Poorly designed Industrial Gear (Beijer Electronics RS422 Adapter Cable)

[ratdude747]

Here's my frustration for today... coming from work.

Background: I'm an engineer at an automotive component supplier; I don't do a lot discreet electronics work in my position (I mainly deal with PLCs, Robots, automation, and the like), but today was a brief exception.

One of our Maitenance techs pulled me aside today because yesterday he managed to roast a couple of PLC serial cables (RS422 to RS232 adapters used for old Misubishi PLCs). Took the one what was confirmed to be dead back to my desk and did some reverse engineering, with the hopes of deriving a pinout so I could test the PLC for a power supply issue on the programming port.

While I was able to derive such, and determine that while the 5V supply is a tad high (5.12V), all of our PLCs that use such are also reading 5.11-5.12V. Not a PLC issue, at least not one that's active at the moment.

What I did find, though, is that these cables we've been using for decades (made by Beijer Electronics), at least the newer one that I tore apart, aren't particularly well designed for the "industrial" build quality/price they're made to. Specifically, they ran the 5V connection from the PLC directly to the three chips with zero protection circuitry. The three chips are a TI MAX202 (RS232 Line Driver), and two TI SN1579B (Differential pair driver/receiver pair). The former is rated for up to 6V supply, but the latter are only rated for Vcc max of 5.25V  . Sure enough, one of those seems to be the fried chip based on such warming up very quickly when powered  (the adapter did overheat when it failed, I'm told)

Maybe I'm being a bit harsh, but this isn't exactly a "Won Hung Lo" product... the "new" cables are made in china, but the older ones were made in Sweden. I haven't had one of the old ones apart to see if the design is any different. On the new ones (and likely the old ones), they even used genuine (or genuine-looking) TI chips. Spend dollars to use name brand parts, but save cents by running the power supply directly into the chip.

Not to say this issue is what killed the cable, but for something built for hard industrial use, such an oversight is a plausible explanation.

End Rant.