1964 Antique MODEM Live Demo

[Ethan Dicks]

On Thu, Jun 4, 2009 at 1:56 PM, Tony Duell <ard at p850ug1.demon.co.uk> wrote:
>> > Have you measured the voltage on the RS232 TxD pin in both cases?
>>
>> Tony is closest ;-)
>>
>> The USB serial adaptor only seems to put out a really tiny current.
>> More than a few microamps (!) into a load makes it put out a train of
>
> In which case it's not even close to being RS232 compatible (I seem to
> remmebr an input impedance of the order of kilohms being OK for RS232
> receivers).

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.

>> 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.

> I saw this years ago on the RS232 card in my Philip P850. That card had a
> potted module on it to provide +/- 12 V (for 1488s) from the 5V logic
> supply. Mione had failed (I assume dried up capacitors) and output some
> interesting waveforms. The result, of course, was garbage on all the
> RS232 outputs.

We had something like that happen with our Qbus COMBOARDs when an
early customer installed theirs into a Qbus backplane that didn't
provide -15V (which is, ISTR, most of them).  Things didn't make sense
until we figured out the lack of negative voltage to the 1488s/1489s.
Every Qboard after that had a DC-DC converter on it (and I still have
dozens of those, new on the slab of foam).

-ethan