Digital Communications
                                 by Hank Volpe

Note: This material is copyrighted 1990 by Hank Volpe. Reprints of this
      article have appeared in more than 50 newsletters around the country.
      For more information or comments, please write to;
                               Hank Volpe
                               P.O. Box 43214
                               Baltimore MD 21236

                           A Modem Primer -- Part 2

Last month we started a discussion about modems and we thought it best to 
approach the topic from a philosophical standpoint with a "beginner's mind". 
We discussed how a modem works, why we need modems, what are Uarts and why do 
we need them , and what are Comm ports. We called this our "Freshman" level 
in our series. Tonight we move onto some sophomore (not sophomoric!) topics, 
starting with "What is a Hayes?"


Modem 201 - What is "Hayes"?
----------------------------
Actually, it should be "who is Hayes"? Dennis Hayes founded Hayes Micromodems 
a few years ago. Some people actually believe that he invented modems. Well 
he did and he didn't. The actual physical modem has been around for quite a 
long time now. What Dennis Hayes did was invent one of the earliest modems 
designed specifically for microcomputers. Hayes modems are very well built, 
but this day and age, the same can be said for almost any modem. The 
circuitry for most modulation and demodulation sections of modems are 
contained on standard chip sets made by Rockwell, XR, and a few other 
manufacturers. What made Hayes modems the "standard" was the programming 
interface that Dennis Hayes created.

As I sit here and type, it occurs to me that one of the most frequently used 
words around computers is "Interface". It is spilling over into all of our 
modern "lingo" where people who sell cars talk about interfacing to clients. 
An interface in electronic terms is simply another word for a connecting 
method. Connectors can be physical or virtual. Virtual means that it acts 
like a real connection, but it is not physically real. An example of a 
virtual connection is a software switch. It acts like a switch that might be 
attached to a light, and it does turn something on or off, but it's just a 
character on a screen, it's not a real, touchable switch. A programming 
interface is a virtual connection between devices or modules in a program. 
Hayes modems came with a programming interface that allowed people to write 
programs that used modems, but to be able to do it in a manner where the 
programmer did not have to actually write commands to dial, connect, change 
baud rates, or disconnect. The Hayes modems contained a microprocessor. This 
in function was no different than your computer you use everyday, however 
it's only purpose was to look at the characters being sent to it. If a line 
started with the letters "AT", the Hayes modem knew that the next group of 
characters it would see would contain a command for it. This command could 
tell it to answer the phone (ATA), dial a phone number (ATDT832-1398), turn 
off it's speaker (ATM0), turn on it's speaker (ATM1) and a host of other 
commands. Likewise, it interfaced back to the computer program using it by 
sending real English messages like "OK", or "CONNECT 2400", "RINGING", "BUSY" 
and the like.  Programmers loved working with it because it offered two 
advantages to them; First, all they needed to do was send characters to the 
modem to do all kinds of complex telecommunication activities, and secondly 
as long as a modem used this "AT" command structure, it didn't matter if the 
end user went out and bought a new and improved modem, for as long as it used 
the "AT" instruction set, it would continue to work with the software the 
programmer had written. Hayes had succeeded at something IBM failed to do, 
for IBM originally had thought of the ROM-BIOS in it's machines as a virtual 
connection between a program and the computer. IBM could have then changed 
the way the computer worked in a manner transparent to the software, as long 
as the software used ROM-BIOS calls. Well as anyone who has been around 
computers knows, the ROM-BIOS in the original PC's worked so slowly that 
programmers took short-cuts around it. This doomed the PC and future PC 
makers to forever stay hardware compatible with the original machine. Dennis 
Hayes's "AT" set however became the standard for personal computer 
communications in the PC, Macintosh, Atari, and nearly all other digital 
communication devices. Many people make modems that are compatible with the 
Hayes 300/1200 and Hayes 2400B modem command sets, with some expanding or 
creating what computer people call "supersets" of the original Hayes "AT" 
instruction set. 

When you are shopping for a modem, you will see them touted as "Hayes 
Compatible". What that means is that they use the "AT" instruction set and 
follow the same command structure as did the original Hayes 300/1200 or Hayes 
2400 series modems (there is a slight difference between the 300/1200's and 
the 2400 commands). This does not mean they are as good as a Hayes, in fact 
they might be better. All it means is that you can use 99% of all the 
communication software developed for your PC with this modem. Ok, so if 99% 
work, where is that 1% incompatibility? Well, there are storage registers on 
a Hayes modem and most compatibles. These storage registers (or S-registers) 
hold vital communications information. They store all the parameters the 
modem uses to communicate. These registers can be accessed, just like a 
programmer can write directly to your screen and bypass the ROM-BIOS. If a 
program does this, then it obviously expects the display adapter to use a 
standard address. If a programmer writes or reads an S-register, it expects 
the modem to use that S-register for the same thing that the modem it was 
written for originally had used. Although a modem claims to be "Hayes 
Compatible", it is doubtful that it is 100% Hayes "hardware" compatible, for 
the only way to do that is to own a Hayes. If your software was written in 
such a way that it expects Hayes "hardware" compatibility, then that program 
may need to be modified when used with a different brand of modem. Many of 
the early BBS programs were written for this hardware compatibility and would 
not operate properly without it.  What modem should you buy then? It depends 
on the application. Quite frankly, most communication programs written during 
the last 2 years will work well with any modem that uses the "AT" instruction 
set. There are other considerations in buying a modem that we will discuss 
when we get to the end of our series, for we have a lot more ground to cover. 
For now though, remember the buzz word "Hayes Compatible" and that it truly 
means "AT instruction-set compatible" (interfaces like a Hayes to your 
software). 

Modem 202- Learning to speak mnemonics
--------------------------------------
Like any good college program, a foreign language needs to be included so 
that the student's eyes can be opened to a wider world. Language is one of 
the interfaces between human beings, but there are others: music, video, 
still pictures, and smells also serve as languages. Some of these, especially 
still pictures and smells can be a more powerful messenger than written 
correspondence. Languages can also pose problems in the areas of translation, 
and when translations are made between languages (like for example in phrase 
books) the true meaning of the original language can be lost or 
misinterpreted. Lately we have been noticing a rise of what have been termed 
"international symbols". A picture of a dog with a line through it means "no 
dogs allowed". I guess that someone could still misinterpret this sign on a 
restaurant to mean that "dogs are not served on the menu", but these symbols 
for the most part can be fairly straight-forward message bearers between 
languages and people. In the computer world, such messages are called 
mnemonics. A popular example of a mnemonic is an icon that you access with a 
mouse. Another example would be the command names used in assembly language 
programming for op-codes, still another the name you assign to a range of 
cells in a spreadsheet program. Mnemonics are kind of a pseudo-language that 
can be easier to understand than a full-blown language. Now there are many 
different ways of looking at the AT instruction set, but I think the best one 
is to think of it as a series of mnemonics that help you control your modem.
As I explained earlier, the AT of the AT instruction set means "attention". 
It is a way for the microprocessor in the modem to distinguish between normal 
characters being sent to it and a command. There are also two modes that a AT 
compatible modem operates in; the command mode and the on-line mode. When you 
first turn on your computer and load a communications program, you are 
communicating to the modem in the command mode. When your modem connects to a 
BBS or other service, the modem switches automatically to the on-line mode. 
When you are in the command mode, you can talk to your modem using AT 
commands and change many of the ways that the modem functions. When you are 
in the on-line mode, you cannot communicate to the modem with AT commands 
(there is a way to go back to the command mode while on-line, but we'll talk 
about that later). The beauty of the command mode is not only appreciated by 
people who write communications programs, but it can be appreciated and used 
by you too.

After you have loaded your communications program (lets say Procomm, Qmodem 
or Telix for the sake of discussion) you will notice that you can type 
characters on the keyboard and have them displayed on the screen. How does 
this happen? No, computers don't echo what you type to the screen like a 
typewriter, you have to instead have a program that will take a character and 
send it to the screen. Good proof of that is try typing characters when your 
computer is hung-up. Nothing happens! MS-DOS does the echoing for command.com 
and programs can use MS-DOS or their own echoing schemes. When you are 
communicating with a modem you send characters by typing on your keyboard. 
These characters are then sent to the modem. The modem then sends these 
characters back to the computer, where the program you are using places the 
character on the screen. This is technically referred to as "Full-Duplex" 
operation and "Local Command Echo". In this mode, you can change many of the 
operating characteristics of your modem by typing a few letters. All you need 
to do is follow some simple rules; First, all commands must start at the 
beginning of a line and the first 2 letters must be AT. Second, you can place 
multiple commands all on the same line up to about 40 characters. Third, all 
the characters you type must be legitimate commands  for your type of modem, 
otherwise you will see the word ERROR get echoed. Try sitting at the computer 
with your communications program loaded and type the command ATH1 and press 
return. This will take your phone off-hook and you should hear a dial tone. 
To place the phone back on-hook, type ATH0 and press return. Voila! You are 
now speaking mnemonics to your modem. More importantly, you've just taken the 
first steps to letting the modem work for you!

Using AT commands, you can set your modem up for special applications. For 
example, most BBS programs do not like to see local command echo, so there is 
a command ATE0 that can be used to turn this echo off. If you do this, you 
will notice that from that point on every character you type to the modem 
will not be displayed. The modem will still respond and send the word OK or 
ERROR back to the screen, but it will not echo the character you type back to 
the computer. To turn the local echo on again, just press return (to make 
sure you are at the beginning of a new line because you don't know for sure 
since the echo was off), type ATE1 press return. The modem responds OK and 
now you can see your characters echoed back again unless you type another 
ATE0. You could start your program and tell the modem that you want to turn 
the speaker off all the time by typing ATM0 and pressing return. To turn the 
speaker back on, type ATM1 and press return. Simple eh? It really is. There 
are many AT commands that can be used by you to program your modem to work 
the way you would like it to. Take a look at the book that came with your 
modem, or if you don't have a modem (or lost your book), there are some 
reference books you can pick up at your book store that list these commands. 
Now say that  you would like to turn off the speaker each time you start your 
communications program because you hate hearing dial-tones. You can start 
your program, then type ATM0 and press return. This works fine. But lets 
assume you wanted to do some other creative things too. Typing this over and 
over can be bothersome, and of course you could type an ATM3 when you meant 
an ATM0 by mistake. It's time to explore a built-in feature of most 
communication programs call the initialization string.

Modem 203 - Using initialization strings
----------------------------------------
What is an initialization string? Well, it's nothing more than some AT 
commands that are sent automatically to your modem by your communication 
program each time you start it up. A proper initialization string can be 
important to some applications. For example, a bulletin board program usually 
does not operate properly if the incorrect initialization string is being 
used. Most communications program allow you to custom set your modem each 
time you start it up. What  you put in that initialization string is totally 
up to you. In our previous example, if you wish to turn off the speaker each 
time the communication program is loaded, then your initialization string is 
ATM0. If you wish to return extended result codes that will tell you if the 
modem sees a busy signal, your initialization string is AT X4. If you wish to 
both turn the speaker off and see extended codes, your initialization string 
is AT M0 X4. Any legal command for your modem can be placed in this 
initialization string. You can also manipulate the contents of something 
called S-Registers. An S-Register is a memory location in the modem. Each 
location is marked with a number S0 for example records how many times the 
phone has rung. S7 sets a timer that counts down a pre-determined number of 
seconds that the modem will wait until it quit's trying to make a carrier 
connection. S11 controls the speed that your touch-tone dialing commands get 
sent out to the phone company. S-Registers are different from AT commands, 
primarily because how many S-Registers you have and how they are used can 
vary from manufacturer to manufacturer (or in other words, they may or may 
not be hardware compatible with Hayes). The first 16 S registers however have 
been universally implemented to work the same as the original Hayes 1200's. 
Your modem may have more or less than a Hayes. Some modems have close to 99 
of these S-Registers. Fortunately, you do not need to be as familiar with S-
Registers as you do with AT commands. Some simple tricks with S-Registers is 
all you need to know in order to use your modem properly. Put these S-
Register commands in your initialization string and you have a powerful, yet 
easy tool for setting up your modem.

To change the contents of an S-Register, you just type (on a new line of 
course) ATSx= nnn (x = an S-Register number, nnn = a 3 digit parameter). For 
example, to change the time a modem waits to connect from it's default time 
of 
30 seconds to 255 seconds, type ATS7=255 and press return. To read an S-
Register value, you type ATSx? (where x again is the number of an S-
Register). to see the value of the S7 register, just type ATS7? and press 
return. The value currently in the S7 register will be displayed in a 3 digit 
form (030 is the default, for 30 seconds). All S-Registers have default 
values, but there can be reasons for changing these defaults. For example, 
suppose you wished to wait for 1 minute to connect to a BBS instead of 30 
seconds. You have to change the value of the S-Register 7 in order for your 
modem to work properly. So your initialization string (including what we 
already mentioned above) now becomes AT M0 X4 S7=60. The only limit to the 
number of commands you can use is usually 40 total characters (for most 
modems). 

Recess time
-----------
Well that is about all the space we have for this month's look at our series, 
a Modem Primer.  Next month, we'll go to the junior level and talk more about 
using initialization strings, serial cables, internal vs external modems, and 
handshaking signals.