Path: utzoo!dciem!nrcaer!scs!spl1!laidbak!att!mtunx!rutgers!gatech!purdue!decwrl!ucbvax!agate!saturn!eshop From: eshop@saturn.ucsc.edu (Jim Warner) Newsgroups: comp.protocols.appletalk Subject: Re: PhoneNet cabling Message-ID: <3642@saturn.ucsc.edu> Date: 7 Jun 88 00:15:24 GMT Article-I.D.: saturn.3642 References: <614@mtxinu.UUCP> <3611@saturn.ucsc.edu> <3090@polyslo.UUCP> Reply-To: eshop@saturn.ucsc.edu (Jim Warner) Organization: University of California, Santa Cruz Lines: 55 In article <3090@polyslo.UUCP> dorourke@polyslo.UUCP (David O'Rourke) writes: >In article <3611@saturn.ucsc.edu> eshop@saturn.ucsc.edu (Jim Warner) writes: >>>No. Apple's LocalTalk is RS-422 balanced differential signalling. > > I was under the impression that differential signalling requires two wires >for each signal. So that would require Apple's LocalTalk to use 5 wires, 2 >for Transmitt, 2 for Recieve, and 1 for ground. The five wires you list exist in the cable leading from the Mac to the transformer. They are combined in the transformer box. There are two active wires in LocalTalk, plus the shield that is connected to ground through 1 Kohm in parallel with 0.1 uF. PhoneNet dispenses with the shield. >Well LocalTalk only has 3 wires, 1 transmit, 1 Recieve, 1 ground. LocalTalk is a party line. Any station can send to any other station. The sender transmits on both wires and everyone else listens on the same wires. The full duplex scheme you propose (separate wires for send and receive) won't work for more than two stations. LocalTalk is half duplex -- one data path is used for both sending and receiving. >Besides I was told by an EE person >who looked at Apple's Transformer specs in inside AppleTalk that their >wiring scheme reaks more of RS-423 (RS-432??). >In any after looking at the specs for the LocalTalk box, and the number of >wires in the LocalTalk cable, the 20 or so people I've talked to seem to think >apple isn't using the RS-422 at all. In "Inside AppleTalk" in the Electrical Specifications section it says: "The use of the EIA RS-422 signalling standard for transmission and reception over AppleTalk provides significantly higher data rates over longer distances than that of the EIA RS-232C standard." >And after looking at how they wired >the 8530 these people were even move convinced that Apple isn't using the >RS-422 standard at all, they seem to be using a tweaked version somewhere >between RS-232 & RS-422. "Inside AppleTalk" says, "The recommended driver is the 26LS30 used with both +5V and -5V as power supplies, and the mode control connected to give differential outputs." Perhaps this is the source of confusion of your 20 friends. Dual power supplies are not required for RS-422. I suppose that Apple engineers made that choice because they wanted a larger signal. What Apple did isn't "tweaked". It is a straight forward application of the info in the Advanced Micro Devices Linear and Interface Data Book. > In any case I might be wrong, but I don't think LocalTalk is run on top of >RS-422 as you stated. > >-- jim warner