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

Dropbox 193.4.5594 - Cloud backup and sy...
Dropbox is a file hosting service that provides cloud storage, file synchronization, personal cloud, and client software. It is a modern workspace that allows you to get to all of your files, manage... Read more
Google Chrome 122.0.6261.57 - Modern and...
Google Chrome is a Web browser by Google, created to be a modern platform for Web pages and applications. It utilizes very fast loading of Web pages and has a V8 engine, which is a custom built... Read more
Skype 8.113.0.210 - Voice-over-internet...
Skype is a telecommunications app that provides HD video calls, instant messaging, calling to any phone number or landline, and Skype for Business for productive cooperation on the projects. This... Read more
Tor Browser 13.0.10 - Anonymize Web brow...
Using Tor Browser you can protect yourself against tracking, surveillance, and censorship. Tor was originally designed, implemented, and deployed as a third-generation onion-routing project of the U.... Read more
Deeper 3.0.4 - Enable hidden features in...
Deeper is a personalization utility for macOS which allows you to enable and disable the hidden functions of the Finder, Dock, QuickTime, Safari, iTunes, login window, Spotlight, and many of Apple's... Read more
OnyX 4.5.5 - Maintenance and optimizatio...
OnyX is a multifunction utility that you can use to verify the startup disk and the structure of its system files, to run miscellaneous maintenance and cleaning tasks, to configure parameters in the... Read more

Latest Forum Discussions

See All

Zenless Zone Zero opens entries for its...
miHoYo, aka HoYoverse, has become such a big name in mobile gaming that it's hard to believe that arguably their flagship title, Genshin Impact, is only three and a half years old. Now, they continue the road to the next title in their world, with... | Read more »
Live, Playdate, Live! – The TouchArcade...
In this week’s episode of The TouchArcade Show we kick things off by talking about all the games I splurged on during the recent Playdate Catalog one-year anniversary sale, including the new Lucas Pope jam Mars After Midnight. We haven’t played any... | Read more »
TouchArcade Game of the Week: ‘Vroomies’
So here’s a thing: Vroomies from developer Alex Taber aka Unordered Games is the Game of the Week! Except… Vroomies came out an entire month ago. It wasn’t on my radar until this week, which is why I included it in our weekly new games round-up, but... | Read more »
SwitchArcade Round-Up: ‘MLB The Show 24’...
Hello gentle readers, and welcome to the SwitchArcade Round-Up for March 15th, 2024. We’re closing out the week with a bunch of new games, with Sony’s baseball franchise MLB The Show up to bat yet again. There are several other interesting games to... | Read more »
Steam Deck Weekly: WWE 2K24 and Summerho...
Welcome to this week’s edition of the Steam Deck Weekly. The busy season has begun with games we’ve been looking forward to playing including Dragon’s Dogma 2, Horizon Forbidden West Complete Edition, and also console exclusives like Rise of the... | Read more »
Steam Spring Sale 2024 – The 10 Best Ste...
The Steam Spring Sale 2024 began last night, and while it isn’t as big of a deal as say the Steam Winter Sale, you may as well take advantage of it to save money on some games you were planning to buy. I obviously recommend checking out your own... | Read more »
New ‘SaGa Emerald Beyond’ Gameplay Showc...
Last month, Square Enix posted a Let’s Play video featuring SaGa Localization Director Neil Broadley who showcased the worlds, companions, and more from the upcoming and highly-anticipated RPG SaGa Emerald Beyond. | Read more »
Choose Your Side in the Latest ‘Marvel S...
Last month, Marvel Snap (Free) held its very first “imbalance" event in honor of Valentine’s Day. For a limited time, certain well-known couples were given special boosts when conditions were right. It must have gone over well, because we’ve got a... | Read more »
Warframe welcomes the arrival of a new s...
As a Warframe player one of the best things about it launching on iOS, despite it being arguably the best way to play the game if you have a controller, is that I can now be paid to talk about it. To whit, we are gearing up to receive the first... | Read more »
Apple Arcade Weekly Round-Up: Updates an...
Following the new releases earlier in the month and April 2024’s games being revealed by Apple, this week has seen some notable game updates and events go live for Apple Arcade. What The Golf? has an April Fool’s Day celebration event going live “... | Read more »

Price Scanner via MacPrices.net

Apple Education is offering $100 discounts on...
If you’re a student, teacher, or staff member at any educational institution, you can use your .edu email address when ordering at Apple Education to take $100 off the price of a new M3 MacBook Air.... Read more
Apple Watch Ultra 2 with Blood Oxygen feature...
Best Buy is offering Apple Watch Ultra 2 models for $50 off MSRP on their online store this week. Sale prices available for online orders only, in-store prices may vary. Order online, and choose... Read more
New promo at Sams Club: Apple HomePods for $2...
Sams Club has Apple HomePods on sale for $259 through March 31, 2024. Their price is $40 off Apple’s MSRP, and both Space Gray and White colors are available. Sale price is for online orders only, in... Read more
Get Apple’s 2nd generation Apple Pencil for $...
Apple’s Pencil (2nd generation) works with the 12″ iPad Pro (3rd, 4th, 5th, and 6th generation), 11″ iPad Pro (1st, 2nd, 3rd, and 4th generation), iPad Air (4th and 5th generation), and iPad mini (... Read more
10th generation Apple iPads on sale for $100...
Best Buy has Apple’s 10th-generation WiFi iPads back on sale for $100 off MSRP on their online store, starting at only $349. With the discount, Best Buy’s prices are the lowest currently available... Read more
iPad Airs on sale again starting at $449 on B...
Best Buy has 10.9″ M1 WiFi iPad Airs on record-low sale prices again for $150 off Apple’s MSRP, starting at $449. Sale prices for online orders only, in-store price may vary. Order online, and choose... Read more
Best Buy is blowing out clearance 13-inch M1...
Best Buy is blowing out clearance Apple 13″ M1 MacBook Airs this weekend for only $649.99, or $350 off Apple’s original MSRP. Sale prices for online orders only, in-store prices may vary. Order... Read more
Low price alert! You can now get a 13-inch M1...
Walmart has, for the first time, begun offering new Apple MacBooks for sale on their online store, albeit clearance previous-generation models. They now have the 13″ M1 MacBook Air (8GB RAM, 256GB... Read more
Best Apple MacBook deal this weekend: Get the...
Apple has 13″ M2 MacBook Airs available for only $849 today in their Certified Refurbished store. These are the cheapest M2-powered MacBooks for sale at Apple. Apple’s one-year warranty is included,... Read more
New 15-inch M3 MacBook Air (Midnight) on sale...
Amazon has the new 15″ M3 MacBook Air (8GB RAM/256GB SSD/Midnight) in stock and on sale today for $1249.99 including free shipping. Their price is $50 off MSRP, and it’s the lowest price currently... Read more

Jobs Board

Early Preschool Teacher - Glenda Drive/ *Appl...
Early Preschool Teacher - Glenda Drive/ Apple ValleyTeacher Share by Email Share on LinkedIn Share on Twitter Read more
Senior Software Engineer - *Apple* Fundamen...
…center of Microsoft's efforts to empower our users to do more. The Apple Fundamentals team focused on defining and improving the end-to-end developer experience in Read more
Relationship Banker *Apple* Valley Main - W...
…Alcohol Policy to learn more. **Company:** WELLS FARGO BANK **Req Number:** R-350696 **Updated:** Mon Mar 11 00:00:00 UTC 2024 **Location:** APPLE VALLEY,California Read more
Medical Assistant - Surgical Oncology- *Apple...
Medical Assistant - Surgical Oncology- Apple Hill WellSpan Medical Group, York, PA | Nursing | Nursing Support | FTE: 1 | Regular | Tracking Code: 200555 Apply Now Read more
Early Preschool Teacher - Glenda Drive/ *Appl...
Early Preschool Teacher - Glenda Drive/ Apple ValleyTeacher Share by Email Share on LinkedIn Share on Twitter Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.