TweetFollow Us on Twitter

ATalk Connections
Volume Number:2
Issue Number:4
Column Tag:The Electrical Mac

AppleTalk Connections

By Jeff Mitchell, President, Digital Solutions, MacTutor Contributing Editor

The Appletalk Physical Layer

This month I'm going to describe the physical (hardware) layer of the AppleTalk network protocol. I'll also respond to some mail and announce a new direction for this column, which I think everyone, including myself, will enjoy.

Appletalk Protocol Architecture

The AppleTalk protocol is based on a set of functional layers corresponding to the ISO (International Standards Organization) OSI (Open Systems Interconnection) reference model. Apple has provided a specific set of protocols for layers 1 through 5 (Physical, Data Link, Network, Transport, and Session) for the AppleTalk network. Being only an engineering type, I won't pretend to understand anything but layer 1, the physical layer, which is the electrical interface between devices.

The Appletalk Physical Layer

Each AppleTalk node has exactly one physical layer. It is here that the actual ones and zeros are passed between nodes after the data has been massaged into the correct format by the higher level layers. The electrical interface between nodes is an FM0 encoded serial bit stream operating at 230.4 Kbits/sec over a transformer-isolated, multi-drop, shielded twisted-pair cable. [Note: This is the spec value. Actual value may vary slightly. See note at the end of this article. -Ed.]

Data is encoded in SDLC (Serial Data Link Control) FM0 format. SDLC defines the very low level data protocol and formats the data in a manner which assures that it can be recovered at the receiving node. FM0 defines how ones and zeros are encoded.

Most low-speed serial communication encodes the ones and zeros in an NRZ (Non-Return to Zero) format. This is how the Macintosh normally talks to printers, modems, and the like. In NRZ encoding a high level represents a one and a low level represents a zero (Figure 1). With this encoding method clocking information is only available on bit-cell boundaries when a transition occurs. Because these transitions cannot be guaranteed when transmitting random data, a start bit is usually prefixed to the beginning and a stop bit is appended to the end of every data byte. The start and stop bit provide a known bit-transition to the receiving device, which it can use to synchronize its clock to the data.

In FM0 encoding, a transition occurs at the boundary of every bit-cell. If the bit is a zero, there is an additional transition in the middle of the cell (Figure 1). These transitions provide enough information for the clock to be recovered at the receiving device.

The hardware required to support SDLC and FM0 transmission, including the Phase-locked-loop used to recover the clock from the bit-cell boundary transitions, is all contained on the 8530 Serial Communications Controller (SCC) chip in the Macintosh. This makes switching between peripherals such as the printer and the AppleTalk network just a matter of reprogramming the control registers on the SCC (and much software, of course).

Transmission Medium

AppleTalk devices send and receive data over a single shielded, 22 AWG twisted-pair cable. Twisted-pair is adequate for the signalling rate and distances involved in AppleTalk and provides a very low-cost interconnect. Nodes are transformer coupled to the cable to provide DC isolation and reduce RFI and noise susceptibility. There can be a maximum of 32 devices per bus and a total maximum cable length of 300 meters. The cable must be terminated at each end with a 100 ohm resistor for proper operation.

Appletalk Connection Module

Each node connects to the AppleTalk cable via a connection module. This module contains a transformer, two 3-pin miniature DIN connectors each with a coupled switch, some passive components and a DE-9 connector which mates to the Macintosh 512K (Does anyone but me care that there is no such thing as a DB-9 connector?) or DIN-8 connector on a Mac Plus.

Figure 2 is a schematic representation of an AppleTalk connection module. J1 and J2 are the 3-pin miniature DIN connectors which are coupled to switches connected to R2. This resistor automatically provides the 100 ohm bus termination if there is only one cable plugged into the module. R1 is a drain for the cable shield and R5 and C1 form a high pass filter to reduce EMI and isolate the grounds of AppleTalk devices from each other. R3 and R4 increase the noise immunity of the receivers.

The connection module is a completely passive device. If a node should fail it will not affect the integrity of the network as long as it fails either high or low and is not broadcasting garbage. The node is driven directly from the outputs of the Macintosh RS-422 drivers and feeds directly through R3 and R4 to the RS-422 receivers. No additional active circuitry is required to connect a Macintosh to the AppleTalk network.

The coupling transformer is a 1:1 turns ratio transformer with the primary wound as two windings of 32 AWG wire in series with one wound below the secondary and one above it on a Siemans B65651-K000-R030 core (Figure 3).

MAIL

The mail I've received generally asks one of two questions: 1) "How can I modify my Macintosh to do _____?" and 2) "How can I interface my Macintosh to _____?". My answer to the first question is it is my policy (not necessarily MacTutor's) that I will not encourage anyone to modify their Mac.

My answer to the second question hopefully will make up for the first, for all you hardware hackers out there. In keeping with the Macintosh concept of the serial ports and the AppleTalk network creating a "Virtual Bus" I will soon have an article on constructing a "Virtual Card" which will plug into this bus. Once you have the "Virtual Card" it can be expanded to add whatever type of peripheral device you'd like.

Starting next month, this column will take a more hands-on approach to hardware. We'll start building things instead of just talking about them. If you have any projects you'd like to see done, send a letter to me care of MacTutor.

Letters From Our Readers

SCC Timing Off

Dave M. Gaylee-Lincer

Rainin Research Group

Emeryville, CA

The SCC clock timing as described in the Oct 1985 MacTutor differs somewhat in practice. I have observed the following:

1. The measured frequency is 3.672 Mhz when measured with a frequency counter.

2. I examined the waveform over a number of cycles and found that the timing goes shorter and longer. When I examined the waveform in more detail, I found that in 64 clocks of the 15.6672 Mhz, that there were 15 SCC clocks. This reatio is not 4.25 to 1 as specified in the documentation, but works out to 4.266666666.

This changes several things that could be important to third party developers (like ourselves) in that the Appletalk baud rate is now: 3.672 Mhz / 16 = 229.5 Kbaud.

This is approximately .4% different from the electrical specifications given in the book Inside Appletalk by Apple Computer Inc. [Spec is 230.4 Kbaud.] This is OK if you are precisely at nominal, but it reduces the margin if you, perhaps, aren't perfect.

Power Supply Problems

Dave Roberts

Santa Barbara, CA

Recently I had my 128K Mac power supply blow out. A friend of mine's Mac also had it's power supply fail. I have also heard of several other people's power supplies failing. MacTutor has printed several letters of people with similar experiences. Now just what is the deal with the power supply? I have had an Apple II+ for over 4 years and have not had so much as a hick-up out of it. What is the deal with the Mac? For almost $100 that is kind of an expensive thing to happen. The store that I got it fixed at delayed getting mine fixed because the power supplies they had gotten in from Apple were D.O.A. You would think with all that talent they could give it a power supply that wouldn't fail so often!

[Perhaps we can cover the power supply circuit in a future column and point out some of it's short comings. -Ed]

Hardware Review Wanted

M. Joseph El-Sayed

Montreal, Quebec

You should, little suggestion here, put out a bi-annual (or something around that frequency) hardware product review. The review should be based on compiled user opinion and may be tests you guy's ran (if you have the time). By the way, some roses, your Journal is certainly THE Mac-mag to read. I have access to subscriptions of MacWorld (yech), MacUser (Better), and MacNibble (the other mag to look at). Anyways, keep up the definitly superior work.

[Hardware reviews are expensive, (you have to have the hardware!) but we'll see what we can do in that area. -Ed]

Serial Port Changes in Mac Plus?

David Smith

MacTutor

I have a question for the hardware column. Recently I bought a Spinwriter 8810 printer from Nec along with their Macintosh kit (cable and software). The software was done by Assimilation Process under contract and is very similar to their printer utilities. It modifies the Imagewriter Driver file to work in draft mode with the Nec printer. The cable and software work perfectly on a 512K Mac using all versions of the Imagewriter file. However, when I try it on my Mac Plus, the printer buffer fills up and an overflow error stops printing. Only the gnd, handshake, recieve and transmit lines are being used from the printer. We have also discovered that several computer stores are selling Mac DB-9 cables to DIN-8 connectors that are wired backwards, so beware! Help?

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Pinegrow 6.23 - Mockup and design web pa...
Pinegrow (was Pinegrow Web Designer) is desktop app that lets you mockup and design webpages faster with multi-page editing, CSS and LESS styling, and smart components for Bootstrap, Foundation,... Read more
WhatsApp 2.2149.4 - Desktop client for W...
WhatsApp is the desktop client for WhatsApp Messenger, a cross-platform mobile messaging app which allows you to exchange messages without having to pay for SMS. WhatsApp Messenger is available for... Read more
Microsoft Remote Desktop 10.7.4 - Connec...
Microsoft Remote Desktop for Mac is an application that allows connecting to virtual apps or another PC remotely. Discover the power of Windows with Remote Desktop designed to help you manage your... Read more
ffWorks 2.6.7 - Convert multimedia files...
ffWorks, focused on simplicity, brings a fresh approach to the use of FFmpeg, allowing you to create ultra-high-quality movies without the need to write a single line of code on the command-line.... Read more
Opera 82.0.4227.58 - High-performance We...
Opera is a fast and secure browser trusted by millions of users. With the intuitive interface, Speed Dial and visual bookmarks for organizing favorite sites, news feature with fresh, relevant content... Read more
Day One 6.15 - Maintain a daily journal.
Day One is an easy, great-looking way to use a journal / diary / text-logging application. Day One is well designed and extremely focused to encourage you to write more through quick Menu Bar entry,... Read more
Default Folder X 5.6.3 - Enhances Open a...
Default Folder X attaches a toolbar to the right side of the Open and Save dialogs in any OS X-native application. The toolbar gives you fast access to various folders and commands. You just click on... Read more
OmniOutliner Pro 5.9.2 - Pro version of...
OmniOutliner Pro is a flexible program for creating, collecting, and organizing information. Give your creativity a kick start by using an application that's actually designed to help you think. It's... Read more
OmniOutliner Essentials 5.9.2 - Organize...
OmniOutliner Essentials (was OmniOutliner) is a flexible program for creating, collecting, and organizing information. Give your creativity a kick start by using an application that's actually... Read more
QuickBooks 19.0.11.984 - Financial manag...
QuickBooks helps you manage your business easily and efficiently. Organize your finances all in one place, track money going in and out of your business, and spot areas where you can save. Built for... Read more

Latest Forum Discussions

See All

The Best Wordle Clone in Town – The Touc...
In this week’s episode of The TouchArcade Show we dig into the drama of the moment which is the cloning and subsequent gloating about the cloning of the lovely little free word game Wordle. This leads into some additional drama about how PUGB Mobile... | Read more »
TouchArcade Game of the Week: ‘Cards Inf...
There’s nothing I love more than a perfect mobile game. What do I mean by that? Well, no game is actually perfect, but there’s something special about a game you know you can just whip out at a moment’s notice and dive into, and you know it will... | Read more »
‘Micro RPG’ Bringing Streamlined RPG Goo...
Originally announced on our forums more than 3 years ago, Micro RPG is an upcoming mobile game from a two-person studio that goes by the name JoliYeti Games and, as the title implies, it looks to offer all the fun of an RPG but in a more condensed... | Read more »
SwitchArcade Round-Up: ‘Kensei: The Seco...
Hello gentle readers, and welcome to the SwitchArcade Round-Up for January 14th, 2022. Yesterday was a big day, but today shows that we’re still warming up the engines for this year. There are a handful of new releases, but nothing nearly as... | Read more »
Mobile MMORPG Shooter ‘Avatar: Reckoning...
Archosaur Games, Tencent, Lightstorm Entertainment, and Disney have just revealed a mobile MMORPG shooter Avatar: Reckoning. Avatar: Reckoning will be published by Level Infinite when it hits iOS and Android. It is an official Avatar game developed... | Read more »
‘Crashlands+’ Is Out Now on Apple Arcade...
The brilliant Crashlands from Butterscotch Shenanigans was confirmed to arrive on Apple Arcade as an App Store Great in the form of Crashlands+ () a little while ago and it has just released worldwide. If it isn’t live yet, it should roll out in... | Read more »
SwitchArcade Round-Up: ‘Eschatos’, ‘To B...
Hello gentle readers, and welcome to the SwitchArcade Round-Up for January 13th, 2022. It’s a Thursday, and we’ve got a pretty hefty bag of new releases to dig into. There are always some fun surprises, and this week that came in the form of SNK Vs... | Read more »
‘Crush the Castle Legacy Collection’ Lau...
Ever since Angry Birds broke into the mainstream and became a household name more than a decade ago, there’s always been a small niche of people on the sidelines who would pipe up to remind everybody that “Crush the Castle did it first!" Indeed, the... | Read more »
Non-Violent Stealth Game ‘El Hijo – A Wi...
Over a year ago, Handy Games brought the non-violent stealth game El Hijo – A Wild West Tale to Switch, PS4, Xbox, PC, and Stadia. El Hijo – A Wild West Tale has been developed by Honig Studios and Quantumfrog. You play as El Hijo, a six year old,... | Read more »
‘ZED BLADE’ from SNK and Hamster Is Out...
After a bit of a break likely due to the holiday season, we’ve gotten a new title in the ACA NeoGeo series on iOS and Android. SNK and Hamster originally brought the series to mobile with Samurai Shodown IV, Alpha Mission II, and Metal Slug 5.... | Read more »

Price Scanner via MacPrices.net

Get an Apple Watch Series 7 for $50 off MSRP,...
Amazon has Apple Watch Series 7 models on sale for $50 off MSRP including free shipping. Their prices are the lowest available for Apple Watch Series 7 models today: – 41mm Apple Watch Series 7 GPS... Read more
Here are the details of Apple’s 2022 Educatio...
Need a new Apple Mac or iPad for school? Whether you’re a student, teacher, or staff member, you can use your .edu email address when ordering at Apple Education to take up to $400 off the price of a... Read more
Amazon is blowing out 2020 21″ iMacs for only...
Amazon has clearance 2020 21″ iMacs (2.3GHz Dual-Core i5, 8GB RAM, 256GB SSD) on sale right now for $599.99 including free shipping. Original MSRP for this model was $1099. Amazon expects delivery in... Read more
Find the best deal on an Apple MacBook using...
In the market for a new 13″ MacBook Air, 13″ MacBook Pro, 14″ MacBook Pro, or 16″ MacBook Pro with M1, M1 Pro, or M1 Max Apple Silicon? Use our Apple award-winning and exclusive price trackers to... Read more
Red Pocket Mobile is offering the Apple iPhon...
Switch to Red Pocket Mobile and get an Apple iPhone 13 Pro for $50 off MSRP, plus get free 6 months of Unlimited nationwide 5G service with the purchase of any iPhone 13. Red Pocket Mobile is a... Read more
24″ M1 iMacs on sale for $1249, $50 off Apple...
Amazon has base 24″ M1 iMacs (8-Core CPU/7-Core GPU/8GB RAM/256GB SSD) on sale today for $1249 shipped. Their price is $50 off Apple’s MSRP, and it’s the lowest price available for a new 24″ M1 iMac... Read more
Open-Box 16″ M1 Pro MacBook Pros available fo...
QuickShip Electronics has open-box return 16″ M1 Pro MacBook Pros in stock and on sale for $200-$300 off MSRP on their eBay store right now with free express delivery. According to QuickShip, “The... Read more
Stock Alert! Order a new 16″ M1 Pro MacBook P...
New 16″ MacBook Pros with Apple’s M1 Pro and M1 Max CPUs have been very hard to find, largely due to current global supply constraints. However, B&H Photo is reporting stock of Space Gray... Read more
Apple has maxed-out 13″ M1 MacBook Airs (16GB...
Save $250 on maxed-out 13″ M1 MacBook Airs today at Apple (16GB RAM/1TB SSD) with Certified Refurbished models available for $1399 in Space Gray and Gold colors. Regular price for this configuration... Read more
New promo at Xfinity Mobile: $400 off any App...
Xfinity Mobile is offering any new Apple iPhone for $400 off MSRP for new customers. This includes the iPhone 13. Price for the phone, including the discount, is spread monthly over a 24 month term... Read more

Jobs Board

Registered Nurse (RN) Employee Health PSJH -...
…is calling for a Registered Nurse (RN) Employee Health PSJH to our location in Apple Valley, CA.** We are seeking a Registered Nurse (RN) Employee Health PSJH to be Read more
Systems Administrator - Pearson (United State...
…and troubleshoot Windows operating systems (workstation and server), laptop computers, Apple iPads, Chromebooks and printers** + **Administer and troubleshoot all Read more
IT Assistant Level 1- IT Desktop Support Anal...
…providing tier-1 or better IT help desk support in a large Windows and Apple environment * Experience using IT Service Desk Management Software * Knowledge of IT Read more
Human Resources Business Partner PSJH - Provi...
…**is calling a** **Human Resources Business Partner, PSJH** **to our location in Apple Valley, CA.** **Applicants that meet qualifications will receive a text with Read more
Manager Community Health Investment Programs...
…is calling a Manager Community Health Investment Programs PSJH to our location in Apple Valley, CA.** **Qualified candidates will be invited to do a self-paced video Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.