Aircraft systems RS232 voltage levels?

While checking my new Garmin 496 power/data cable, I put a scope on the
NEMA DATAOut line. Just like the EI instruments, the Garmin puts out
TTL (0V and 4V) logic levels instead of the normal RS232 voltage levels
(-5V / -12V to +5V /+12V)

Do all aircraft systems that communicate via "RS232" use the TTL
levels? How about if I drag my Labtop out to the computer to read some
diagnostics or upload a new database?

mikem wrote:
While checking my new Garmin 496 power/data cable, I put a scope on the
NEMA DATAOut line. Just like the EI instruments, the Garmin puts out
TTL (0V and 4V) logic levels instead of the normal RS232 voltage levels
(-5V / -12V to +5V /+12V)

I had a gadget (Caller ID interface) that was designed to plug into the
serial port of a PC. It didn't have an onboard way to generate the
negative voltage - no dual power supply, nor a MAX232 or similar chip.
Instead, it depended (IIRC) on one of the handshake lines on the PC
being set to mark (negative voltage), and used a transistor to switch
this voltage onto the data line when a mark was needed. I think you
could even use the transmit line from the PC as the source of the
negative voltage (it idles in mark), as long as you could guarantee that
the software on the PC wouldn't try to talk and listen at the same time -
in other words, half duplex.

If you have an RS232 breakout box, or a cable you don't mind cutting up,
you might connect the Garmin to your laptop through the box/cable and see
what the voltage levels look like then.

Matt Roberds

mikem wrote:
While checking my new Garmin 496 power/data cable, I put a scope on the
NEMA DATAOut line. Just like the EI instruments, the Garmin puts out
TTL (0V and 4V) logic levels instead of the normal RS232 voltage levels
(-5V / -12V to +5V /+12V)

Do all aircraft systems that communicate via "RS232" use the TTL
levels? How about if I drag my Labtop out to the computer to read some
diagnostics or upload a new database?


Is the idle line up or down?

If it's cheated RS-232 (negative half missing),
the idle level is down. If it's logic level,
the idle line (marking) is up.

For a logic-level line you need a MAX232 or clone
to create correct RS-232 levels. For a cheat RS
you need additionally a logci inverter ahead of
the buffer.

HTH

--

Tauno Voipio (OH-PYM, PA28RT201T)
tauno voipio (at) iki fi

Tauno Voipio wrote:
....
Is the idle line up or down?

Down, which I think you are saying means that the signal I see is the
same polarity as full-blown RS232 levels, but the amplitude is reduced
to only 4V p-p. (0 to 4V).

Obviously, the trip point of the receiver of such a reduced amplitude
signal would have to be at about 1.5V (TTL like).

Thanks,

Peter wrote:
TTL (0V and 4V) logic levels instead of the normal RS232 voltage levels
(-5V / -12V to +5V /+12V)

No idea. There are some suspect practices in avionics though, like
wiring multiple receivers to a single RS232 transmitter. They should
be using RS422 for that.

RS-232 is being BADLY abused these days. The spec only requires more
than 3 Vdc or less than -3 Vdc, and a lot of modern laptops settle on
3.3 as "okay." It's rare to find one that actually puts out +/- 12 any
more.

I too have "cheated" for quick and dirty interfaces. TTL out will
usually drive an RS-232 input (although not always). Inputs? Put a
couple of protection diodes and a resistor in front of a CMOS gate and
you can receive RS-232 without any special stuff - but you give up half
your noise immunity.

I agree - for aircraft differential RS-422 is much better (or ARINC
429).

jmk

Peter wrote:
What interface is ARINC? Isn't it RS422?

"jmk" wrote
I agree - for aircraft differential RS-422 is much better (or ARINC
429).

No. ARINC-429 is the most common avionics bus on commercial airliners.
Electrically it is +/- 10 Vdc, return-to-zero, differential pair.
There are two speeds, one around 100 kbps and one down around 12,500
bps. Very tight limits on rise and fall times.

Software wise, it's pretty much fixed message length (packets), with
most of the message types pre-defined (international standard).

It's not hard to implement, but if you want to meet the drive specific
conditions (rise/fall times, loading) you either have to implement your
own circuitry with a bunch of op amps, or use one of the very few -429
driver chips out there. There is a good tutorial at:


http://www.condoreng.com/support/dow...NCTutorial.pdf