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

DaisyDisk 4.4 - $9.99
DaisyDisk allows you to visualize your disk usage and free up disk space by quickly finding and deleting big unused files. The program scans your disk and displays its content as a sector diagram... Read more
BetterTouchTool 2.07 - Customize Multi-T...
BetterTouchTool adds many new, fully customizable gestures to the Magic Mouse, Multi-Touch MacBook trackpad, and Magic Trackpad. These gestures are customizable: Magic Mouse: Pinch in / out (zoom... Read more
BetterTouchTool 2.07 - Customize Multi-T...
BetterTouchTool adds many new, fully customizable gestures to the Magic Mouse, Multi-Touch MacBook trackpad, and Magic Trackpad. These gestures are customizable: Magic Mouse: Pinch in / out (zoom... Read more
PDFpen 8.3.2 - $74.95
PDFpen allows users to easily edit PDF's. Add text, images and signatures. Fill out PDF forms. Merge or split PDF documents. Reorder and delete pages. Even correct text and edit graphics! Features... Read more
DiskCatalogMaker 6.5.20 - Catalog your d...
DiskCatalogMaker is a simple disk management tool which catalogs disks. Simple, light-weight, and fast Finder-like intuitive look and feel Super-fast search algorithm Can compress catalog data for... Read more
Things 2.8.9 - Elegant personal task man...
Things is a task management solution that helps to organize your tasks in an elegant and intuitive way. Things combines powerful features with simplicity through the use of tags and its intelligent... Read more
PDFpenPro 8.3.2 - $124.95
PDFpenPro allows users to edit PDF's easily. Add text, images and signatures. Fill out PDF forms. Merge or split PDF documents. Reorder and delete pages. Create fillable forms and tables of content... Read more
Things 2.8.9 - Elegant personal task man...
Things is a task management solution that helps to organize your tasks in an elegant and intuitive way. Things combines powerful features with simplicity through the use of tags and its intelligent... Read more
DiskCatalogMaker 6.5.20 - Catalog your d...
DiskCatalogMaker is a simple disk management tool which catalogs disks. Simple, light-weight, and fast Finder-like intuitive look and feel Super-fast search algorithm Can compress catalog data for... Read more
PDFpenPro 8.3.2 - $124.95
PDFpenPro allows users to edit PDF's easily. Add text, images and signatures. Fill out PDF forms. Merge or split PDF documents. Reorder and delete pages. Create fillable forms and tables of content... Read more

Latest Forum Discussions

See All

Pokémon GO Generation 2 evolution guide
At long last, Niantic Labs finally unleashed the Generation 2 Pokémon into the wild. Pokémon GO trainers are scrambling to grab up this new set of 80 Pokémon. There are some special new tricks required to catch all of these new beasties, though.... | Read more »
The best new games we played this week
It feels as though the New Year got off to a creaking start as far as mobile games go, but that's changed over the past few weeks. The last few days alone have seen the debut of a number of wonderful games, so we thought we'd take the time to... | Read more »
Recruit more scallywags and discover new...
Get ready to show off your sea legs all over again in Oceans & Empires’ new grand update, which aims to make the act of rising to the role of seven seas ruler even more fresh and appealing, thanks to a richness of new content on both iOS and... | Read more »
Mage the Ascension: Refuge (Games)
Mage the Ascension: Refuge 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: The groundbreaking roleplaying game Mage: The Ascension manifests in our turbulent present with Refuge, an... | Read more »
Vampire: Prelude (Games)
Vampire: Prelude 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: The classic roleplaying game Vampire: The Masquerade returns to digital games with a Prelude of things to come. Experience a... | Read more »
Digby Forever Guide: How to dig to the d...
Digby Forever is a sparkling homage to arcade classics, and while you may be tiring of the number of arcade games being thrown at you, this endless digger finds many ways to stand out from the rest of the pack. The game manages to be challenging... | Read more »
The best sales on the App Store this wee...
It's been quite the week in mobile games, but if the latest releases(there were some pretty darn good ones, in case you missed out) aren't really doing the trick, perhaps some of these discounted games will. Many of these premium games had their... | Read more »
Why the new Fire Emblem Heroes update sh...
It’s exciting to see Nintendo delving into the mobile sphere, regardless of whether it’s to give fans another platform to enjoy their fans or simply a sound business venture. Two of the company's announced mobile games have finally come to... | Read more »
New Fire Emblem Heroes update adds new h...
Fire Emblem Heroes received a sizeable update first thing this morning. The update features a batch of fresh content along with a few updates to the game's systems. [Read more] | Read more »
The Deep Paths (Games)
The Deep Paths 1.0 Device: iOS iPhone Category: Games Price: $3.99, Version: 1.0 (iTunes) Description: 25% off launch sale!!! The Deep Paths: Labyrinth Of Andokost is a first-person, dungeon crawling RPG, with traditional grid-based... | Read more »

Price Scanner via MacPrices.net

15-inch Touch Bar MacBook Pros on sale for up...
Amazon has 15″ Touch Bar MacBook Pros in stock today and on sale for up to $150 off MSRP including free shipping: - 15″ 2.7GHz Touch Bar MacBook Pro Space Gray: $2720.38 $79 off MSRP - 15″ 2.7GHz... Read more
Apple’s Education discount saves up to $300 o...
Purchase a new Mac or iPad using Apple’s Education Store and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free: -... Read more
13-inch 1.6GHz/256GB MacBook Air on sale for...
Newegg has the 13″ 1.6GHz/256GB MacBook Air (MMGG2LL/A) on sale for $1029.99 including free shipping. Their price is $170 off MSRP, and it’s the lowest price available for this model. Choose Newegg... Read more
Save up to $600 with Apple refurbished Mac Pr...
Apple has Certified Refurbished Mac Pros available for up to $600 off the cost of new models. An Apple one-year warranty is included with each Mac Pro, and shipping is free. The following... Read more
12-inch 1.1GHz Retina MacBooks on sale for $1...
B&H has 12″ 1.1GHz Retina MacBooks on sale for $150 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 12″ 1.1GHz Space Gray Retina MacBook: $1149 $150 off MSRP - 12″ 1.1GHz... Read more
InTouch Health Expands iOS And Windows Produc...
Specialty telehealth enterprise provider InTouch Health has announced an expanded range of FDA Class I listed medical devices and software solutions for ambulatory, non-acute and non-emergent... Read more
iMobie Airs World’s 1st iCloud Manager with M...
iMobie Inc., an Apple-related software company, announced their newly-updated iPhone manager AnyTrans with exclusive feature to sync and manage contents across multiple iCloud accounts. With it,... Read more
New Proactive Apple Support Professional Cert...
Watchman Monitoring has announced Proactive Support Professional Certification at MacTech Pro. Watchman Monitoring is a premier Proactive Support Software as a Service (SaaS) tool for IT... Read more
13-inch 2.7GHz Retina MacBook Pro on sale for...
B&H Photo has the 2015 13″ 2.7GHz/128GB Retina Apple MacBook Pro on sale for $100 off MSRP. Shipping is free, and B&H charges NY tax only: - 13″ 2.7GHz/128GB Retina MacBook Pro (MF839LL/A): $... Read more
Back in stock: Apple refurbished 13-inch Reti...
Apple has Certified Refurbished 2015 13″ Retina MacBook Pros available for up to $360 off original MSRP, starting at $1099. An Apple one-year warranty is included with each model, and shipping is... Read more

Jobs Board

*Apple* Retail - Multiple Positions - Apple,...
SalesSpecialist - Retail Customer Service and SalesTransform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* macOS Systems Integration Administra...
…most exceptional support available in the industry. SCI is seeking an Junior Apple macOS systems integration administrator that will be responsible for providing Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Lead Senior *Apple* (Mac) Administrator - S...
…but also to protect them. Position Summary: SC3 is actively seeking a Senior Apple (Mac) Administrator that will be responsible for providing Apple Mac Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.