RS-232 standard?


Thread Starter


Is it my imagination or is RS-232 a de facto data comm standard for PLC's? If so, why?

I recently purchased two small fixed I/O PLC's of different manufacturers and each had a 9-pin D-type connector for 232 data comm. Neither plc offered a more contemporary data comm alternative such as 485, 488 or even better, USB. Since the programming will be performed on a USB-based machine and since the 232 to USB adaptor can frequently add an additional layer of aggravation, why does rs-232 continue to appear on new PLC's? Thanks.
USB to some extent, and especially Ethernet, are becoming common on current generation PLC's.

RS232, or at least on option for it will probably be on PLC's for some time, as some of the things the PLC needs to talk to are standardized on RS232, but many of those things are shifting to Ethernet as well, and that trend will no doubt continue.

Bob Peterson

RS485 is almost as old as RS232. They have both been around a long time. The problem is both are relatively limited in what data rate they can support. RS232 is not especially noise resistant either.

A fair number of PLCs no longer come standard with RS232 ports, and it is hard to find a laptop computer these days that has one, although most times a USB to RS232 adapter works OK (other than for AB DH485 protocol).

USB and Ethernet seem to be the standard for anything other than the very low-end PLCs these days, although some still have RS232.
As mentioned this is changing, but I think you are not thinking the problem through fully - there is no 'standard' for how a USB link works. On a Windows PC (or tablet) the complexity of the USB link is often hidden in complex plug-n-play handlers. so dozens of MB of custom code select the correct USB driver from many vendors.

Plus what if you want to link a 900Mhz radio modem? USB is unlikely to work, or if it does, it only works for brand X but not Y or Z. RS-232 works on all brands.

That said, I agree with others that things are changing. RS-232 is very sensitive to grounding and surges, plus as you point out, few notebook include RS232 anymore. If one wants a safe-secure RS-232 with a few thousand volt isolation, then Ethernet is a far cheaper solution which include 1500v transformer isolation. Plus even low-end devices now often have ARM 32-bit or equivalent processors, so most can handle Ethernet as easily as RS-232.

In fact, several product managers at large PLC firms have told me that on NEW products, they layer comms like this:
1) ultra low end - USB to a Windows PC via cable purchased at extra cost. Use for radios or anything but Windows PC voids warranty.
2) all other ranges - Ethernet is default
3) if anyone wants an RS-232 port, they must buy an add-on card which features a fully isolated, surge protected RS-232 port.

So they see RS-232 as something to avoid in new designs. So while USb is coming, it has been demoted away from being a 'general PLC interface' to being a dedicated 'programming' port designed for Windows PC. It may work on other things, but that is an accidental benefit (or liability).

- Lynn A Linse,

Curt Wuollet

Call it installed base or least common denominator, but RS232 is still prevalent for some very good reasons. It's well understood, and has been widely deployed. The connectors are inexpensive and the wiring is not too critical. And the fact that it is old is one of it's advantages, compared to proprietary flash in the pan offerings that seldom interoperate and as a result achieve only a meager following.

It spans nearly the entire age of automation. It's low cost and would still be the standard for PC's if there weren't a concerted effort by the duopoly to kill it.

Their replacement USB, is only good for a few feet., 485 is of the same generation and 488 is an instrument bus. RS 485 is an excellent solution for automation, many of the big names are based on or derived from it. The spendy purple hose carries RS485 on steroids.

But. in short, RS232 is still around because automation folks are pragmatists, and there's really nothing _better_ for automation purposes to replace it. I predict that RS485 will be around even longer, perhaps under different names, but from an engineering standpoint, RS485 hits all the buttons. RS232 and RS485 are Open and you can see the result. Closed systems never become ubiquitous, they are self-defeating.



Bob Peterson

While Curt is not entirely wrong in his assessment, RS232 has some issues for use in industrial environments that can't just be ignored.

One is that it is not especially noise resistant.

Another is the distance limitations, although that is highly baud rate sensitive.

Neither of these is especially bothersome for small projects (say <50 I/O) that are what the lower end PLCs are used for. It's often a real nuisance for larger projects. I think that is why you see lower end PLCs with RS232 ports still. The software protocols are a lot simpler in many cases so that reduces the raw power the CPU needs to deal with communications.

A third issue that comes to mind is speed. RS232 can run as fast as >100 kbaud, but most hardware implementations and other practical considerations make the limit more commonly something like 19200.

Lastly, for whatever reasons, personal computers rarely have built-in RS232 ports on them anymore. Granted one can add USB to RS232 adapters, or pay extra for a PC with it built in, but why bother if you don't have to.

Ethernet and USB are rapidly becoming the de facto standard interfaces because they avoid these problems that are inherent with RS232. They are also inexpensive to implement and well understood. The cables are cheap and available at every Walmart.

USB makes a lot of sense to me as a local programming and configuration port, while Ethernet makes a lot of sense for general purpose communications, and even interlocking of multiple units.

I suspect as low end PLCs start to get a little more oomph to them, you will see fewer and fewer RS232 ports and more USB and Ethernet ports on the lower cost PLCs.
Much thanks for all of your input, effort and experience. You've provided a range of perspectives and considerations I'll incorporate into future PLC and related hardware purchases - Jon L.

Curt Wuollet

You forgot that it's point to point and some recent implementations have used 5 volt signals. Still it's much more flexible than USB.

If you need noise immunity and distance at speed, you can easily and inexpensively convert to RS485, although I hate adapters. Ethernet is a reasonably good solution, being transformer isolated and ubiquitous.

But, what I don't understand is why I can get an Ethernet card for $10, but it adds $100+ to the price of a PLC.


> But, what I don't understand is why I can get an Ethernet
> card for $10, but it adds $100+ to the price of a PLC.

Everything industrial-rated is far more expensive, from the PCB components to the layout requirements and enclosures. Add to it the lower volumes and the prices are just plain higher.

Moral of the story - verify a part is industrial-rated before putting it in an industrial environment, otherwise you didn't really just save money.
Just asking around the office none of our guys have seen a USB port on a PLC. You need a simple interface to set up the processor to communicate over Ethernet. RS-232 is as simple as you can get, a multi-meter test identifies the signal pins, a few wires soldered into a connector and you're off to the races.

I haven't seen any DIY USB connectors, are they available?

RS-232 will disappear some day but not in the near future.
> Just asking around the office none of our guys have seen a USB port on a PLC.

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