TweetFollow Us on Twitter

Serial Port Demo
Volume Number:4
Issue Number:6
Column Tag:Pascal Procedures

Serial Port Demo

By Tom Scheiderich, Fullerton, CA

Just try to find out anything about how to talk to the Macs serial ports. I have looked in about 10-15 books and they all seem to forget the these devices exist. You can see how to write to the screen, the disk, the printer, the system clock etc. What happened to the serial ports!!! They aren’t so mysterious, as you will see. The only place I found any information about them is in Inside Macintosh. Anyone who has read Inside Macintosh knows that it is very difficult to follow as a tutorial, even by professional programmers. In many cases, a good example program, along with some discussion of the subject, will better help most programmers grasp the new techniques. “A Picture is worth a Thousand Words”!!! I learned about serial ports from a very basic terminal emulator. I just played with it and talked to other people about it. I will try to impart what I have learned to those interested. The sample program is derived from a Data Analyser program I wrote which can be used to examine data going back and forth between two serial devices. For example, from between a printer and a computer or a modem and a computer. Our sample program displays one window for each serial port, and sends data typed at the keyboard out one port and in the other, where it is displayed in the port’s window. This completely demonstrates both reading and writing to the Macintosh serial ports.

A serial port is one of the computer’s means to talk to the outside world in which the data is sent one bit at a time to some device such as a modem or printer. The Mac has 2 such ports at its disposal. Each port has two default buffers of 64 characters, an input and an output buffer. It seems that each buffer is set up in a circular fashion.

Fig. 1 Output of our sample Serial Port Program

General Circular Buffer Procedure

A circular buffer works logically as shown in the graphic. Logically there is no end to the buffer. It just keeps going around in a circle.

What actually happens in this type of buffer is that as a byte is received, it is put in the next position in the buffer. If the end of the buffer is reached, the byte is put at the start of the buffer overwriting whatever byte was there before. The users extracts these bytes from this buffer and put them into a work area (a character, byte or array). As each byte is received, a check must be made to see if the number of bytes in the buffer match the maximum number of bytes allowed in the buffer. Since we don’t want to over-write these bytes, the program must then tell the device (modem, for example) not to send any more data until later. The device will then “go to sleep” until requested to continue. This is called handshaking. There is no shifting of bytes as the data is read from and written to the buffer. The bytes will stay in the buffer as is, until over-written by new data.

In the following example, we will assume that data is coming from a modem and is sending the word “TESTING”. We have already received “TEST” and we are just going to read our first byte from the buffer. There are four control variables needed to handle the buffer.

1. Buffer Size - This is the total number of bytes available in our buffer.

2. Get Byte - The next byte position in the buffer to read.

3. Put Byte - The next byte position in the buffer available to put a byte.

4. Byte Count - The number of bytes available in the buffer to read. This will be (Put Byte - Get Byte). If < 0 then add Buffer Size.

At this point there are 4 bytes in the Buffer. The next byte we are going to read is a “T” which is in byte #1. The next byte coming from the modem will go into byte #5.

After the byte is taken from the circular buffer and put into a buffer specified by the program, the Byte Count will be set to 3, which says there are now three bytes in the buffer to be read. The next byte to be read is in the 2nd byte position. This will be the letter “E”. You will notice that nothing has happened to the “T” in position 1.

After the byte is put into the circular buffer, the Byte Count is then set to 4, which says there are now four bytes in the buffer to be read. The next byte read from the modem will be put into the 6th byte position.

Here we are also putting a byte into the buffer but have run out of room. The next position would have been 65. Since this is greater than the Buffer Size, we now wrap around and put the byte in byte position 1, over-writing the “T”. Also, you will notice that the Byte Count equals the Buffer size. We cannot accept any more bytes until some bytes are taken out of the buffer, so we must perform some sort of handshaking to stop the modem from sending any more data. In the real world, this would happen before the buffer was full. The data could be coming so fast that by the time the modem has found out it is supposed to stop, it may have sent 20 more bytes.

Macintosh Serial Ports

Now we will see how this all applies to the Mac, which of course is our only reason for living.

The Mac has two serial ports to choose from. Each of these ports has an input buffer and an output buffer. These are “our” circular buffers. Each buffer has a default size of 64 bytes. This can be changed to whatever size you want by a call to SerSetBuf. You don’t have to worry about the control variables from our previous discussion of the circular buffers as these are handled internally by the Serial Drivers. The control variables were just to demonstrate how a circular buffer is handled. You may, however, want to know how many bytes are in the buffer at any one time. This is accomplished by a call to SerGetBuf. If the count returned is not equal to 0, then a call to FSRead is executed. FSRead will take the number of bytes requested out of the buffer, puts them into a buffer specified by the program and updates the buffers control words. The program can then do what it wishes with the data. For example, if running a terminal emulator, the bytes received might be text and can now be displayed to the screen. If receiving a file, the bytes might now be written to the disk.

The sample program was written in Lightspeed Pascal. You should be able to see that dealing with the serial ports is very simple and not a mystery at all. The whole serial flow can be broken down to the following steps:

1. Open and set up the Serial Port.

2. Get any bytes that might be in the input buffer and deal with the data.

3. Check if any keys have been pressed and write out the serial port

4. Go back to # 2 and repeat.

The actual flow is summarized here in the flowchart shown at the end of the article.

In the example program, characters entered by the keyboard are written out one serial port and read into the other serial port and displayed on one of the two screens.

Before we start looking at the program some important Serial port variables need to be understood:

1. inBuffPtr - this is the 2k circular buffer that we are going to replace the default 64k buffer with. We set up the size and pointer to it (NewPtr) and then pass the pointer to the driver routine by way of SerSetBuf . We don’t deal with it anymore from this point on. All of our dealings will be with inRefNum and outRefNum

2. filterBuffPtr - the data returned from FSRead is put into this buffer. Since we will only be reading one character at a time, we only set up the buffer as one byte long. If we were going to read more than one byte from the buffer, you must set it up to the maximum number of bytes you might read. You may want to set the size the same as inBuffPtr . Unlike the circular buffers, the data being transfered will be put in the buffer starting from the first byte and continuing until all the bytes requested are transferred.

3. inRefNum - this is the input channel (reference) number. This number will be either a -6 (modem) or -8 (printer). All reading from the serial port will be done by referencing this number. It gets set up in RAMSDopen.

4. outRefNum - this is the output channel (reference) number. This number will be either a -7 (modem) or -9 (printer). All writing to the serial port will be done by referencing this number. It gets set up in RAMSDopen.

Each of these variables is set up as either “A” variables for the modem ports or “B” variables for the printer ports. For example, there is an inRefNumA for the modem and an inRefNumB for the printer. These are needed in our routine since we will be reading and writing to both of the ports.

To change the default settings (9600 baud, 8 data bits, 2 stop bits and no parity), set SerConfig, which is an integer that has its bits set for the appropriate parameters and passed along with the reference number to SerReset.

Handshaking is needed to prevent data overruns in the circular buffers. If no handshaking or the wrong handshaking is defined, you could get this overrun. Many of you have seen this with your printers when you are struggling to get the correct parity and stop bits set up. You will see a lot of garbage on your paper until you get it correct. This is caused many times because some data is getting lost when the printer has told your computer to stop sending, but because of incorrect settings, the computer kept sending data anyway and some of it was lost.

You will need to set up a record of control bytes to tell the Mac what type of handshaking you want it to do and call SerHShake while passing this record. The record contains:

1. fXOn - XOn/XOff output buffer flow control flag. If this is nonzero, then XOn/XOff flow control is enabled for the output buffer.

2. fCTS - If nonzero, then hardware flow control is enabled. The handshaking will then be controlled by lines 4 and 5 of the serial port.

3. xOn - What XOn character to use (usually a control-Q).

4. xOff - What XOff character to use ( usually a control-S).

5. errs - Tells the driver which type of errors cause input request to be aborted (parity, hardware overruns, or framing errors).

6. evts - This byte tells whether changes in CTS or Break status will cause the driver to post device driver events. This option is discouraged because interrupts are disabled for a long time while events are being posted

7. fInX - XOn/XOff input buffer flow control flag. If this is nonzero then XOn/XOff flow control is enabled for the input buffer.

The type of handshaking we use here doesn’t matter because everytime we send a character we also read it, so there is no way to get an overrun. But I have it set up as XOn/XOff for the input buffer only, using a control-S and a control-Q as our XOn/XOff characters for demonstration purposes.

Before starting the program you will need to a special cable. They are very easy to build. Since we are dealing with XOn/XOff, we need only 3 wires; the transmit, receive and ground wires. We are building a null modem cable which is just a cable which reverses the transmit and receive lines. When we are transmitting out the modem port, we will be receiving through the printer port and vice versa.

Pin Assignments on the Serial Ports

Following are the cable setups needed for the included program. To use the DB-9’s, if you are using a Mac Plus, Mac SE or a Mac II, you will need to purchase two - DB-9 to mini-8 conversion cables. The DB-9 connectors are easier to deal with than the mini-8 connecters and both DB-9 connectors are male.

To wire the cable, take the two male DB-9 connectors and wire them as shown in the figure below. Wire pin 3 to pin 3, ground. Wire pin 5 on one to pin 9 on the other for transmit. And finally, wire pin 9 on one to pin 5 on the other for receive. Then plug each male DB-9 connector into an Apple mini-8 to DB-9 cable and plug the two mini-8 connectors into the modem and printer ports on the back of the Mac. The Apple cables are available at Apple dealers to convert the mini-8 serial ports to the old style DB-9 connector devices used by the Mac 128 and Mac 512 computers. If you are a wiz, you can try to make a mini-8 cable instead, but we don’t recommend it.

The first thing that is done in our program is to open the serial ports which is done in the Init_Serial routine. We are using the RAM serial driver. Note that AppleTalk must be turned off at the chooser in order for this to work. All buffers and pointers are set up here. Next we will set up the two windows shown in figure 1.

The window selected is the port the data will be transferred out of and the other window is the port that will be doing the receiving. The non-selected window will be the one we display the text on.

Now we will just go into our normal “mainloop” routine. The first thing we will do is check for any bytes in the serial buffer (Get_comm_input). SerGetBuf is called to see if there are any bytes in our serial buffer. If there are, we will call FSRead (from GetChar) to get a character out of the buffer. Before writing to the window we need to call SetPort to select the correct window. If PortA is true, we are writing out the modem port and are displaying on window B, so we would then call SetPort(windowB) to select the second window and draw the character onto the screen (Put_Char).

Last of all is to see if a key was pressed. If so, we need to write the character out the output port (Handle_keys). A call would be made to FSWrite to transfer the character.

The only other routine of interest would be RSerBuf which is called to reset the buffers and pointers and to reverse the input and output ports. This is done whenever a new window is selected or at the start of the program. Clicking in the content region of the unselected window, then reverses which window and port is doing the output and input of our characters typed on the keyboard.

One last thing. We are using the RAM Serial Driver for our program, but we could just as easily have used the ROM Serial Driver. The main problem is that the ROM Driver doesn’t support XOn/XOff, so if this is necessary use the RAM Driver. If the ROM driver is needed then all that need be done is:

Replace

 errl:=RAMSDOpen(sPortA)

With

 errI := OpenDriver(‘.Ain’, inRefNum);
                    errI := OpenDriver(‘.Aout’, outRefNum);

You need to explicitly open the input and the output Drivers.

Wiring our null modem cable for port to port communications

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Kodi 17.1. - Powerful media center tool...
Kodi (was XBMC) is an award-winning free and open-source (GPL) software media player and entertainment hub that can be installed on Linux, OS X, Windows, iOS, and Android, featuring a 10-foot user... Read more
Bookends 12.8 - Reference management and...
Bookends is a full-featured bibliography/reference and information-management system for students and professionals. Bookends uses the cloud to sync reference libraries on all the Macs you use.... Read more
Apple iTunes 12.6 - Play Apple Music and...
Apple iTunes lets you organize and stream Apple Music, download and watch video and listen to Podcasts. It can automatically download new music, app, and book purchases across all your devices and... Read more
Default Folder X 5.1.4 - 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
Amazon Chime 4.1.5587 - Amazon-based com...
Amazon Chime is a communications service that transforms online meetings with a secure, easy-to-use application that you can trust. Amazon Chime works seamlessly across your devices so that you can... Read more
CrossOver 16.2 - Run Windows apps on you...
CrossOver can get your Windows productivity applications and PC games up and running on your Mac quickly and easily. CrossOver runs the Windows software that you need on Mac at home, in the office,... Read more
Adobe Creative Cloud 4.0.0.185 - Access...
Adobe Creative Cloud costs $19.99/month for a single app, or $49.99/month for the entire suite. Introducing Adobe Creative Cloud desktop applications, including Adobe Photoshop CC and Illustrator CC... Read more
MegaSeg 6.0.2 - Professional DJ and radi...
MegaSeg is a complete solution for pro audio/video DJ mixing, radio automation, and music scheduling with rock-solid performance and an easy-to-use design. Mix with visual waveforms and Magic... Read more
Bookends 12.8 - Reference management and...
Bookends is a full-featured bibliography/reference and information-management system for students and professionals. Bookends uses the cloud to sync reference libraries on all the Macs you use.... Read more
Adobe Creative Cloud 4.0.0.185 - Access...
Adobe Creative Cloud costs $19.99/month for a single app, or $49.99/month for the entire suite. Introducing Adobe Creative Cloud desktop applications, including Adobe Photoshop CC and Illustrator CC... Read more

The best deals on the App Store this wee...
Deals, deals, deals. We're all about a good bargain here on 148Apps, and luckily this was another fine week in App Store discounts. There's a big board game sale happening right now, and a few fine indies are still discounted through the weekend.... | Read more »
The best new games we played this week
It's been quite the week, but now that all of that business is out of the way, it's time to hunker down with some of the excellent games that were released over the past few days. There's a fair few to help you relax in your down time or if you're... | Read more »
Orphan Black: The Game (Games)
Orphan Black: The Game 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: Dive into a dark and twisted puzzle-adventure that retells the pivotal events of Orphan Black. | Read more »
The Elder Scrolls: Legends is now availa...
| Read more »
Ticket to Earth beginner's guide: H...
Robot Circus launched Ticket to Earth as part of the App Store's indie games event last week. If you're not quite digging the space operatics Mass Effect: Andromeda is serving up, you'll be pleased to know that there's a surprising alternative on... | Read more »
Leap to victory in Nexx Studios new plat...
You’re always a hop, skip, and a jump away from a fiery death in Temple Jump, a new platformer-cum-endless runner from Nexx Studio. It’s out now on both iOS and Android if you’re an adventurer seeking treasure in a crumbling, pixel-laden temple. | Read more »
Failbetter Games details changes coming...
Sunless Sea, Failbetter Games' dark and gloomy sea explorer, sets sail for the iPad tomorrow. Ahead of the game's launch, Failbetter took to Twitter to discuss what will be different in the mobile version of the game. Many of the changes make... | Read more »
Splish, splash! The Pokémon GO Water Fes...
Niantic is back with a new festival for dedicated Pokémon GO collectors. The Water Festival officially kicks off today at 1 P.M. PDT and runs through March 29. Magikarp, Squirtle, Totodile, and their assorted evolved forms will be appearing at... | Read more »
Death Road to Canada (Games)
Death Road to Canada 1.0 Device: iOS Universal Category: Games Price: $7.99, Version: 1.0 (iTunes) Description: Get it now at the low launch price! Price will go up a dollar every major update. Update news at the bottom of this... | Read more »
Bean's Quest Beginner's Guide:...
Bean's Quest is a new take on both the classic platformer and the endless runner, and it's free on the App Store for the time being. Instead of running constantly, you can't stop jumping. That adds a surprising new level of challenge to the game... | Read more »

Price Scanner via MacPrices.net

Updated iPad Price Trackers
Scan our Apple iPad (and iPod touch) Price Trackers for the latest information on sales, bundles, and availability on systems from Apple’s authorized internet/catalog resellers. We update the... Read more
12-inch 32GB Space Gray iPad Pro on sale for...
B&H Photo has 12″ Space Gray 32GB WiFi Apple iPad Pros on sale for $50 off MSRP including free shipping. B&H charges sales tax in NY only: - 12″ Space Gray 32GB WiFi iPad Pro: $749 $50 off... Read more
2.6GHz Mac mini on sale for $559, $140 off MS...
Guitar Center has the 2.6GHz Mac mini (MGEN2LL/A) on sale for $559 including free shipping. Their price is $140 off MSRP, and it’s the lowest price available for this model. Read more
SSD Speeder RAM Disk SSD Life Extender App Fo...
Fehraltorf, Switzerland based B-Eng has announced they are making their SSD Speeder app for macOS publicly available for purchase on their website. SSD Speeder is a RAM disk utility that prevents... Read more
iPhone Scores Highest Overall in Smartphone D...
Customer satisfaction is much higher among smartphone owners who use their device to operate other connected home services such as smart thermostats and smart appliances, according to the J.D. Power... Read more
Swipe CRM Free Photo-Centric CRM Sales DEal C...
Swipe CRM LLC has introduced Swipe CRM: Visual Sales 1.0 for iPad, an app for creating, managing, and sharing visually stunning sales deals. Swipe CRM is targeted to small-and-medium creative... Read more
13-inch 2.0GHz Apple MacBook Pros on sale for...
B&H has the non-Touch Bar 13″ 2.0GHz MacBook Pros in stock today and on sale for $150 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 13″ 2.0GHz MacBook Pro Space Gray (... Read more
15-inch Touch Bar MacBook Pros on sale for up...
B&H Photo has the new 2016 15″ Apple Touch Bar MacBook Pros in stock today and on sale for up to $150 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 15″ 2.7GHz Touch Bar... Read more
Apple’s iPhone 6s Tops Best-Selling Smartphon...
In terms of shipments, the iPhone 6s from Apple bested all competitors for sales in 2016, according to new analysis from IHS Markit, a world leader in critical information, analytics and solutions.... Read more
Logitech Rugged Combo Protective iPad Case an...
Logitech has announced its Logitech Rugged Combo, Logitech Rugged Case, and Logitech Add-on Keyboard for Rugged Case for Apple’s new, more affordable $329 9.7-inch iPad, a complete solution designed... Read more

Jobs Board

*Apple* Mobile Master - Best Buy (United Sta...
**492472BR** **Job Title:** Apple Mobile Master **Location Number:** 000470-Seattle-Store **Job Description:** **What does a Best Buy Apple Mobile Master do?** Read more
*Apple* Mobile Master - Best Buy (United Sta...
**492562BR** **Job Title:** Apple Mobile Master **Location Number:** 000853-Jackson-Store **Job Description:** **What does a Best Buy Apple Mobile Master do?** 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
Fulltime aan de slag als shopmanager in een h...
Ben jij helemaal gek van Apple -producten en vind je het helemaal super om fulltime shopmanager te zijn in een jonge en hippe elektronicazaak? Wil jij werken in 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.