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

Dashlane 4.8.4 - Password manager and se...
Dashlane is an award-winning service that revolutionizes the online experience by replacing the drudgery of everyday transactional processes with convenient, automated simplicity - in other words,... Read more
FileZilla 3.27.1 - Fast and reliable FTP...
FileZilla (ported from Windows) is a fast and reliable FTP client and server with lots of useful features and an intuitive interface. Version 3.27.1: Fixed Vulnerabilities: Change client... Read more
f.lux 39.984 - Adjusts the color of your...
f.lux makes the color of your computer's display adapt to the time of day, warm at night and like sunlight during the day. Ever notice how people texting at night have that eerie blue glow? Or wake... Read more
Merlin Project 4.2.7 - $289.00
Merlin Project is the leading professional project management software for OS X. If you plan complex projects on your Mac, you won’t get far with a simple list of tasks. Good planning raises... Read more
Sound Studio 4.8.10 - Robust audio recor...
Sound Studio lets you easily record and professionally edit audio on your Mac. Easily rip vinyls and digitize cassette tapes, or record lectures and voice memos. Prepare for live shows with live... Read more
Google Chrome 60.0.3112.101 - 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
Microsoft Office 2016 15.37 - Popular pr...
Microsoft Office 2016 - Unmistakably Office, designed for Mac. The new versions of Word, Excel, PowerPoint, Outlook and OneNote provide the best of both worlds for Mac users - the familiar Office... Read more
Sketch 46.2 - Design app for UX/UI for i...
Sketch is an innovative and fresh look at vector drawing. Its intentionally minimalist design is based upon a drawing space of unlimited size and layers, free of palettes, panels, menus, windows, and... Read more
beaTunes 5.0.4 - Organize your music col...
beaTunes is a full-featured music player and organizational tool for music collections. How well organized is your music library? Are your artists always spelled the same way? Any R.E.M. vs REM?... Read more
Yasu 4.0.4 - System maintenance app.
Yasu was 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 quickly within a... Read more

Portal Walk (Games)
Portal Walk 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: Portal Walk is adventure and relaxing platform game about Eugene. Eugene stuck between worlds and trying to find way back home.... | Read more »
Technobabylon (Games)
Technobabylon 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: City of Newton, 2087. Genetic engineering is the norm, the addictive Trance has replaced almost any need for human interaction,... | Read more »
5 reasons why 2v2 is the best mode in Cl...
Supercell has been teasing fans with 2v2 windows that allow players to team up for limited periods of time. The Summer of 2v2 was just this past July, but players are already clamoring for more of that sweet, sweet team-based action. The fans have... | Read more »
The best deals on the App Store this wee...
It seems like the week's only just started, and yet here we are with a huge pile of discounted games to sort through. There are some real doozies on sale this week. We're talking some truly stellar titles. Let's take a look at four of the best... | Read more »
Cat Quest Guide - How to become a purrfe...
Cat Quest is an absolutely charming open-world RPG that's taken the gaming world quite by storm. This game about a world populated by furry kitty warriors is actually a full-length RPG with sturdy mechanics and a lovely little story. It's certainly... | Read more »
Silly Walks Guide - How to strut your st...
Silly Walks is an all new adventure game that lives up to its name. It sees you playing as a variety of snack foods as you teeter-totter your way to rescue your friends from the evil blender and his villainous minions. It's all very . . . well... | Read more »
The best mobile point-and-click adventur...
Nostalgia for classic point-and-click adventure games has reached an all-time high in recent years, and the rise of mobile games have provided a perfect platform for this old-school genre. This week we're going to take a look at some of the best... | Read more »
Titanfall: Assault Beginner's Guide...
Respawn's dazzling mech battler Titanfall has gotten its first mobile game at long last -- Titanfall: Assault. Assault transforms that parkour FPS action into a card battler not dissimilar to Clash Royale. Titanfall: Assault isn't just another... | Read more »
Unobstruct (Utilities)
Unobstruct 1.0 Device: iOS Universal Category: Utilities Price: $.99, Version: 1.0 (iTunes) Description: You’re already reading web pages on a pretty small screen. Don’t let floating social media sharing toolbars and the like get in... | Read more »
Burgle Bros (Games)
Burgle Bros 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: Burgle Bros requires stealth, planning and a little bit of luck. In the tradition of classic heist movies like Ocean's 11 and The... | Read more »

Price Scanner via MacPrices.net

New iOS 11 Productivity Features Welcome But...
The iOS community is in late summer holding mode awaiting the September arrival of the iPhone 8 and iOS 11. iOS 11 public betas have been available for months — number six was released this week —... Read more
Samsung Electronics Launches New Portable SSD...
Samsung Electronics America, Inc. has announced the launch of Samsung Portable SSD T5 – its newest portable solid state drive (PSSD) that raises the bar for the performance of external memory... Read more
TrendForce Reports YoY Gain of 3.6% for 2Q17...
Market research firm TrendForce reports that the global notebook shipments for this second quarter registered a sequential quarterly increase of 5.7% and a year-on-year increase of 3.6%, totaling 39.... Read more
Sale! 10-inch iPad Pros for $50 off MSRP, no...
B&H Photo has 10.5″ iPad Pros in stock today and on sale for $50 off MSRP. Each iPad includes free shipping, and B&H charges sales tax in NY & NJ only: – 10.5″ 64GB iPad Pro: $599, save $... Read more
Sale! 2017 13-inch Silver 2.3GHz MacBook Pro...
Amazon has new 2017 13″ 2.3GHz/128GB Silver MacBook Pro on sale today for $100 off MSRP including free shipping. Their price is the lowest available for this model from any reseller: – 13″ 2.3GHz/... Read more
WaterField Unveils Collaboratively-Designed,...
In collaboration with customers and seasoned travelers, San Francisco maker WaterField Designs set out to create the preeminent carry-on system to improve the experience of frequent fliers. The... Read more
Miya Notes Mac-Client for Google Keep (Launch...
MacPlus Software has announced te launch of Miya Notes for Google Keep 1.0, a powerful Mac-client for Google Keep. Millions of people use Google Keep on their phones and online, but a convenient Mac... Read more
Apple refurbished iMacs available starting at...
Apple has previous-generation Certified Refurbished 2015 21″ & 27″ iMacs available starting at $849. Apple’s one-year warranty is standard, and shipping is free. The following models are... Read more
2017 13-inch MacBook Airs on sale for $100 of...
B&H Photo new 2017 13″ MacBook Airs on sale today for $100 off MSRP, starting at $899: – 13″ 1.8GHz/128GB MacBook Air (MQD32LL/A): $899, $100 off MSRP – 13″ 1.8GHz/256GB MacBook Air (MQD42LL/A... Read more
12-inch MacBooks on sale for $100 off MSRP
Amazon has 2017 12″ Retina MacBooks on sale for $100 off MSRP. Shipping is free: 12″ 1.2GHz Space Gray MacBook: $1199.99 $100 off MSRP 12″ 1.2GHz Silver MacBook: $1198 $101 off MSRP 12″ 1.2GHz Gold... Read more

Jobs Board

SW Engineer *Apple* TV - Apple Inc. (United...
Job Summary The Apple TV team is looking for an experienced engineer with a passion for delivering first in class home entertainment solutions. The candidate will Read more
Development Operations and Site Reliability E...
Development Operations and Site Reliability Engineer, Apple Payment Gateway Job Number: 57572631 Santa Clara Valley, California, United States Posted: Jul. 27, 2017 Read more
Frameworks Engineering Manager, *Apple* Wat...
Frameworks Engineering Manager, Apple Watch Job Number: 41632321 Santa Clara Valley, California, United States Posted: Jun. 15, 2017 Weekly Hours: 40.00 Job Summary 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
*Apple* Solutions Consultant - Apple Inc. (U...
Job Summary As an Apple Solutions Consultant, you'll be the link between our future customers and our products. You'll showcase your entrepreneurial spirit as you Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.