TTL / RS-232 conversion circuit

[Will]

Hi All,

I am interfacing a ham radio to a computer to program it and I have been
looking at various TTL / RS-232 conversion circuits. However I happen to
have a commercial ADMS programming cable I use with my Yaesu VX-7R radio
(ham radio). This cable also handles this type of conversion so I am
thinking why reinvent the wheel when I can utilize this existing circuit
which is already integrated into the D-sub shell of this cable.

Thus my question:

These generic circuits appear to RS-232 logic levels (+/-12VDC) on pins 2 &
3 of a DB9 connector and converts them to 0 & 5VDC. Which TTL voltage
corresponds to a logic 0 and which to a logic 1? On the RS-232 side, the
data pins use negative logic, so +3 to +12 VDC is logic 0 and -3 to -12VDC
is logic 1. Does the TTL data input use positive logic instead ( logic 1 is
+5VDC and logic 0 is 0VDC)?

What is the industry convention when terminating the TTL side of this cable
to a stereo plug. ... is the tip conductor logic 1 or logic 0? Once I
determine this, then I will use this existing conversion cable and simply
wire the stereo plug conductors to their respective TXD and RXD pins on my
Yaesu FT-857.

Lastly, what happens if one connects the TTL level TX conductor to the TXD
pin on the radio side accidentally instead of the RXD pin on the radio?
i.e. wires crossed in error

Thanks for any and all info!

73's Will

 

[CheapscateDave]

I can't say for sure if it's a standard but cables for RS232/TTL to
glucose testers use the tip as RX and small ring is TX and last is
common or ground .If you want a simple 1 chip solution look at the
DS275 from Dallas.

Dave

 

[CheapscateDave]

You could try a wrap jumper..With TTL side connected jumper RS232 TXD
to RXD and scope signals. Then disconnect TTL and connect RS232 and
jumper TTL TXD to RXD and scope signals. You should see the original
and converted signal in each direction.
Dave