>>  Strictly speaking, the RS422 voltage levels should be more in line
>> with typical RS485 signals (differential on top of a positive common
>> mode voltage), but the Mac doesn't seem to adhere to this convention.
>> Most models seem to use asymmetrical signals about 0v, like RS232.

> That's not consistent with what you just described.  Can you explain
> more clearly how the Mac does not "adhere"?  I suspect you mean it
> will "tolerate" RS-232 rather than "use" RS-232 levels.

No.  The Macintosh serial port hardware was designed as RS422
but certain details were chosen to provide easy and complete
EIA/TIA-232 (aka RS232C) compatibility.

See Inside Macintosh, volume III, pages 22 to 25.  I quote:

    Macintosh serial ports conform to the EIA standard RS422,
    which differs from the more common RS232C standard.  While
    RS232C modulates a signal with respect to a common ground
    ("single-ended" tranmission), RS422 modulates two signals
    against eash other ("differential" tranmission).  The RS232C
    receiver senses whether the received signal is sufficiently
    negative with respect to ground to be a logic "1", whereas
    the RS422 receiver simply senses which line is more negative
    than the other.  This makes RS422 more immune to noise and
    interference, and more versatile over longer distances.

    If you ground the positive side of each RS422 receiver and
    leave unconnected the positive side of each transmitter,
    you've converted to EIA standard RS423, which can be used
    to communicate with most RS232C devices over distances up
    to fifty feet or so.


> I'd use the resistor, 220 ohms, in the RS-422 input just in case.

Apple makes no mention of a resistor in the adapter cable.
I've never used such a resistor in over 15 years of Macintosh
network consulting and support.  But then I've connected Macs
to networking hardware, not PIC programmers.


> The RS422-to-RS232 conversion works because all standard RS232 chips
> have a threshold set at 1.5 volts not 0V as the standard implies.  Since
> the RS422/RS485 driver is a simple buffer and outputs a guaranteed 0.5
> to 2.5V swing, this works fine.

The Macintosh, using the cable described, meets both the
transmitter and receiver specs of EIA/TIA-232.  It doesn't
depend on the non-standard input thresholds obtained using
recevier chips with floating threshold control pins.

In the original Macintosh system, Apple's designers used AMD
26LS30 driver and 26LS32 receiver.  The driver's power supplies
were +5V (Vcc) and -5V (Vee) [volume III, page 26, figure 4.
Diagram of Serial Ports].  So the RS422 TxD- output pin was
very close to the letter of the EIA/TIA-232 voltages at the
transmitter pin and certainly met the voltage requirements of
EIA/TIA-232 at the receiver pin with any reasonable cable.

Succeeding Macintosh models have, to the best of my knowledge,
followed this serial port hardware model.

                                                Lee Jones

-------------------------------------------------------------------
Jones Computer Communications             lee@frumble.claremont.edu
509 Black Hills Dr, Claremont, CA 91711         voice: 909-621-9008
-------------------------------------------------------------------