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Al Williams

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USB, Too

September 26, 2010

I'm always amused at how much of my career has had RS-232 in it. My first "real" job was at a company that made multiport serial switches and we had 1488 and 1489 chips that did the RS232. These were workhorses, but required +/-12V supplies.

Everyone wanted to get rid of those blasted supplies. Dallas had a chip that "stole" voltage from the remote serial port so it only needed a 5V supply. But, of course, two of the chips couldn't talk to each other and you couldn't do full duplex, either since to transmit you needed the receiver to feed you a steady voltage.

But Maxim had a neat answer: the popular MAX232 chip uses a clever capacitor switching scheme to generate plus and minus voltage from a single 5V supply. Such a clean answer, that these chips took over -- or at least versions of them (I usually use the ones from TI -- they cost less). They also made a different chip that didn't need external capacitors, but that never really took off -- probably because four capacitors are cheaper than the price difference.

Today, everyone has gone USB. But a lot of embedded systems that use USB still look like RS232 internally. FTDI was an early innovator in making chips that are "fake" RS232 ports for USB. I prototyped a converter with one of their early offerings and it needed quite a few parts. More recent chips don't need much other than the chip, a connector, and maybe a crystal. Others have jumped on the bandwagon (I like SiLab's chipset which is similar).

I noticed in a recent press release that Microchip is now providing similar types of chips. I had them send me a couple of samples. I doubt they are really much different from the FTDI or SiLab offerings -- looking at the datasheet they look pretty similar. But the advantage to Microchip -- judging by their other products -- is that they are available. Everywhere. All the time. I can't remember the last time I wanted a PIC microcontroller and found out they were on allocation or were otherwise unavailable. I painfully remember that with some other processors.

The Microchip USB UART chip is US$1.40 in quantity and should be highly obtainable. Now if only someone would make one that would plug into a MAX232 socket.... hmm.....

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