I have test equipment with optically isolated RS232 ports.
Seems modern RS232 ports do not support +/-12V.
They all seem to be less than +/-8V, many only +/-5V.

Anybody know of a cheap source for +/-12V RS232 PCI serial boards?

What ever happened to 1488/1489 serial drivers?

On Tue, 26 May 2009 08:55:15 -0700, Rumpelstiltskin <r...@abc.biz>
wrote:

>I have test equipment with optically isolated RS232 ports.
>Seems modern RS232 ports do not support +/-12V.
>They all seem to be less than +/-8V, many only +/-5V.

A +/-5 V transmitter is not RS-232, a transmitter generating +/-6 V to
+/-25 V might be RS-232, 
a receiver with threshold levels between +/-3 V is acceptable.

>What ever happened to 1488/1489 serial drivers?

These chips require some uncomfortable voltages like +/-15 V, which
are not readily available in current 5 or 3.3 V systems.

Paul

[Apologies if this is double posted]

On 2009-05-26, Rumpelstiltskin <r...@abc.biz> wrote:
> I have test equipment with optically isolated RS232 ports.
> Seems modern RS232 ports do not support +/-12V.
> They all seem to be less than +/-8V, many only +/-5V.
>
> Anybody know of a cheap source for +/-12V RS232 PCI serial boards?
>
> What ever happened to 1488/1489 serial drivers?

RS232 minimum voltages have progressively got lower with each new
revision of the standard.  Now +/-5V is the minimum.  From memory
that is at the receiver end though, so if a card is only giving 5V
that doesn't allow anything for losses but it is still within spec.

Have you considered dropping in an active buffer between the PC
and the isolator?  For moderate speed RS-232 this needn't be
difficult.  Back of an envelope calculations show a TL084 used as
a comparator will slew plenty fast enough over 24V for 38400,
though you will probably want to artifically lower the input
impedance to give a degree of noise resistance.

Of course, the driver will be fairly simple to knock up, but
providing a +/-15V supply is going to be cumbersome.

-- 
Andrew Smallshaw
a...@sdf.lonestar.org

> Anybody know of a cheap source for +/-12V RS232 PCI serial boards?
>

B&B Electronics (www.bb-elec.com) has all types of serial stuff: PCI, USB, 
PCMCIA; 1-port, 2-, 3...8-port, optically isolated or not.  They also have 
stand-alone isolators, some of which are port powered.  Reliable company, 
most products ship from stock.  I have no direct relationship with this 
company, but I've been a completely satisfied customer for the past decade 
or so.

> What ever happened to 1488/1489 serial drivers?

A lot of designs use the MAX232 class of devices, with on-chip charge pumps 
(using external capacitors) to generate the RS232 mark and space voltages. 
Technically, +/- 5volts is acceptable (barely - the spec allows down to 
+/-3), but most of these charge pump devices seem to run around +/- 9 volts 
or so.
--
Mark Moulding