Re: [avr-gcc-list] Butterfly RS232?

[Joerg Wunsch]

"Mike Young" <address@hidden> wrote:

> Looking at the schematic for the Butterfly, it's not readily
> apparent how it manages to translate levels for outbound data.

Well, it's a bit tricky, but it can be seen.

The incoming RxD is normally held at negative level by the external
transmitter (i.e. the PC).  This negative potential is used to charge
a capacitor (1 µF) via a Schottky diode.  The butterfly's transmitter
uses the stored charge as a negative bias to produce its own negative
level on output.

Of course, this is a bit tricky and cannot guarantee enough negative
level to match the RS-232 or V.28 specs (in particular not if the
sender sends signals with a lot of positive bits), but then, most PCs
these days don't care as they only use a single +5 V supply for the
receiver side, so 0 V on input (which is in V.28's "undefined range")
would normally be considered to be like a negative input voltage
anyway.

-- 
cheers, J"org               .-.-.   --... ...--   -.. .  DL8DTL

http://www.sax.de/~joerg/                        NIC: JW11-RIPE
Never trust an operating system you don't have sources for. ;-)