4 Jun
2009
4 Jun
'09
10:56 a.m.
On Tue, 2009-06-02 at 18:54 +0100, Tony Duell wrote:
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).
Okay,
let's try again. I have a circa-70s 300-baud modem, that will
generate incorrect tones when it's plugged into a USB-to-serial
Waht's the make and model of the modem?
converter, but not when it's plugged into a
real serial port.
Have you measured the voltage on the RS232 TxD pin in both cases? 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.
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.
I will have to admit I've still not fixed it. There are pluggable jumpers
on this board to disconnect this module and instead power the 1488s from
pins on the serial cable connecotr. I did that, and provided a simple
+/-12V unregulated supply. Works fine...
-tony