>>  just to make sure I understand this, the DB9 (J2) is connected
>> to a peripheral NOT to a PC (because a PC would be driving pin 3
>> and reading pin 2).

RS232, now more properly EIA/TIA-232, is handed.  The PC serial
port is DTE (data terminal equipment) sex.  Modems and (usually)
peripherals are DCE (data circuit-terminating equipment) sex.

>> On the incoming side, who cares about -12?  No problem.  But on
>> the TD pin, you are actually driving it between <+12 and ground
>> not +12 and -12.  Right?  And this works?

Spec says that the circuit output voltage at the transmitter is
lower than -5V or higher than +5 volts.  Receiver is supposed to
accept below -3V or above +3V.


> Resuming all the things about RS232... long time ago in a long
> and distant galaxy, people used the RS232C physical layer specs
> that define the levels as "mark" and "space", this was very long
> time ago, when teletype machines use 75 and 134.5 bps... (read
> below about it).  The teletypes needed to work with magnets to
> allow selection of =mark= or =space=

Teletypes originally used a current loop interface (which predated
RS232).  Loop current was 60mA then 40mA.  Direction of current
flow drove the magnets which activated clutches on a rotating shaft.

This is also why early async serial protocols needed 2 stop bits.
The extra time was required for the mechanism to "reset" at the
end of each cycle.

In the Teletypes I worked with (ASR33, KSR33, KSR37), the RS232
interface was an add-in board that converted the signal levels
from single-ended voltage to current-loop.

Ahhh, hearing a Teletype ASR33 clattering and watching it "walking"
around the floor during a long printout...  Modern laser printers
may be efficient but they just don't have the same panache. :-)


> But, since the "new age" of silicon electronics, I never found a RX
> RS232 chip that doesn't commute its TTL output from - to + when the
> RS232 input signal goes down from +4.5 to zero volts.

I have.  The MC1489 quad line receiver in one; check the data sheet.
On that chip, each receiver has a "response control" pin.  If the pin
is left floating, then you get the behavior Wagner has seen.  This is
a quite common setup.

Some designers add the external resistor and/or capacitor network as
laid out in the data sheet.  The resistor is used for "Input Threshold
Voltage Adjustment" (figure 5, Motorola MC1489AD data sheet).  The
capacitor is used for noise filtering.

I have run into equipment, rarely, that would disregard the input
signal unless it went below -3 volts.  I have also seen equipment
that had a pull-down resistor on each EIA-232 input pin to set it
to a negative voltage (I suspect for cases where nothing was
plugged into the interface connector).

                                                Lee Jones