TweetFollow Us on Twitter

MacBox
Volume Number:4
Issue Number:5
Column Tag:Mac Hardware

MacBox and the Serial Sermon

By Rev. Willis, MacFugNews

Once upon a time in a galaxy far, far away...a humble servant of Big Byte took upon himself the serial expansion of the blessed Mac. However, before enlightenment was acheived, much wisdom had to be revealed unto and understood by the humble servant. In fact, much self-flagellation, usually with an RS-232 cable, was necessary to gain the wisdom needed to successfully expand the Mac’s serial ports.

All hi-tech parables aside, my travail resulted from the desire to have my Mac talk to my modem, an X-10 Powerhouse appliance controller and a Radio Shack Model 100 portable computer, and do so without going through a cable swapping boog-a-loo. This was accomplished with an inexpensive switch and some simple wiring. I’ve included some pictures and diagrams to help the more ambitious (and cheap, Mac switch boxes are commercially available) of you duplicate the task.

Excluding the SCSI port and the second disc drive port, the Mac talks to the world through two serial ports. One of these serial ports is usually configured through the Chooser software to talk to the Imagewriter. The other port can be configured by Chooser as an Appletalk connection, or, using communications software such as Red Ryder, as a modem port. Also, using an adaptor box and MIDI software, these serial ports can talk to musical instruments such as a DX-7 sythesizer. Finally, one of these ports can be used to talk directly to another computer without the modem/phone line connection assuming both machines are in close proximity. Some computers, such as the Incredibly Bad Machine and the Model 100, normally talk to printers through a parallel port; however, the Imagewriter/Mac combo do it serially.

The RS-232 data communications standard is used by most personal computer systems for data transfer. There is also Ethernet, Appletalk and others, but we’re talking proletarian stuff here, not big-time University networking. The Mac uses an improvement of the RS-232 standard called RS-422. This improvement allows for longer cable lengths to carry digital data, necessary for Appletalk networks. Thankfully, RS-422 and RS-232 protocols are generally compatible. The RS-232 standard defines the DB-25 connector pinouts (check the diagrams included) as well as describes data terminal equipment (DTE) sending digital information to data communications equipment (DCE) over an analog phone line to another DCE-DTE combo (digital data poops out after 100 feet or so and must be converted to an analog signal for the long haul). DTEs are usually computers, while DCEs are modems. They are connected with straight cables, that is, the TXD pin on the DTE is connected to the same TXD pin on the DCE. The internal wiring of the equipment makes sure everthing is copesetic. The DTE/DCE pairs use software handshaking to control the communications connection, usually XON/XOFF protocol. If one pair wants to pause communications, it issues a little ASCII data code ,Control Q, to tell the other pair to stop transmitting; a Control S resumes communications. If you ever used Red Ryder to access Compu$erve or Genie, you may have used the Q and S radio buttons to control the information zipping across the screen at a rate faster than you can read it.

However, as occurs with all good standards, they get modified to accomplish new tasks. A problem arises when two computers are hooked back-to-back with no modem/phone link inbetween. Now you have a DTE-DTE link which confuses the signal lines on the interconnection cable. So you need a cross or “null” cable to connect the equipment. In such a cable, the TXD pin from one DTE goes to the RXD pin of the other DTE, and vice versa. If these DTE-DTE combos are a back-to-back computer pair, they usually use a form of software handshaking. However, if a computer is talking to a printer (which usually acts as a DTE), then they may use a form of hardware handshaking: two control lines are crossed and the pins toggled with a logic “Hi” to indicate the DTEs are ready to communicate. If one DTE toggles its control pin “Lo”, communications are paused until the pin is again toggled “Hi”. The control lines usually link the RTS-CTS or DTR-DSR pins. In the case of the Mac and the Imagewriter, the handshaking lines are CTS-DTR. If the IMW needs to pause, the “Hi” on its DTR pin goes “Lo”; the Mac detects this new condition on its CTS pin and stops sending data until its CTS pin is toggled “Hi” again. Got it?

Another problem, and a further bastardization of the RS-232 standard, occurs when two DTEs want to talk to each other using software (XON/XOFF) handshaking but have pins on their serial ports that are expecting hardware handshaking. This situation requires the old “fake-out” handshaking method. Pins that are normally “Hi” are jumpered to the pins that expect a “Hi” before they will allow data transmission to begin. RS-232 cables for the IBM are notorious for this and require jumpering several pins to get the serial port to work. Usually, the normally “Hi” RTS pin is jumpered to the normally “Lo” CTS pin, while the normally “HI” DTR pin is jumpered to both DSR and CD. This “fake-out” method is very common in situations where multi-conductor cable is not available or where the exact method of hardware handshaking is not known. All in all, the RS-232 standard has become a very unstandard standard.

Anyway, enough sermonizing and on to some constuction details for the ultimate in Mac serial port switching...MACBOX!!! The first detail that needed to be resolved was the 8-pin miniDIN socket used by the newer Macs. The required plugs are impossible to find and wouldn’t work well panel mounted into a box. Since my Mac was an updated 512 machine, I had a couple miniDIN to DB-9 adaptor cables that I extended to reach MacBox. Panel mount DB-9s are readily available, so I used two male DB-9s as inputs from the Mac and four female DB-9s as outputs to peripherals. Note when wiring the pins that they are numbered in opposite directions on the male and female connectors (I didn’t). The protective and signal grounds are now tied together within the Mac, so only three wires are normally needed to connect serial devices. I included a handshaking line since it was wired that way in my modem cable and is needed for the IMW (the other devices handshake through software XON/XOFF).

Initially everything worked great until I decided to get the M100 to talk to the IMW. First of all, the M100 usually talks to printers through a parallel port. Thankfully, the built-in BASIC accesses the serial port. Also, I have SuperRom installed in my M100 which contains a word processor that accesses the serial port. So, another switch and a little more wiring later, I was soon printing away on the IMW from the M100...sort of. After about a page of data, everything would turn to garbage. Obviously I was overloading the IMW’s input buffer, no amount of toggling the IMW’s DTR pin was shutting off the M100 from continuing to send data. I soon discovered that while the handshaking pins were active on the M100, there was no firmware (ROM routines) to recognize hardware handshaking. About then Big Byte stepped in and handed me a set of golden tablets containing a M100 mod that would give the M100 adequate hardware handshaking. The mod details involved the piggybacking of a quad nand gate chip inside the M100 to use the DSR line to read the IMW’s DTR line. To be more consistent to the Mac’s way of doing things, I could have easily use the CTS line instead. (See references below)

Soon the M100-IMW connection was yakking away with no problems...except now the M100 wasn’t talking to the Mac. I soon realized that I needed to run another wire from the Mac’s DTR pin to the M100 DSR pin to “fake out” the M100 even though it was talking to the Mac using software handshaking. Tracing the wiring in the accompanying diagram should reveal the necessary connections.

References:

MacAccess, Hayden Books,1987

Understanding Data Com., TI Library

Imagewriter II Owner’s Manual

Portable 100 Magazine, 12/87

Modern Electronics Magazine, 5/86

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

calibre 2.17 - Complete e-library manage...
Calibre is a complete e-book library manager. Organize your collection, convert your books to multiple formats, and sync with all of your devices. Let Calibre be your multi-tasking digital librarian... Read more
OmniGraffle Pro 6.1.2 - Create diagrams,...
OmniGraffle Pro helps you draw beautiful diagrams, family trees, flow charts, org charts, layouts, and (mathematically speaking) any other directed or non-directed graphs. We've had people use... Read more
OmniGraffle 6.1.2 - Create diagrams, flo...
OmniGraffle helps you draw beautiful diagrams, family trees, flow charts, org charts, layouts, and (mathematically speaking) any other directed or non-directed graphs. We've had people use Graffle to... Read more
RoboForm 2.0.2 - Password manager; syncs...
RoboForm is a password manager that offers one-click login, mobile syncing, easy form filling, and reliable security. Password Manager. RoboForm remembers your passwords so you don't have to! Just... Read more
Apple MainStage 3.1 - Live performance t...
Love the sound you got on your recording? MainStage 3 makes it easy to bring all the same instruments and effects to the stage. Everything from the Sound Library and Smart Controls you're familiar... Read more
Freeway Pro 7.0.2 - Drag-and-drop Web de...
Freeway Pro lets you build websites with speed and precision... without writing a line of code! With its user-oriented drag-and-drop interface, Freeway Pro helps you piece together the website of... Read more
A Better Finder Rename 9.44 - File, phot...
A Better Finder Rename is the most complete renaming solution available on the market today. That's why, since 1996, tens of thousands of hobbyists, professionals and businesses depend on A Better... Read more
Stacks 2.6.9 - New way to create pages i...
Stacks is a new way to create pages in RapidWeaver. It's a plugin designed to combine drag-and-drop simplicity with the power of fluid layout. Features: Fluid Layout: Stacks lets you build pages... Read more
Sid Meier's Civilization: Beyond Ea...
Sid Meier's Civilization: Beyond Earth is a new science-fiction-themed entry into the award-winning Civilization series. Set in the future, global events have destabilized the world leading to a... Read more
Logic Pro X 10.1 - Music creation and au...
Apple Logic Pro X is the most advanced version of Logic ever. Sophisticated new tools for professional songwriting, editing, and mixing are built around a modern interface that's designed to get... Read more

Choice Provisions is Set to Launch Destr...
Choice Provisions is Set to Launch Destructamundo on iOS This Month Posted by Tre Lawrence on January 23rd, 2015 [ permalink ] Choice Provisions – home stable to | Read more »
King of Thieves – An Interview With Zept...
Ahead of the release of ZeptoLab’s King of Thieves, we were able to ask ZeptoLab’s co-founder, Semyon Voinov, a few questions about the inspiration behind the game and what that means for the Cut the Rope franchise. | Read more »
Handle Review
Handle Review By Jennifer Allen on January 23rd, 2015 Our Rating: :: SPEEDY ORGANIZINGUniversal App - Designed for iPhone and iPad Handle is a very convenient way of juggling your emails, To Do list, and Calendar all through one... | Read more »
The New Disney Inquizitive App Offers a...
The New Disney Inquizitive App Offers a Place for Fans to Take Disney Quizzes Posted by Tre Lawrence on January 23rd, 2015 [ permalink ] | Read more »
Hands-On With Cut the Rope Developer Zep...
Marking quite a departure from ZeptoLab’s past successes, namely the Cut The Rope series, King of Thieves is shaping up to be quite promising. Due for release in February, we were lucky enough to have some time with a preview build to see exactly... | Read more »
Fast Fishing Review
Fast Fishing Review By Jennifer Allen on January 23rd, 2015 Our Rating: :: LIVES UP TO ITS NAMEUniversal App - Designed for iPhone and iPad Fishing is far from relaxing in Fast Fishing, but it is fun.   | Read more »
Head Back to Dark World of Arnashia in B...
Head Back to Dark World of Arnashia in Blood Brothers 2 Posted by Jessica Fisher on January 23rd, 2015 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Saved Review
Saved Review By Jennifer Allen on January 23rd, 2015 Our Rating: :: SIMPLE BUDGETINGiPhone App - Designed for the iPhone, compatible with the iPad Saved is a convenient and speedy way of adding expenses and keeping track of your... | Read more »
Wooga Announces Agent Alice, a New Hidde...
Wooga Announces Agent Alice, a New Hidden Object Game Posted by Jessica Fisher on January 23rd, 2015 [ permalink ] Wooga, makers of | Read more »
One Tap Desert Hero Review
One Tap Desert Hero Review By Campbell Bird on January 23rd, 2015 Our Rating: :: DRY GAMEPLAYiPhone App - Designed for the iPhone, compatible with the iPad This one-touch arcade game uses random elements and unlocks to try to keep... | Read more »

Price Scanner via MacPrices.net

College Student Deals are back, additional $5...
Take an additional $50 off all MacBooks and iMacs at Best Buy Online with their College Students Deals Savings, valid through April 11, 2015. Anyone with a valid .EDU email address can take advantage... Read more
iPhone 6 and 6 Plus GIve Apple Half Of US Mob...
Chicago-based Consumer Intelligence Research Partners, LLC (CIRP) have released analysis of the results of its research on mobile phone manufacturers for the calendar quarter that ended December 31,... Read more
Save $100 on MacBook Airs with 256GB of stora...
B&H Photo has 256GB MacBook Airs on sale for $100 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 11″ 1.4GHz/256GB MacBook Air: $999 $100 off MSRP - 13″ 1.4GHz/256GB MacBook... Read more
21-inch 2.7GHz iMac on sale for $1179, save $...
B&H Photo has the 21″ 2.7GHz iMac on sale for $1179 including free shipping plus NY sales tax only. Their price is $120 off MSRP, and it’s the lowest price available for this model from any... Read more
iPhone Usage Rates by State Correlate With Ed...
Chitika Insights notes that despite iPhones being the largest source of smartphone Internet traffic in North America, their latest study finds a relatively high degree of variation of iPhone usage... Read more
ProGearX Extendable Pole “Pov/Selfie Stick” M...
There’s something inescapably narcissistic about the concept of selfies as they’ve developed as a smartphone-driven social (particularly social media) phenomenon that rubs me the wrong way. However,... Read more
iPad Air 2 on sale for up to $100 off MSRP, 2...
 Best Buy has iPad Air 2s on sale for up to $100 off MSRP on their online store for the next two days. Choose free shipping or free local store pickup (if available). Sale prices available for online... Read more
Roundup of Apple refurbished MacBook Pros and...
The Apple Store has Apple Certified Refurbished 2014 MacBook Pros and MacBook Airs available for up to $400 off the cost of new models. An Apple one-year warranty is included with each model, and... Read more
Sale! 13-inch 2.8GHz Retina MacBook Pro for $...
 B&H Photo has the 13″ 2.8GHz Retina MacBook Pro on sale for $1599 including free shipping plus NY sales tax only. Their price is $200 off MSRP, and it’s the lowest price available for this model... Read more
Next OS X/iOS Version Upgrades Should Concent...
On stage at Apple’s World Wide Developers’ Conference in June 2009, Bertrand Serlet, the company’s Senior Vice President of Software Engineering at the time, announced that the forthcoming OS X... Read more

Jobs Board

*Apple* Acura/Subaru Service Technicians - A...
Apple Automotive is one of the fastest growing dealer…and it shows. Consider making the switch to the Apple Automotive Group today! At Apple Automotive , Read more
Business Development Manager - *Apple* Pay...
**Job Summary** Apple Pay is seeking an experienced business development manager to support the identification, recruitment, negotiation and ongoing management of Read more
*Apple* Solutions Consultant (ASC)- Retail S...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
*Apple* Solutions Consultant - Retail Sales...
**Job Summary** As an Apple Solutions Consultant (ASC) you are the link between our customers and our products. Your role is to drive the Apple business in a retail Read more
*Apple* Lead Operator, GSOC - Apple (United...
**Job Summary** Apple is seeking an exceptional, customer service oriented and experienced persons to fulfill the role of Apple Lead Operator (ALO) as part of the Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.