[time-nuts] TBOLT communication

[Lux, James P]

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
>