1964 Antique MODEM Live Demo

[Tony Duell]

> One wonders if the in-house product test was more rigorous than
> connecting two devices with a 2m cable and sending a few characters at
> 9600 bps, or more likely, the engineers said that the design as given
> wouldn't meet the formal spec without spending $0.25 on more robust
> components and were told to keep it cheap since it would probably work
> for most customers, or at least work well enough that few customers
> would return the item.

Or, given that many people seem to have problems with RS232 interfaces 
[1], they'd simply claim the cable was wired wrongly, or you haven't set 
the baud rate/parity/word length correctly, or... And of coruse support 
is non-existant, as it seems to be for all thigns these days :-(

[1] I don't seem to have too many problems. But then of course I don't 
just wire the cable and hope, I read the manuals and more imporatantly 
stick a breakout box or prootcol analyser on the devives first.

> >> spikes at about 20kHz more like a sawtooth wave, than nice neat pulses.
> >
> > Is it possible that the DC-DC converter is playing up? I asusme this
> > USB-RS"32 converter is powered from the 5V on the USB port so presumably
> > it contains some kinde of DC-DC converter to get RS232 levels, possible
> > inside a MAXnnnn IC.Perhaps that can't supply enough current to the drivers.
> 
> The USB serial devices I've seen have been a 1-chip design -
> presumably USB, UART, and level converters all on one die, with a few
> discretes for the DC-DC converter.  I don't remember seeing any
> separate Maxim devices in them.

Ah... I thought I heard of a USB-TTL level async serial chip (i.e. you 
need to feed the asynch side through a MAXnnn or similar) and assumed 
that's what was used in these adapters. Probably not.

-tony