[ColorForth] Why I'm Here

[Roger Ivie <IVIE@xxxxxxxxxx>]

Arthur Green wrote:
> The device I look to plug into the USB interface [eventually] is not a USB
> device. It's a "standard" serial device, so that was my real concern. 
> Assuming I "match" the electrical differences of both devices, 

That's a fairly large assumption. If by "standard" serial device you mean
a normal, garden-variety RS-232 thingie, the USB port is the wrong place to
put it.

RS-232 is designed to deal with single-ended signals ranging from +-3V to 
+-15V. This is radically different from the differential signals with at 
least a 200mV difference used by USB. It's going to take some sort of
circuitry to paste the two together, and I suspect that would be better
spent on a UART for the machine you're trying to use than pulling your
hair out over USB.

There are products in the Mac marketplace to convert USB to
RS-232. There are plenty of hits over at http://www.macmall.com/ if
you search for "usb serial". Bear in mind that such devices are USB
devices, so you have to speak via whatever USB class profile they
use.


> I'm back to 
> original
> question. Can I plug this device in, without any "additional" circuitry
> and just read and write raw from the USB ports, much like a standard 
> serial port?

I do not believe so. The whole point of the USB controller is to do a
lot of the USB housekeeping for the CPU. There's an awful lot of
intelligence sitting between you and the USB wires; I don't think you
can get directly at the wires.

> Thanks for the information. Actually, this is pretty useful, but for the
>  moment, I am just trying to take advantage of what already existed on 
> the motherboard I will be using. So, I guess I have to look into that some 
> more, somehow.

Tell us more about the motherboard you're using. Most motherboards I've
seen have serial ports on them. Failing that, there's usually a parallel
port; that would be much easier to interface a serial device to (you'd
have to implement a software UART and have some level translation 
circuitry out there...).
-- 
Roger Ivie
ivie@xxxxxxxxxx
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