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From: boortz@sics.se (Kent Boortz)
Newsgroups: comp.protocols.appletalk,comp.sys.mac.comm
Subject: Re: do I need phone net connectors? <summary>
Message-ID: <1991Jan9.175029.23835@sics.se>
Date: 9 Jan 91 17:50:29 GMT
References: <1991Jan8.090043.25113@sics.se>
	<1991Jan8.112500.14143@d.cs.okstate.edu>
Sender: news@sics.se
Organization: Swedish Institute of Computer Science, Kista
Lines: 42
In-Reply-To: minich@d.cs.okstate.edu's message of 8 Jan 91 11:25:00 GMT


by Robert Minich

> A straight null-modem cable should do fine for two LocalTalk devices however,
> Kent is wrong about (mac<->mac) == (mac<->modem) == (LW|IW<->mac). [Did
> that come out right?]  The exception is the modem. Serial devices come in
> two flavors, Data Terminal Equipment (DTE) and Data Comm. Equ. (DCE).
> When wiring DTE<->DCE, the pins labeled Tx (transmit) on both devices
> are connected together while DCE<->DCE cables have Tx<->Rx wiring,
> eliminating the need for a modem, thus the null-modem cable.

IF the modem has an MiniDIN-8 connector, otherwise I can't use an
ImageWriter cable, I assume I was right even for mac<->modem. Apple 
did the right thing leaving this messy RS-232C DTE, DCE... You have 
never to worry about connecting output to output using two equipment 
with the Apple MiniDIN-8 standard and cables. I don't even think 
there is something you can call a NULL-cable in this standard because 
there is only one cable wiring needed.



by Bill Woodcock

>    Actually, the main reason for using a LocalTalk transciever box
> like a Farallon PhoneNet connector is to prevent your SCC chip and
> potentially your motherboard from being fried by static and higher-
> than-5v-charges coming in off the port.  
>    LocalTalk transcievers by and large do two things: they combine
> the T+ and T- and R+ and R- onto a single balanced pair, and they
> provide surge suppression and protection from voltage spikes.
                         
>                        -Bill Woodcock
>                         BMUG NetAdmin

The transformer inside the LocalTalk transciever box is isolating
from DC current but static will be like AC. The transformer will
happily give it to your SCC chip. Or is the transformer to
small to transfer that much energy or to slow for responding to
static?

Kent Boortz
boortz@sics.se