TweetFollow Us on Twitter

Serial Ports as Slots
Volume Number:1
Issue Number:8
Column Tag:The Electrical Mac

"Macintosh Serial Ports"

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

The Macintosh has frequently been criticized for its lack of hardware expansion capability. Apple's response has been that the two high-speed serial ports act as 'virtual slots' which give the user practically unlimited expansion capability. Although the merits of this approach may be debatable, it is nevertheless our only point of access to the internals of the Macintosh.

In this article, I will describe both the RS-232-C and the RS-422-A standards and examine their compatibility. I will then examine in detail Apple's implementation of RS-422 in the Macintosh serial ports and describe how to interface them with standard RS-232 peripherals, along with precautions to be taken in order to maintain data integrity and protect your equipment from damage.

RS-232-C

Electronic Industries Association (EIA) Standard RS-232-C, Interface Between Data Terminal Equipment and Data Communication Equipment Employing Serial Binary Data Interchange, released in August 1969, defines how data terminal equipment (DTE) and data communication equipment (DCE) exchange information serially.

Originally, DTE was intended to be a dumb terminal, and DCE was intended to be a modem. The large number of handshake lines were included to provide control over what were entirely hardware driven devices, with no local intelligence. As microprocessors became popular and devices became capable of implementing handshaking protocols in software, many of the handshake lines became superfluous. This, combined with the popularity of using RS-232 to connect all types of different devices not even imagined at the time the standard was written, makes signal definition somewhat subjective and is what causes us so much grief when trying to make interconnecting cables.

EIA Standard RS-232-C defines electrical, mechanical, and signal characteristics. Electrically, a mark, or binary one, is defined as less than 3 volts with respect to signal ground, while a space, or binary zero, is greater than 3 volts. The region between +3 volts is defined as the transition region, where the signal state is not uniquely defined. The maximum open-circuit voltage is not to exceed +25 volts. In addition, there are some restrictions on source and load impedances, load capacitance, and slew rates.

Mechanically, the standard assigns signal definitions to 25 pins (including 3 unassigned pins), although no particular connector type is specified. However, the 25 pin subminiature-D type connector (DB-25) has emerged as a de-facto standard. Cable length is recommended to be kept to less than 50 feet, although the load capacitance of 2500 picofarads is really the limiting factor.

Signal characteristics are defined in some detail in the standard, although if not applied to a strict terminal/modem configuration, they tend to become open to interpretation. Data rates are restricted to a nominal 20,000 bits per second.

Standards notwithstanding, how is RS-232 generally implemented? Electrically, most equipment uses MC1488 (or equivalent) drivers operating from +12 volt supplies, and MC1489 (or equivalent) receivers operating from +5 volts. The drivers accept a standard TTL level in and convert it to about +10 volts out, inverting it in the process to meet the RS-232 mark/space requirements.

The receivers do not operate in strict accordance with RS-232, since their input switching threshold is around +1 volt. Therefore these receivers will correctly respond not only to RS-232 input levels, but TTL level inputs also. Like the drivers, the output is inverted from the input.

RS-422-A, RS-449

EIA Standard RS-422-A, Electrical Characteristics of Balanced Voltage Digital Interface Circuits, released in December 1978, defines the interchange of serial binary signals between DTE and DCE, or in any point-to-point interconnection of serial binary signals between digital equipment.

There are several significant differences between RS-232 and RS-422. Most importantly, RS-422 is a balanced interface. Secondly, note that this standard defines electrical characteristics only. EIA Standard RS-449 defines the mechanical and signal characteristics and will be included in this discussion. A final, more subtle point: notice that the description of the standard has been expanded to include connection of any digital equipment, not only DCE and DTE.

The RS-422 specification defines a balanced interface. This means that instead of a voltage on one signal line being referenced to a common ground, as in RS-232, the signal is carried on two lines which are always in complementary states (see Fig.1). The threshold criteria in a balanced system is the voltage difference between the two signals, not the absolute value of either with respect to ground. The open circuit voltage between the two outputs is not to exceed 6.0 volts, and cannot be less than 2.0 volts when driving a load of 100 + 1%. In addition, there are restrictions on offset voltages, short circuit output current, and receiver specifications, among others.

Mechanically, RS-449 defines a 37 pin and a 9 pin interface, where the 9 pin interface is defined as a secondary channel for the 37 pin interface. As with RS-232, signals are given pin assignments. Unlike RS-232, standard connectors are specified. In recognition of the de-facto DB-25 standard for RS-232, RS-449 defines DS-37 and DE-9 connectors (The D indicates subminiature-D type, the B, S, and E indicate shell size, and the 25, 37, and 9 are number of pins. Note that a DB-9 would be a DB-25 with 16 pins missing.). Maximum cable length using 24 AWG twisted pair cable varies according to signalling rate, from 50 feet at 10 Mbits/sec to 4000 feet at 100 Kbits/sec or less.

Signal characteristics are described in RS-449, again with the emphasis on a terminal/modem configuration. A table showing equivalent RS-232 signals is provided in the standard.

The industry standards for RS-422 drivers and receivers are the Am26LS31 (or equivalent) and the Am26LS32 (or equivalent), respectively. Both of these devices operate off a single +5 volt supply, and meet all the requirements set forth in the EIA standard.

Compatibility

As defined, how compatible is RS-422 with RS-232? During development of the RS-422 and RS-449 standards, the EIA gave close attention to an orderly transition from existing RS-232 equipment. For systems designed strictly to the letter of the standards, interoperability is assured. Realistically, most systems will be compatible electrically, but signal definitions and mechanical connections are another matter.

Remember, however, that RS-422 is differential, and the only relation it has to signal ground is that the midpoint of the output voltage swing must be within +3 volts of ground and the input to either receiver input terminal cannot exceed +10 volts with respect to ground. For systems operating off of standard supply voltages and using industry standard parts, this is not a problem. In addition, RS-422 does not meet the voltage swing requirement of RS-232, but generally suffices at the short cable lengths found in RS-232 installations. The specifics of how to interface a single-ended to a differential signal will be covered below.

Do you need to worry about damaging your Mac or peripherals while trying to connect a new piece of equipment? RS-232 specifies that drivers be able to withstand an open circuit, a short to any other signal line, or a short to ground without damage. Receivers must be able to withstand +25 volts without damage. A short between any two signal lines should not result in a current of more than 1/2 amp. RS-232, therefore, is essentially bulletproof.

RS-422 leaves fail-safe operation more to the system designer, but specifies that an output shorted to ground be limited to 150 mA and that receivers shall operate correctly over a common mode voltage of +7 volts and with a maximum differential voltage of 12 volts. These requirements result in a fail-safe system as long as it remains an RS-422 interface.

For a mixed RS-232/RS-422 system, there is a possibility of device damage since the maximum RS-232 voltage levels are +25 volts. The 26LS32 receivers have been designed to withstand this input voltage range, meaning that they can be connected to RS-232 drivers with relative impunity. RS-422 drivers can be damaged by shorting them to voltages above 5.5 volts or below ground (26LS31), or to voltages greater than +15 volts (26LS30). To summarize, RS-422 cannot damage RS-232, but RS-232 can damage RS-422 under some conditions.

Macintosh Serial Ports

In order for their concept of the 'virtual slot' to be accepted as a viable alternative to true hardware expandability, Apple had to forego the industry standard RS-232 in favor of an interface that would support a much higher data rate, thus the choice of RS-422. They also chose to compromise the standard somewhat in order to make the output voltages more compatible with RS-232.

For output drivers, the Macintosh uses 26LS30's running from +5 volt supplies, rather than the single +5 volt supply recommended by the manufacturer for RS-422 operation. The result is a symmetrical open-circuit voltage swing of about 8 volts centered around ground. This drops to about 4 volts when loaded with 100 . Centering the output signal around ground makes it RS-232 compatible, but the 8 volt open circuit voltage exceeds the RS-422 restriction of a maximum of 6 volts.

The receivers are 26LS32's and meet all RS-422 requirements. Since the Macintosh handshake line is a single-ended input, the inverting input on that receiver is tied to ground, making that input RS-232 compatible. The TXD and RXD lines are differential but are compatible with RS-232 levels when connected to the inverting (-) terminals. The 26LS32 is designed so the non-inverting (+) input floats at about +2.7 volts when open and the inverting input floats at about +1.8 volts. Therefore, when an RS-232 signal is connected to the inverting input the receiver will switch around +2.7 volts if the non-inverting input is left open, which is within the +3 volt RS-232 spec.

Between the drivers/receivers and the connectors, the signal lines go through an RC filter network. This is not part of the RS-422 specification, but is included to meet FCC EMI requirements and to provide signal deglitching.

The Macintosh serial connector pinout is as follows:

PIN # FUNCTION

1 GND

2 +5 V

3 GND

4 TXD +

5 TXD -

6 +12 V

7 HSK

8 RXD +

9 RXD -

For RS-232 compatibility, use Gnd, TXD-, and RXD- (remember, RS-232 signals are inverted). The HSK line goes to both the CTS and the TRxC inputs on the 8530 chip in the Macintosh. Depending upon how the chip is configured by the serial driver, this line may be used as a handshake line or as an external clock in order to increase data transfer speeds. The +5 volt and +12 volt outputs come directly from the power supply inside the Macintosh. Apple warns that the +5 volt line may turn into an output handshake line at some future date and that it should not be used. They also warn that the +12 volt line is for detecting power only. When connecting peripherals or making cables, exercise caution, but you'll probably not be able to damage any of the interface circuitry. Pay special attention to the power pins (2 & 6), however, since these are tied directly to the power supply and are not protected from accidental shorting.

Electrically, like most of the Macintosh hardware, the serial ports are quite simple. Some compromises to the RS-422 standard were made in Apple's implementation, but generally speaking, these are not significant. The end result has been to provide users with a much more flexible interface, able to communicate with nearly all RS-232 peripherals in addition to supporting much higher data transfer rates for applications that require it, such as external disk drives and networking.

 
AAPL
$102.50
Apple Inc.
+0.25
MSFT
$45.43
Microsoft Corpora
+0.55
GOOG
$571.60
Google Inc.
+2.40

MacTech Search:
Community Search:

Software Updates via MacUpdate

Path Finder 6.5.5 - Powerful, award-winn...
Path Finder is a file browser that combines the familiar Finder interface with the powerful utilities and innovative features. Just a small selection of the Path Finder 6 feature set: Dual pane... Read more
QuarkXPress 10.2.1 - Desktop publishing...
With QuarkXPress, you can communicate in all the ways you need to -- and always look professional -- in print and digital media, all in a single tool. Features include: Easy to Use -- QuarkXPress is... Read more
Skype 6.19.0.450 - Voice-over-internet p...
Skype allows you to talk to friends, family and co-workers across the Internet without the inconvenience of long distance telephone charges. Using peer-to-peer data transmission technology, Skype... Read more
VueScan 9.4.41 - Scanner software with a...
VueScan is a scanning program that works with most high-quality flatbed and film scanners to produce scans that have excellent color fidelity and color balance. VueScan is easy to use, and has... Read more
Cloud 3.0.0 - File sharing from your men...
Cloud is simple file sharing for the Mac. Drag a file from your Mac to the CloudApp icon in the menubar and we take care of the rest. A link to the file will automatically be copied to your clipboard... Read more
LibreOffice 4.3.1.2 - Free Open Source o...
LibreOffice is an office suite (word processor, spreadsheet, presentations, drawing tool) compatible with other major office suites. The Document Foundation is coordinating development and... Read more
SlingPlayer Plugin 3.3.20.505 - Browser...
SlingPlayer is the screen interface software that works hand-in-hand with the hardware inside the Slingbox to make your TV viewing experience just like that at home. It features an array of... Read more
Get Lyrical 3.8 - Auto-magically adds ly...
Get Lyrical auto-magically add lyrics to songs in iTunes. You can choose either a selection of tracks, or the current track. Or turn on "Active Tagging" to get lyrics for songs as you play them.... Read more
Viber 4.2.2 - Send messages and make cal...
Viber lets you send free messages and make free calls to other Viber users, on any device and network, in any country! Viber syncs your contacts, messages and call history with your mobile device,... Read more
Cocktail 7.6 - General maintenance and o...
Cocktail is a general purpose utility for OS X that lets you clean, repair and optimize your Mac. It is a powerful digital toolset that helps hundreds of thousands of Mac users around the world get... Read more

Latest Forum Discussions

See All

Rhonna Designs Magic (Photography)
Rhonna Designs Magic 1.0 Device: iOS Universal Category: Photography Price: $1.99, Version: 1.0 (iTunes) Description: Want to sprinkle *magic* on your photos? With RD Magic, you can add colors, filters, light leaks, bokeh, edges,... | Read more »
This Week at 148Apps: August 25-29, 2014
Shiny Happy App Reviews   | Read more »
Qube Kingdom – Tips, Tricks, Strategies,...
Qube Kingdom is a tower defense game from DeNA. You rally your troops – magicians, archers, knights, barbarians, and others – and fight against an evil menace looking to dominate your kingdom of tiny squares. Planning a war isn’t easy, so here are a... | Read more »
Qube Kingdom Review
Qube Kingdom Review By Nadia Oxford on August 29th, 2014 Our Rating: :: KIND OF A SQUARE KINGDOMUniversal App - Designed for iPhone and iPad Qube Kingdom has cute visuals, but it’s a pretty basic tower defense game at heart.   | Read more »
Fire in the Hole Review
Fire in the Hole Review By Rob Thomas on August 29th, 2014 Our Rating: :: WALK THE PLANKUniversal App - Designed for iPhone and iPad Seafoam’s Fire in the Hole looks like a bright, 8-bit throwback, but there’s not enough booty to... | Read more »
Alien Creeps TD is Now Available Worldwi...
Alien Creeps TD is Now Available Worldwide Posted by Ellis Spice on August 29th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Dodo Master Review
Dodo Master Review By Jordan Minor on August 29th, 2014 Our Rating: :: NEST EGGiPad Only App - Designed for the iPad Dodo Master is tough but fair, and that’s what makes it a joy to play.   | Read more »
Motorsport Manager Review
Motorsport Manager Review By Lee Hamlet on August 29th, 2014 Our Rating: :: MARVELOUS MANAGEMENTUniversal App - Designed for iPhone and iPad Despite its depth and sense of tactical freedom, Motorsport Manager is one of the most... | Read more »
Motorsport Manager – Beginner Tips, Tric...
The world of Motorsport management can be an unforgiving and merciless one, so to help with some of the stress that comes with running a successful race team, here are a few hints and tips to leave your opponents in the dust. | Read more »
CalPal Update Brings the App to 2.0, Add...
CalPal Update Brings the App to 2.0, Adds Lots of New Stuff Posted by Ellis Spice on August 29th, 2014 [ permalink ] | Read more »

Price Scanner via MacPrices.net

Are We Now In The Post-Post-PC Era?
A longtime and thoroughgoing laptop aficionado, I was more than a little dismayed by Steve Jobs’s declaration back in 2010 when he sprang the iPad on an unsuspecting world. that we’d entered a “post-... Read more
Apple now offering refurbished 21-inch 1.4GHz...
The Apple Store is now offering Apple Certified Refurbished 21″ 1.4GHz iMacs for $929 including free shipping plus Apple’s standard one-year warranty. Their price is $170 off the cost of new models,... Read more
Save $50 on the 2.5GHz Mac mini, on sale for...
B&H Photo has the 2.5GHz Mac mini on sale for $549.99 including free shipping. That’s $50 off MSRP, and B&H will also include a free copy of Parallels Desktop software. NY sales tax only. Read more
Save up to $300 on an iMac with Apple refurbi...
The Apple Store has Apple Certified Refurbished iMacs available for up to $300 off the cost of new models. Apple’s one-year warranty is standard, and shipping is free. These are the best prices on... Read more
The Rise of Phablets
Carlisle & Gallagher Consulting Group, a businesses and technology consulting firm focused solely on the financial services industry, has released an infographic depicting the convergence of... Read more
Bad Driver Database App Allows Good Drivers t...
Bad Driver Database 1.4 by Facile Group is a new iOS and Android app that lets users instantly input and see how many times a careless, reckless or just plain stupid driver has been added to the... Read more
Eddy – Cloud Music Player for iPhone/iPad Fre...
Ukraine based CapableBits announces the release of Eddy, its tiny, but smart and powerful cloud music player for iPhone and iPad that allows users to stream or download music directly from cloud... Read more
A&D Medical Launches Its WellnessConnecte...
For consumers and the healthcare providers and loved ones who care for them, A&D Medical, a leader in connected health and biometric measurement devices and services, has launched its... Read more
Anand Lal Shimpi Retires From AnandTech
Anand Lal Shimpi, whose AnandTech Website is famous for its meticulously detailed and thoroughgoing reviews and analysis, is packing it in. Lal Shimpi, who founded the tech site at age 14 in 1997,... Read more
2.5GHz Mac mini, Apple refurbished, in stock...
The Apple Store has Apple Certified Refurbished 2.5GHz Mac minis available for $509, $90 off MSRP. Apple’s one-year warranty is included, and shipping is free. Read more

Jobs Board

*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
Senior Event Manager, *Apple* Retail Market...
…This senior level position is responsible for leading and imagining the Apple Retail Team's global event strategy. Delivering an overarching brand story; in-store, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.