[Tfug] Tiny "serial console"

Bexley Hall bexley401 at yahoo.com
Sun Feb 16 16:30:25 MST 2014

Hi Harry,

[PM me when you know what your "free" schedule looks like]

On 2/16/2014 11:46 AM, Harry McGregor wrote:

> The big problem is a lot of USB "rs232" adapters don't use proper voltage.

Well, that's been the case with "real" serial ports for quite a while.
The "5 volt only" transmitters with on-board charge pumps were already
stressing the terms of the *original* EIA232 specification.  But,
manufacturers have continued to cut corners whenever they can
rationalize (often, "to themselves"!) such a change.

[I.e., moving to a 9 pin port already rationalized away much of the
original intent of the RS232!]

> They try and get away with +/- 5 volt instead of +/- 12 volt, and some
> really crappy ones try and do +5 volt and 0 volt, instead of going negative.

Or, won't drive a low enough line impedance (especially for longer

> You can find some USB serial adapters that have a DC/DC converter in
> them that do the full +/- 12 volt

But they still have timing problems.  I.e., many external "protocol
converters" rely on characteristics of a "genuine" serial port to
implement features (via software) that might otherwise require
additional (though trivial!) hardware.

E.g., when talking to a party-line (485) bus, often the modem control
signals are used to control the "direction" of the bus transceiver
in the (external to the serial port) adapter.  I.e., "It is now time
for *me* to use the bus so I want to enable the transmitter in the
adapter by asserting <some_modem_control_signal>".

When you put another protocol in the middle (i.e., USB) between the
processor and the actual serial port hardware, you add uncertainty
and timing skews/variations to this that the software (in the PC)
isn't aware of -- it *thinks* it is talking to a real UART!

Or, the driver fails to completely virtualize the UART so software
that goes looking for a particular "UART" doesn't *find* it!

[A fair bit of work went into making the "enhanced/deep-buffered"
UARTs compatible with their "dumb" ancestors.  That care does not
seem to be present in the USB<->serial (or parallel) converters.]

> If you want a relatively small laptop with a true RS232 port, look at a
> toughbook CF-19, you can find some on ebay in the<$200 range.

That's at least as large as the current "laptop+port expander"
solution I'm using.

I was really hoping for something along the lines of one of those
little *tiny* (like hand-sized) computers that were en vogue a
while back (useful for little more than email!).  This isn't
the sort of application where I need a nice, big, comfortable screen
and keyboard.  Rather, something that is used *just* long enough
to figure out why something doesn't boot.  Or, to adjust some
configuration parameter, etc.

I'll see what I can find and maybe cobble a true serial port into
one "after the fact" (I only need to make *one*, after all!  :> )


More information about the tfug mailing list