piclist 1999\10\10\220436a >
Thread: Warp-13 and MacPIC
face BY : Dennis Plunkett email (remove spam text)

At 07:20 11/10/99 +1000, you wrote:
>Hello to Jim (Nagy that is).
>>  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.
>  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.

RS 422 and RS 485 are not the same, you will find that 422 will swing to
+6V nominaly on transmit (Can be up to 21V offset (Not used often at all))
This is just the unbalanced version of RS423 which will swing (Nominally)
from +/- 6V (OK so some will argue the ground ref bit too).

See end

Also if the RS 422 driver only puts out 0.5V how woul this fit into the
1.5V min? Also the swing out of an RS 422 driver is dependant on the load
(Around 50 ohms), and must produce more than 200mV lateration at the
termination resistor

>  The RS232-to-RS422 conversion works because all standard RS422 chips
>are rated for a common-mode voltage to well below zero and tolerate up
>to the RS-232 -12V input.  A resistor might be nice just in case.

Dont need a resistor at all

{Quote hidden}

Need to terminate these, if the driver is RS232 then around 3k will do, so
series resistance is normally required. Not sure about the protection diode
stuff, will have to check up

Got it wrong RS 422 is balanced, where RS 423 is unbalanced!


>  Cheers,
>        Paul B.


See also: www.piclist.com/techref/microchip/devices.htm?key=pic
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