[PLUG] USB-to-Serial Converter?

Aaron Ten Clay aaron at madebyai.com
Wed Jul 4 04:49:42 UTC 2007


On Tuesday 03 July 2007 17:37, Tony Schlemmer wrote:
> On Friday 29 June 2007 13:08, Eric Wilhelm wrote:
> > # from Tony Schlemmer
> >
> > # on Friday 29 June 2007 11:43 am:
> > >> I suspect the reason is that USB can deliver significant power
> > >> (e.g., enough to power a hard drive) along with signaling, while
> > >> RS-232 and PS/2 jacks cannot -- but that's just a guess.
> > >
> > >IIRC RS-232 delivers 12 volts over the line.
> >
> > Voltage can apparently vary between 4-15 (up to 25!?).  It seems that
> > most laptops put out about 5.
> >
> > Current (and therefore power) is very limited though (maybe 5mA.)
> > Compare to the default 100mA at 5V of standard-compliant usb bus power.
> > That's roughly 0.025W vs 0.5W -- you'll definitely be talking about an
> > externally powered device.
>
> [snip]
>
> I'm dated in my RS-232C knowledge and have only begun using a laptop 4
> years ago. I've never had any serial ports on either of my laptops so I
> could take a voltage measurement but I assumed it was 12 volts like it was
> years ago.. I've been modem free for 7 1/2 years now so I don't have any
> use for serial ports anymore.
>
> I do have an old Palm Pilot V that I still use and I did have to buy a USB
> to DB9 cable to use KPilot. Linux recognized the USB Serial port as soon as
> I plugged it in. I also use it for my GPS as well and can use "gpsdrive"
> with it.
>
> > --Eric
>
> Tony

As far as RS-232 goes, I've observed most devices output +/- 10V. I believe 
the spec calls for a minimum swing of +/- 7V, but I could be wrong. The spec 
is from days when RS-232 lines were run hundreds (thousands?) of feet long 
and needed a large differential to overcome noise, etc. I have observed many 
UARTs these days are tolerant of 0-5V because laptops tend to generate these 
voltage levels. Although I don't think this is technically called RS-232 
anymore because it doesn't fit the spec.

Also I believe the spec calls for a ground connection between the devices, 
relative to the signals on the line. (meaning this need not be a system 
ground). So connecting the ground on a USB adapter's RS-232 side to the 
ground on the USB side could cause it problems, if the spec works how I think 
it does.

If you have a device (like a USB serial adapter) generating 0-5V, you can 
build a simple circuit to drive those voltages to "proper" RS-232 levels with 
free samples of Maxim Semiconductor's MAX233A which doesn't even require 
capacitors for the charge pump.

-Aaron
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