Re: a USB for the Apple II smoke test

[et472@FreeNet.Carleton.CA (Michael Black)]

The Wizard of Oz (wizard@emeraldcity.gov) writes:

>> Actually, USB has about as much in common with RS-232 as a
>> giraffe has with a grapefruit.
> 
> 	Bad analogy. A better one would be like what a bison has to do with a
> cow. The hardware protocols can't be a lot different. After all there is a
> direct path from RS-232 to USB. I bought a couple of RS-232 to RS-232-C
> (25 pin to 9 pin) adaptors. Friends have bought RS-232-C to USB adaptors.
> None of these adaptors required circuitry. It was all a matter of wiring.
> 

But what were they used with?  The "adaptors" that come with mice are
merely a rewiring of the connectors.  But the scheme counts on circuitry
within the mouse.  In other words, the mice have dual circuitry, and
the presence of a signal on one of the pins tells it to switch to
the other protocol.

A quick search doesn't turn up the voltage levels for USB, but I
suspect it is different from RS-232.  One level of incompatibility.

USB is differential (ie two lines carrying the same information, but
one line is the mirror image of the other), RS-232 isn't.  A second
level of incompatibility.

USB is a very fast data rate.  I don't think many if any RS-232
serial interfaces run that fast. A third level of incompatibility.

USB is addressed, allowing for multiple devices on the bus.  If the
code doesn't deal with that, it can't access the devices.  I can't
see a device saying anything until it gets some data to tell it
to work, and that's not going to happen with just any serial port.
A fourth level of incompaitiblity.

USB is bidirectional, RS-232 is unidirectional.  The former uses
the same lines for transmit and receive, the latter uses two separate
lines, one for transmit one for receive.  You have to combine the
two somehow, and then you have to toss in circuitry to check whether
it's possible to send or if you have to wait till someone else has
sent what it's sending.  That to me suggests something like packets
of data, again more complicated than is built into the average serial
port.  Likely another level of incompatibility.

USB is a serial interface, but is it the same protocol as used
in traditional serial interfaces?  Ie it's 8bits of data, with
stop and start bits, and asynchronous.  I don't know what USB
uses, but there's no reason to assume it uses the same protocol.
Likely another level of incompatibility.

In the original post, the poster was talking about plugging a header
into a similar connector on the Apple.  The chances of anything lining
up, even if none of the above applied, are pretty unlikely.  Even on
IBM type motherboards, there is not a complete standard for such things,
and here the poster is connecting two very different types of connectors.
You're far more likely to connect a data pin to ground than have
any success.

  Michael