[time-nuts] TBOLT communication

[Predrag Dukic]

Lux,

my hardware experiences confirm that +-6V is marginal. Some devices 
work, some do not.
The problem is a charge pump chip inside the converter and not the 
Prolific interface  chip itself.
(It generates  ttl levels anyway).

All converters I saw till now are two chip solutions (usb chip and 
level translator with internal charge pump to produce +- supply.

Predrag




At 18:12 24.9.2008, you wrote:
>Well.. Non standards conforming RS-232 devices are hardly new..
>
>But.. The standard just says -3 to +3 is a "no defined behavior" 
>zone, and that the receiver should be able to accept +/- 15 V, with 
>a damage threshold of +/- 25V. (bear in mind, also, RS232C is in 
>1969, the latest is TIA-232-F, in 1997.. But I'm pretty sure the 
>voltages haven't changed)
>
>The standard also says that the transmitter should put out +5 to +15 
>for logic zero and -5 to -15 for logic 1.
>
>The other often broken rules are the ones about any pin needing to 
>be able to source or sink 30mA, and the one about any pin (Rx or Tx) 
>being able to tolerate shorts to any voltage from +25 to -25.
>
>It's true that in TTL days, the standard MC1488 driver and MC1489 
>receiver did typically work off 12V rails, and if you just hook up a 
>volt meter with no load, you'll get about 10.5 volts. The output Z 
>is 300 ohms though, if there's some load at the receiver end (and 
>there should be, the standard assumes 3-7kohms and 2500pF cable C), 
>there will be some voltage drop.
>
>BTW the MC1488 (at least the one from TI) can only source 9mA (typ)
>Also bear in mind that there's some voltage drop in the wiring, 
>accommodated by the higher swing on the spec transmit side vs the 
>reduced range on the receive side.
>
>In any case, +/- 6V swing is barely compliant.
>
>James Lux, P.E.
>Task Manager, SOMD Software Defined Radios
>Flight Communications Systems Section
>Jet Propulsion Laboratory
>4800 Oak Grove Drive, Mail Stop 161-213
>Pasadena, CA, 91109
>+1(818)354-2075 phone
>+1(818)393-6875 fax
>
> > -----Original Message-----
> > From: time-nuts-bounces at febo.com
> > [mailto:time-nuts-bounces at febo.com] On Behalf Of Predrag Dukic
> > Sent: Wednesday, September 24, 2008 6:10 AM
> > To: Discussion of precise time and frequency measurement
> > Subject: Re: [time-nuts] TBOLT communication
> >
> >
> >
> > I had the same problem with some other devices, and I took
> > apart several converters.
> > Most of them were unable to generate sufficient levels for a
> > proper communication with rs232.
> > Internal charge pump was generating  +-6V instead of +-12 or
> > +-15 that is normal for rs232.
> >
> > Putting additional +-12 V source on the psu pins of the
> > converter chip might help.
> >
> >
> > Predrag Dukic
> >
>
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