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

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

calibre 2.69.0 - Complete e-book library...
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
Evernote 6.9.1 - Create searchable notes...
Evernote allows you to easily capture information in any environment using whatever device or platform you find most convenient, and makes this information accessible and searchable at anytime, from... Read more
jAlbum Pro 13.5 - Organize your digital...
jAlbum Pro has all the features you love in jAlbum, but comes with a commercial license. You can create gorgeous custom photo galleries for the Web without writing a line of code! Beginner-friendly... Read more
jAlbum 13.5 - Create custom photo galler...
With jAlbum, you can create gorgeous custom photo galleries for the Web without writing a line of code! Beginner-friendly, with pro results - Simply drag and drop photos into groups, choose a design... Read more
Google Chrome 53.0.2785.143 - 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
Chromium 53.0.2785.143 - Fast and stable...
Chromium is an open-source browser project that aims to build a safer, faster, and more stable way for all Internet users to experience the web. Version 53.0.2785.143: [Security Fix] High CVE-2016-... Read more
QuickBooks 2015 16.1.7.1524 R8 - Financi...
Save 20% on QuickBooks Pro for Mac today through this special discount link QuickBooks 2015 helps you manage your business easily and efficiently. Organize your finances all in one place, track... Read more
Sierra Cache Cleaner 11.0.1 - Clear cach...
Sierra Cache Cleaner is an award-winning general purpose tool for macOS X. SCC makes system maintenance simple with an easy point-and-click interface to many macOS X functions. Novice and expert... Read more
Default Folder X 5.0.7 - Enhances Open a...
Default Folder X attaches a toolbar to the right side of the Open and Save dialogs in any OS X-native application. The toolbar gives you fast access to various folders and commands. You just click on... Read more
Safari Technology Preview 10.1 - The new...
Safari Technology Preview contains the most recent additions and improvements to WebKit and the latest advances in Safari web technologies. And once installed, you will receive notifications of... Read more

Pumped BMX 3: Beginner tips and tricks
There’s a whole lot more to Pumped BMX 3 than meets the eye. Your goal is to perform a wide array of sweet flips and tricks, but that’s easier said than done. It takes well practiced timing and coordination, and the game doesn’t really explain that... | Read more »
Cybird’s latest release - BFB Champions...
Launched in the UK in early September, BFB Champions’ newest update is loaded with great new features, and looks set to outshine the original version by taking it out of soft launch and giving it a new lease of life. | Read more »
3 apps to boost your focus
As someone who works from home, my workspace is a minefield of distraction. Cats, tasty snacks, the wind blowing past my window, that cleaning that I suddenly can’t put off any longer. If I let distraction takes its course, I find that soon half... | Read more »
Pumped BMX 3 (Games)
Pumped BMX 3 1.0 Device: iOS Universal Category: Games Price: $3.99, Version: 1.0 (iTunes) Description: The final instalment of the smash hit #1 rated BMX game is here! Following on from the insane success of Pumped BMX 2, Pumped 3... | Read more »
4 games like Burly Men at Sea to inspire...
Burly Men at Sea is out today and it looks a treat. It tells the tale of three Scandinavian fishermen who leave the humdrum of their daily lives to go exploring. It’s a beautiful folksy story that unfurls as you interact with the environment... | Read more »
3 reasons you need to play Kingdom: New...
Developed by a tag team of indie developers - Thomas "Noio" van den Berg and Marco "Licorice" Bancale - Kingdom is a vibrant medieval fantasy adventure that casts players as a king or queen who must expand their empire by exploring the vasts lands... | Read more »
JoyCity have launched a brand new King o...
Great news for all of you Game of Dice fans out there - JoyCity have just released a brand new limited edition pack with a really cool twist. The premise of Game of Dice is fairly straightforward, asking you to roll dice to navigate your way around... | Read more »
Burly Men at Sea (Games)
Burly Men at Sea 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: Burly Men at Sea is a folktale about a trio of large, bearded fishermen who step away from the ordinary to seek adventure. | Read more »
3 tips for catching the gnarliest waves...
Like a wave breaking on the shore, Tidal Rider swept its way onto the App Store charts this week settling firmly in the top 10. It’s a one-touch high score-chaser in which you pull surfing stunts while dodging seagulls and collecting coins. The... | Read more »
The beginner's guide to destroying...
Age of Heroes: Conquest is 5th Planet Games’ all new turn-based multiplayer RPG, full of fantasy exploration, guild building, and treasure hunting. It’s pretty user-friendly as far as these games go, but when you really get down to it, you’ll find... | Read more »

Price Scanner via MacPrices.net

CAZE Annouces New Zero 5 Case for Jet Black i...
Hong Kong basd CAZE has announced Zero 5 case for iPhone 7/ 7 Plus, one of the world’s thinnest clear hard cases, measuring just 0.5 millimeters. CAZE has been producing and improving the Zero 5... Read more
Nest Egg Inventory App for iOS Offers Conven...
Campbell, California based Winprogger LLC has announced the release and immediate availability of Nest Egg – Inventory 4.1.22, an important update to their easy-to-use, yet comprehensive inventory... Read more
Factor4, LLC Launches Apple iOS and Android G...
Factor4, LLC, which offers gift and loyalty services to the SMB marketplace, has released free mobile applications that enable merchants to process via all Apple and Android devices. The Apple and... Read more
15-inch Retina MacBook Pros on sale for $200...
B&H Photo has 15″ Retina Apple MacBook Pros on sale for $200 off MSRP. Shipping is free, and B&H charges NY tax only: - 15″ 2.2GHz Retina MacBook Pro: $1799 $200 off MSRP - 15″ 2.5GHz Retina... Read more
Apple refurbished iMacs available for up to $...
Apple has Certified Refurbished 2015 21″ & 27″ iMacs available for up to $350 off MSRP. Apple’s one-year warranty is standard, and shipping is free. The following models are available: - 21″ 3.... Read more
Check Apple prices on any device with the iTr...
MacPrices is proud to offer readers a free iOS app (iPhones, iPads, & iPod touch) and Android app (Google Play and Amazon App Store) called iTracx, which allows you to glance at today’s lowest... Read more
Apple price trackers, updated continuously
Scan our Apple Price Trackers for the latest information on sales, bundles, and availability on systems from Apple’s authorized internet/catalog resellers. We update the trackers continuously: - 15″... Read more
Apple refurbished 2016 13-inch MacBook Airs a...
Apple has Certified Refurbished 2016 13″ MacBook Airs available starting at $849. An Apple one-year warranty is included with each MacBook, and shipping is free: - 2016 13″ 1.6GHz/8GB/128GB MacBook... Read more
1.4GHz Mac mini on sale for $449, save $50
Adorama has the 1.4GHz Mac mini on sale for $50 off MSRP including free shipping plus NY & NJ sales tax only: - 1.4GHz Mac mini (Apple sku# MGEM2LL/A): $449 $50 off MSRP To purchase a mini at... Read more
Apple refurbished 2015 13-inch MacBook Airs a...
Apple has Certified Refurbished 2015 13″ MacBook Airs available starting at $759. An Apple one-year warranty is included with each MacBook, and shipping is free: - 2015 13″ 1.6GHz/4GB/128GB MacBook... Read more

Jobs Board

*Apple* Retail - Multiple Positions- South B...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Restaurant Manager (Neighborhood Captain) - A...
…in every aspect of daily operation. WHY YOU'LL LIKE IT: You'll be the Big Apple . You'll solve problems. You'll get to show your ability to handle the stress and Read more
US- *Apple* Store Leader Program - Apple (Un...
…Summary Learn and grow as you explore the art of leadership at the Apple Store. You'll master our retail business inside and out through training, hands-on 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
Restaurant Manager (Neighborhood Captain) - A...
…in every aspect of daily operation. WHY YOU'LL LIKE IT: You'll be the Big Apple . You'll solve problems. You'll get to show your ability to handle the stress and Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.