would be unnecessary)

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Where can you get those? Does a Sasquatch riding a Unicorn deliver them?

It's pretty funny that this was posted almost a year ago and the 5480 is still just picture on the AB website.

NetNathan said:

I remember AB PLC3 days....128MB memory module was over $10,000.

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And the CPU was about an 18" cube!