Re: Appletalk/Phonenet box wiring (Re: ...phoneNet connection to Win2K?)

[mister_boffo@hotmail.com (Mister Boffo)]

On 11 Jul 2001 10:10:12 +1000, david@uow.edu.au (David Wilson) wrote:

>mister_boffo@hotmail.com (Mister Boffo) writes:
>>And would ya know it, I've got a disassembled Appletalk dongle
>>right here. Wiring looks like this inside:
>
>>                          T1
>>BUS Wire 1 --+------+---      ----------+--------- Serial Red
>>             |      |   ) ## (          |
>>          Term R2   |   ) ## (          `--- R4 -- Serial Blu
>>             |      |   ) ## (
>>          Term Sw   |   ) ## (    
>>             |      |   ) ## (
>>BUS Wire 2---+----- | --  ##  --- N/C
>>                    |   ) ## (
>>                    |   ) ## (
>>                   R1   ) ## (
>>                    |   ) ## (          .--- R3 -- Serial Wht
>>                    |   ) ## (          |
>>BUS Ground----------+---      ----------+--------- Serial Grn
>>                    |
>>                    |
>>                    `-----------+-- R5 --+-------- Serial GND
>>                                |        |
>>                                `-- C1 --'
>>                              (noise filter)
>
>>Okay, I don't know what the serial wire colors represent but
>>it seems fairly easy to figure out. The main thing you need to
>>know about RS422 is that you might say the sending line has
>>its own ground, and so does the receiving line. It's usually
>>represented as RX+ RX- and TX+ TX-
>
>And here are the serial connections from page A-4 of "Inside Appletalk"
>Second Edition:
>
>Wire	Sig	MiniDIN-8	DE-9
>-------------------------------------
>Red	TxD-	pin 3		pin 5
>Blu	RxD-	pin 5		pin 9
>
>Wht	RxD+	pin 8		pin 8
>Grn	TxD+	pin 6		pin 4
>
>GND	Case	Shield		Shield
>
>>In order for you to send a signal you have to send from RX+
>>through the transformer and out to RX-. Similarly to receive
>>a signal the transformer must connect to TX+ and TX-.
>
>The +/- signals are differential. When TxD+ is +3volts the TxD-
>will be -3volts giving 6 volts between them. When the signal changes
>from logical 1 to logical zero the polarities of both lines change.

Ah-HAH! So nice to see that my electronics theory is actually valid. I
was also right about the resistors being there to reduce the local
current going to RxD+ and RxD-! :)

As I think about it some more, in a direct serial link, RxD+/- is
normally connected straight to TxD+/- of another computer. So RxD+/-
should be fully capable of accepting whatever current TxD+/- puts out.


It is possible that R3 and R4 don't "protect" RxD+/- but instead are
there to make sure some power flows through the transformer from TxD+
to TxD-. Without R3 and R4, all the output of TxD+/- could be sucked
up by RxD+/- without doing anything in T1. 

Resistor R1 is still a bit of a question mark for me, but I do believe
you can also construct a filter out of an inductor and a resistor. So
in this case R1 and the "secondary" coil of T1 are working together to
form a second noise filter for the Appletalk network bus, in addition
to the R5/C1 noise filter...

I'm crossposting this to sci.electronics.basics, in hopes of getting
some input from an electronics expert on this. :)

-Mr. Boffo