TweetFollow Us on Twitter

Jun 98 Tips

Volume Number: 14 (1998)
Issue Number: 6
Column Tag: Tips & Tidbits

June 1998 Tips & Tidbits

by Steve Sisak

Over the past few months we've had a few tips submitted on how to open a serial port or detect if it is in use. Unfortunately, we haven't received one that was sufficiently correct or complete to publish as a winner. Since this seems like an interesting topic (and one that lots of people get wrong), I'm going to try a new format: a terse mini-article with enough information to get you started and pointers to more information.

If you like this format, have a good idea for a topic but don't know the answer, or have other ideas how to make this space more useful, please send mail to tips@mactech.com. I'll be glad to pay the standard reward and give you credit for a really good question (assuming I can find the answer and it's generally useful).

Serial Port TidBits

If you've ever tried to write an application which uses the serial port on a Macintosh, you've probably discovered that (1) it didn't work on the first try, (2) the information on what to do was scattered all over the place and (3) it still didn't work in all cases.

In case you haven't, the information for how to correctly open and use the serial drivers is scattered across Inside Macintosh, the Communication Toolbox documentation, the ARA SDK and various tech notes. There are also several misleading and obsolete descriptions in Inside Macintosh Vols. I-VI.

The most authoritative sources are Inside Macintosh: Devices and Tech note 1119, by Quinn (The Eskimo!) which pulls most of the relevant information together in one place.

Listing the Serial Ports

In the beginning, every Macintosh had exactly 2 serial ports named "Modem Port" and "Printer Port" and the names of their drivers were hard coded -- these days PowerBooks often have only one port and/or a built-in modem, NuBus and PCI cards make it possible for the user to add ports, and software creates "virtual" ports to make it possible for multiple programs to share the same physical port.

To determine how many ports a machine has and what their human-readable name are, you need to use the Communications Resource Manager (CRM), which is part of the Communications Toolbox (one of those managers that Apple has declared obsolete, but hasn't gotten around to replacing yet).

For each port, the CRM maintains a CRMSerialRecord containing the following information:

typedef struct CRMSerialRecord {
  short         version;
  StringHandle   inputDriverName;
  StringHandle   outputDriverName;
  StringHandle   name;
  CRMIconHandle  deviceIcon;
  long           ratedSpeed;
  long           maxSpeed;
  long           reserved;
} CRMSerialRecord, *CRMSerialPtr;

To iterate over the available ports, you use the function CRMSearch(). The following code fragment finds a port by name -- you can easily adapt it to build a menu, etc.:

CRMSerialPtr FindPortInfo(ConstStr255Param name)
{
  CRMRec      crmRec;
  CRMRecPtr    crm   = &crmRec;

  // Get the search started
  crmRec.crmDeviceType = crmSerialDevice;  crmRec.crmDeviceID   = 0;

  while ((crm = CRMSearch(crm)) != nil)
  {
    CRMSerialPtr portInfo = 
      (CRMSerialPtr) crm->crmAttributes;
    
    if (EqualString(*portInfo->name, name, false, true))
    {
      return portInfo;
    }
  }
  
  return nil;
}

Opening, Initializing and Closing a Serial port

There is a specific sequence of calls you must use to open, configure and close a serial port. It is listed in Inside Macintosh: Devices on page 7-11. If you do not make the calls in this order, strange things will happen.

The sequence is:

  1. Open the output driver, then the input driver; always open both.
  2. (optional) allocate a buffer larger than the default 64-byte buffer and call SerSetBuf.
  3. Set the handshaking mode.
  4. Set the baud rate and data format.
  5. Read and/or write the desired data.
  6. Call KillIO on both drivers to terminate any pending IO.
  7. Restore the default input buffers.
  8. Close the input driver, then the output driver.

Determining If a Serial Driver is Open

Determining if a serial driver is open in use is a little bit tricky and a lot of software gets it wrong. The problem is twofold: first, OpenDriver() doesn't bother to check if a driver is already open -- it just returns noErr and the reference number of the already-open driver. If you use it (or worse, close it when you're done) Bad Things(tm) will happen.

To get around this you must walk the device list in low memory to see if a driver is already open before trying to open it again.

The following routine finds the index of a driver in the unit table (or -1 if it doesn't exist):

short FindDriverIndex(ConstStr255Param name)
{
  StringPtr    driverName;
  short      index;
  AuxDCEHandle  entry;
  AuxDCEHandle*  table = (AuxDCEHandle*) LMGetUTableBase();
  short      count = LMGetUnitNtryCount();
  
  for (index = 0; index < count; index++)
  {
    if ((entry = table[index]) != nil)
    {
      if ((**entry).dCtlFlags & dRAMBasedMask)
      {
        driverName = (**((DRVRHeaderHandle)((**entry).dCtlDriver))).drvrName;
      }
      else
      {
        driverName = (*((DRVRHeaderPtr)((**entry).dCtlDriver))).drvrName;
      }
      
      if (EqualString(driverName, name, false, true))
      {
        return index;
      }
    }
  }
  
  return -1;
}

To check if a port is open, we can write:

Boolean  IsDriverOpen(ConstStr255Param name)
{
  short index = FindDriverIndex(name);
  
  if (index >= 0)
  {
    AuxDCEHandle dce = 
      ((AuxDCEHandle*) LMGetUTableBase())[index];
    
    if ((**dce).dCtlFlags & dOpenedMask)
    {
      return true;
    }
  }
  
  return false;
}

NOTE: LMGetUTableBase() is missing from some versions of the Universal Headers you may have to implement it yourself (or use newer headers).

Now for the second half of the problem -- the Serial Port Arbitrator, included with the Appletalk Remote Access server and other software allows a port to be opened "passively" meaning that a server may have a the port open to look for an incoming call, but will relinquish it if another application wants to use it.

In this case OpenDriver will return portInUse (-97) if the driver is open or noErr if it is not. (in either case, it will return a valid refNum). However, software which walks the device table will incorrectly think that the driver is open and report an error.

The correct procedure here is to use Gestalt to determine if the Serial Port Arbitrator is present and, if it is, then just call OpenDriver(), otherwise, walk the Unit Table:

Boolean  HaveSerialPortArbitration(void)
{
  long  result;
  OSErr  err;
  
  err = Gestalt(gestaltArbitorAttr, &result);
  
  return (err == noErr) && (result & (1 << gestaltSerialArbitrationExists));
}

OSErr OpenSerialDriver(ConstStr255Param name, short* refNum)
{
  if (!HaveSerialPortArbitration())
  {
    short index = FindDriverIndex(name);

    if (index >= 0)  // Driver is already open
    {
      *refNum = ~index;
      return portInUse;
    }
  }
  
  return OpenDriver(name, refNum);
}

Reading from a Serial Port

You can read from a serial port just like a file by using PBRead or FSRead, however you can't seek and if you try to read more bytes are actually available, you will hang the machine in SyncWait() until the number of bytes you requested is actually available.

To avoid this, you can make a status call to the input driver with csCode = 2 to find out how many bytes are available in the drivers buffer and then only request that many bytes.

Correction to May Tip -- Wrapper for ParamText

The tip "Wrapper for ParamText" incorrectly states that one cannot pass nil arguments to ParamText(). In fact, ParamText() has always accepted nil arguments, and leaves the corresponding parameters unchanged.

I use this feature frequently in order to conserve stack space. I can set all 4 parameters using only one Str255, as long as I do it one at a time.

Actually, this example only sets 3 params:

  void
ReportError( short doingIx, const char *what )
{
Str255          str;

  if( spareMem )
    DisposeHandle( spareMem );
  spareMem = NULL;

  CtoPstrcpy( str, (const unsigned char *)what, sizeof str );
  ParamText( str, NULL, NULL, NULL );

  GetErrString( doingIx, str );
  ParamText( NULL, str, NULL, NULL );

  /* Eww. But I don't have to ParamText menu item text every command,
    which might slow things down if we're being scripted.
  */

  if( doingIx == doingMenuCmd ) {
    str[0] = 0;
  if( tg.doingItem )
      GetID( tg.doingItem, str );
    ParamText( NULL, NULL, NULL, str );
  }

  else if( doingIx == doingMenuAct ) {
    str[0] = 0;
    if( tg.doingItem && tg.doingMenu )
    GetMenuItemText( GetMenu(tg.doingMenu), tg.doingItem, str );
    ParamText( NULL, NULL, NULL, str );
  }

  VerifyAlert( 144 );
  InitCursor();
  StopAlert( 144, DefaultFilter );
}

Tony Nelson
tonyn@tiac.net

 
AAPL
$102.08
Apple Inc.
+2.32
MSFT
$44.83
Microsoft Corpora
+0.75
GOOG
$523.95
Google Inc.
+3.11

MacTech Search:
Community Search:

Software Updates via MacUpdate

RapidWeaver 6.0 - Create template-based...
RapidWeaver is a next-generation Web design application to help you easily create professional-looking Web sites in minutes. No knowledge of complex code is required, RapidWeaver will take care of... Read more
NTFS 12.0.39 - Provides full read and wr...
Paragon NTFS breaks down the barriers between Windows and OS X. Paragon NTFS effectively solves the communication problems between the Mac system and NTFS, providing full read and write access to... Read more
RestoreMeNot 2.0.3 - Disable window rest...
RestoreMeNot provides a simple way to disable the window restoration for individual applications so that you can fine-tune this behavior to suit your needs. Please note that RestoreMeNot is designed... Read more
Macgo Blu-ray Player 2.10.9.1750 - Blu-r...
Macgo Mac Blu-ray Player can bring you the most unforgettable Blu-ray experience on your Mac. Overview Macgo Mac Blu-ray Player can satisfy just about every need you could possibly have in a Blu-ray... Read more
Apple iOS 8.1 - The latest version of Ap...
The latest version of iOS can be downloaded through iTunes. Apple iOS 8 comes with big updates to apps you use every day, like Messages and Photos. A whole new way to share content with your family.... Read more
TechTool Pro 7.0.5 - Hard drive and syst...
TechTool Pro is now 7, and this is the most advanced version of the acclaimed Macintosh troubleshooting utility created in its 20-year history. Micromat has redeveloped TechTool Pro 7 to be fully 64... Read more
PDFKey Pro 4.0.2 - Edit and print passwo...
PDFKey Pro can unlock PDF documents protected for printing and copying when you've forgotten your password. It can now also protect your PDF files with a password to prevent unauthorized access and/... Read more
Yasu 2.9.1 - System maintenance app; per...
Yasu was originally created with System Administrators who service large groups of workstations in mind, Yasu (Yet Another System Utility) was made to do a specific group of maintenance tasks... Read more
Hazel 3.3 - Create rules for organizing...
Hazel is your personal housekeeper, organizing and cleaning folders based on rules you define. Hazel can also manage your trash and uninstall your applications. Organize your files using a... Read more
Autopano Giga 3.7 - Stitch multiple imag...
Autopano Giga allows you to stitch 2, 20, or 2,000 images. Version 3.0 integrates impressive new features that will definitely make you adopt Autopano Pro or Autopano Giga: Choose between 9... Read more

Latest Forum Discussions

See All

Tilt to Live: Gauntlet’s Revenge is Almo...
Tilt to Live: Gauntlet’s Revenge is Almost Here Posted by Jessica Fisher on October 21st, 2014 [ permalink ] One Man Left has announced the official release date of Tilt to Live: Gauntlet’s Re | Read more »
Sago Mini Monsters Celebrates Halloween...
Sago Mini Monsters Celebrates Halloween with Fun Costumes and Special Treats. Posted by Jessica Fisher on October 21st, 2014 [ permal | Read more »
Inferno 2 Review
Inferno 2 Review By Andrew Fisher on October 21st, 2014 Our Rating: :: TWIN STICK GOODNESSUniversal App - Designed for iPhone and iPad With tight controls and awesome, stark visuals, Inferno 2 is loads of fun.   | Read more »
Clips Review
Clips Review By Jennifer Allen on October 21st, 2014 Our Rating: :: CONVENIENT PASTINGUniversal App - Designed for iPhone and iPad Making copying and pasting more powerful than usual, Clips is a great way to move stuff around.   | Read more »
MonSense Review
MonSense Review By Jennifer Allen on October 21st, 2014 Our Rating: :: ORGANIZED FINANCESiPhone App - Designed for the iPhone, compatible with the iPad Organize your finances with the quick and easy to use, MonSense.   | Read more »
This Week at 148Apps: October 13-17, 201...
Expert App Reviewers   So little time and so very many apps. What’s a poor iPhone/iPad lover to do? Fortunately, 148Apps is here to give you the rundown on the latest and greatest releases. And we even have a tremendous back catalog of reviews; just... | Read more »
Angry Birds Transformers Review
Angry Birds Transformers Review By Jennifer Allen on October 20th, 2014 Our Rating: :: TRANSFORMED BIRDSUniversal App - Designed for iPhone and iPad Transformed in a way you wouldn’t expect, Angry Birds Transformers is a quite... | Read more »
GAMEVIL Announces the Upcoming Launch of...
GAMEVIL Announces the Upcoming Launch of Mark of the Dragon Posted by Jessica Fisher on October 20th, 2014 [ permalink ] Mark of the Dragon, by GAMEVIL, put | Read more »
Interview With the Angry Birds Transform...
Angry Birds Transformers recently transformed and rolled out worldwide. This run-and-gun title is a hit with young Transformers fans, but the ample references to classic Transformers fandom has also earned it a place in the hearts of long-time... | Read more »
Hail to the King: Deathbat Review
Hail to the King: Deathbat Review By Rob Thomas on October 20th, 2014 Our Rating: :: SO FAR AWAYUniversal App - Designed for iPhone and iPad Hail to the King: Deathbat may feel like “Coming Home” for Avenged Sevenfold’s faithful,... | Read more »

Price Scanner via MacPrices.net

Select MacBook Airs $100 off MSRP, free shipp...
B&H Photo has 2014 a couple of MacBook Airs on sale for $100 off MSRP. Shipping is free, and B&H charges NY sales tax only. They also include free copies of Parallels Desktop and LoJack for... Read more
13-inch 2.5GHz MacBook Pro on sale for $100 o...
B&H Photo has the 13″ 2.5GHz MacBook Pro on sale for $999.99 including free shipping plus NY sales tax only. Their price is $100 off MSRP. Read more
Strong iPhone, Mac And App Store Sales Drive...
Apple on Monday announced financial results for its fiscal 2014 fourth quarter ended September 27, 2014. The Company posted quarterly revenue of $42.1 billion and quarterly net profit of $8.5 billion... Read more
Apple Posts How-To For OS X Recovery
OS X 10.7 Lion and later include OS X Recovery. This feature includes all of the tools you need to reinstall OS X, repair your disk, and even restore from a Time Machine backup. OS X Recovery... Read more
Mac OS X Versions (Builds) Supported By Vario...
Apple Support has posted a handy resource explaining which Mac OS X versions (builds) originally shipped with or are available for your computer via retail discs, downloads, or Software Update. Apple... Read more
Deals on 2011 13-inch MacBook Airs, from $649
Daily Steals has the Mid-2011 13″ 1.7GHz i5 MacBook Air (4GB/128GB) available for $699 with a 90 day warranty. The Mid-2011 13″ 1.7GHz i5 MacBook Air (4GB/128GB SSD) is available for $649 at Other... Read more
2013 15-inch 2.0GHz Retina MacBook Pro availa...
B&H Photo has leftover previous-generation 15″ 2.0GHz Retina MacBook Pros now available for $1599 including free shipping plus NY sales tax only. Their price is $400 off original MSRP. B&H... Read more
Updated iPad Prices
We’ve updated our iPad Air Price Tracker and our iPad mini Price Tracker with the latest information on prices and availability from Apple and other resellers, including the new iPad Air 2 and the... Read more
Apple Pay Available to Millions of Visa Cardh...
Visa Inc. brings secure, convenient payments to iPad Air 2 and iPad mini 3as well as iPhone 6 and 6 Plus. Starting October 20th, eligible Visa cardholders in the U.S. will be able to use Apple Pay,... Read more
Textkraft Pocket – the missing TextEdit for i...
infovole GmbH has announced the release and immediate availability of Textkraft Pocket 1.0, a professional text editor and note taking app for Apple’s iPhone. In March 2014 rumors were all about... Read more

Jobs Board

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
*Apple* Solutions Consultant (ASC) - Apple (...
**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
Project Manager / Business Analyst, WW *Appl...
…a senior project manager / business analyst to work within our Worldwide Apple Fulfillment Operations and the Business Process Re-engineering team. This role will work Read more
*Apple* Retail - Multiple Positions (US) - A...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Position Opening at *Apple* - Apple (United...
…customers purchase our products, you're the one who helps them get more out of their new Apple technology. Your day in the Apple Store is filled with a range of Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.