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Hello again, folks 



Here's January! We can see the light 
at the end of the tunnel now. Our 
various toys seem to be humming right 
along, and will probably continue to do 
so until they break (now there's a 
logical statement). As a matter of 
distinct fact, if you'll look up at. the 
CLOAD logotype in the upper right 
corner, you'll see a blot right after 
the word "magazine". That's an "inc." 
blot, As of January first, it's CLOAD 
Magazine, Inc. We have managed, with 

hard work and understanding subscribers, to arrive at 
it's appropriate to shift from the "garage operation" 




to the club of True Capitalists, 
garage in the process... 



that point where 
form and move up 
If only we could move out of the 



Announcements : 

The Peripheral People (Box 524, Mercer Island, WA 98040) have 
announced Yet Another Upper/Lower Case Conversion. Theirs has, 
however, an Interesting Twist. First, it is switchable, so you can 
return to the "normal" configuration easily. Second, all of the parts 
come from your local Radio Shack (RS catalog numbers are included) . 
And third, the associated word processing software (the Electric 
Secretary) has a hyphenating dictionary. Bad news first: the 
disadvantage of this last point is that to use it, you must have a 
disk system. To explain the advantage of a hyphenating dictionary, we 
must first describe a word processor. 



a wore processor 
writes text by typinq 
keeps a copy of the 
control, the memory c 
the printer, the w 
and carriage returns 
insert a phrase in th 
the lines "come out r 
processing computer 
paragraph will appear- 
Most people like fo 



is a system where the writer (f'ri 

it all up on a computer screen. Th 

text in memory and/or on disk and und 

ontents are sent to the printer. On t 

ord processor software inserts and del 

to "even up" the text. This allows th 

e middle of a page without worrying ab 

ight". This editorial is being writte 

(confession - not a TRS-80). This 

in the text that you read as "right 

r the text to be laid out like this, b 



nstance me) 
e computer 
er software 
he way to 
etes spaces 
e writer to 
out making 
n on a word 
particular 
justified" . 
ut I don't. 



So I just turned it off. What a hyphenating dictionary does is 
make the software more capable of handling this particular trick. 
Suppose the right margin occurs smack in the center of a word like 
"montmorillonite". It's too long to put on the current line, and if 
it's put off to the next line, the current line is spaced just about 
like this. A hyphenating dictionary allows the 
computer to look the word up, and cut it into two pieces at a "legal" 
place . 

Fine. Now what I'd like to see is an auxilliary program called 
"spel" which looks up each word in a chunk of text, and prints out all 
words it can't find, so the spelling can be checked. 



By the way, for those of you who might recognize the Peripheral 
People's address, they used to be CLOAD House. 



A software tip for you level Il'ers who want to run a program 
without changing the value of the variables already computed: Try GOTO 
10 (or wherever) . When you type RUN or RUN nn the computer first 
resets all variable space to zero. There are times when this is a 
pain. 

For those of you with disk systems, there has been a hardware hug 
crawling around causing I/O errors . Turns out that there is a 
hardware "data holder" whose duty it is to hold data coming off the 
disk for up to 80 microseconds. Fine. There is also a heartbeat 
interrupt which will occasionally stop the central processor from its 
appointed rounds for up to 900 microseconds. So we have the possible 
conflict of a heartbeat going flubadub in the middle of a data access, 
getting the disk controller all upset over inattention, and routing 
the data to the bit bucket. There are two fixes available,, Number 
one is to tell the software to keep on trying... POKE 16553,255 : 
DEFINT I, J : FOR 1=0 TO 7 : READ J : POKE 18104+1, J : NEXT I : DATA 
203,87,32,19,254,32,40,17 Chop this up into whatever your style is in 
line numbers, and execute it once at the beginning of your program. 
This puts a machine language patch in Disc BASIC, giving it the word 
to try again if there is an error. It is still (remotely) possible to 
have a case of discular fibrillation, though, and the only recourse 
short of drastic open-case surgery is to momentarily suspend the 
heartbeat immediately before each disk access. How? POKE 18075,243 
prior to the first disk access. This instructs the Z-80 central 
processor to stop listening to the heartbeats while playing with the 
disk. The disadvantage of this technique is that the real time clock, 
like Galileo, uses the heartbeat as a time standard. Continued heart 
stoppages lead to a lethargic clock, possibly several minutes a day. 

Rumors mongered here: 

Tandy Corp appears to be making a parallel printer interface and an 
RS--232 serial interface that will not need the expansion interface. 

Hardware: 

This month, for our hardware talk, I'd like to start things off 
with the information that yes, indeed, the 8255 chip is sold by Radio 
Shack (catalog number 276-2555, price $9.95). A crazed but enthusias- 
tic Tandy supporter broke into our international headquarters, beat me 
severely about the head and shoulders with the jawbone of a used 
computer salesman, and informed me of my error. 'Scuse me. One 
logical common place we all know about is the local Radio Shack store, 
so I'll try to use their parts as much as possible (I don't own any RS 
stock, honest) . 

Next I'd like to introduce the concept of voltage. Voltage, by 
definition, is what a flashlight battery has 1.5 of (stop moaning, you 
purists). It comes in two varieties, positive and negative. We 
connect the negative side of our voltage supply to "ground" and forget 
about it. When we speak of a point in a circuit having 5 volts, we 
are really saying that the electrical force between that point and 
"ground" is 5 volts. It turns out that nowadays the main power supply 
voltage is this single value - not long ago it was three separate 
supplies, but we'll get into that. Ground, for those interested, 
refers to the most common point in the circuit. If the enclosure of 
the circuit is made of metal, it is usually the ground connection. 
All chips have one pin which must be connected to ground, partly 
because ground is one of the power connections, and partly to act as a 
signal reference. 



At this point, I should put in a pitch for Ohm's Law. It is the 
one which summarizes the relationship between voltage, current and 
resistance. A working understanding of these three entities is the 
basis for the field of electronics. Those who do not already know 
this relationship should consult a basic electronics book before 
actually getting" to the point of building something. For this 
discussion we will use voltage almost exclusively, and resistance and 
current will have to shift for themselves. We won't get overly 
concerned about them at first. 

There are five voltage levels which we are interested in. Tne 
first, is (nominally) 5 volts. This is the main power source for the 
various chips. The maximum value for this supply is 5.25 volts; the 
minimum is 4.75. If the power supply is outside of this range, the 
circuit might not work. The rest of the voltage levels all refer to 
signal levels. Starting at the low end, we have .5 volts. This is 
the upper limit for a logical "0" or "low" at any output pin of a chip 
(74LS type outputs, for you purists). The next voltage level is .8 
volts. This level is the highest that any input pin is guaranteed to 
accept as a logical / low. We now look at the high side. 2.0 volts 
Is the lowest voltage that any input pin is guaranteed to accept as a 
Logical 1 / hi . 2.8 volts is the' lowest voltage that any qutput pin 
will transmit for a logical 1 / hi. 

These signal levels can be summarized as follows: 






to 


.5 


volts - 


a solid 


low 


.5 


to 


.8 


volts - 


a shaky 


low 


.8 


to 


2.0 


volts - 


invalid 


level 


2.0 


to 


2.8 


volts - 


a shaky 


hi 


2.8 


to 


5 


volts - 


a solid 


hi 



Now I'd like to discuss the philosophy of Power On Clear (POC) . 
When starting up a system which is hooked up to motors, pumps, valves 
and such, the general desirability is to apply power and have the 
system "wait" in a relatively docile mode while the computer program 
is being loaded. Example - if there is a valve controlling a gasoline 
supply, it should start up in the closed position, and remain closed 
until actively commanded to open. When the controller (that's what 
we're hypothetically building) is turned on, the TRS-80 hooked into it 
may or may not be ready to start control, and the power circuitry to 



Sodd's Second 



'ided 

applied to our controller, we will let this pin go "high" (anything 
between 2.8 and 5 volts will do). After a sufficient amount of time 
has passed to allow the circuitry to stabilize, this pin is pulled 
"low" (anything between and .8 volts) and the 8255 is now in its 
initial state - all registers cleared, and all ports in the "input" 
mode. Note that this does not insure that the whole project is 
properly initialized - each subsystem must still .'be considered, and if 
there is a "dangerous" mode of operation (to you or it) , some form of 
initialization is required. 



Why all this palaver? Isn't it enough to simply steer clear of 
dangerous applications? Well, I'll concede that not too many of our 
subscribers will be controlling bombs with their computers, but even 
fairly simple ideas, like door and window management in a house, have 
aspects which are uncomfortable if not actually dangerous. So you get 
one more paragraph on the subject, entitled "Fail Safe". 

In any mechanism or system, there are three general categories of 
operation. One, working correctly. Two, working incorrectly. Trree, 
not working at all. All systems that are presently in category ore- 
have a finite chance of becoming category two or category three 
systems. Sometimes it doesn't make much difference which way they go 
( f ' r instance your TRS-80). Sometimes it is important for them to 
continue to work, even if incorrectly (lights over an operating table) 
and sometimes it is important for them to stop completely (nuclear 
reactors). In these last two cases, the designer must take steps to 
assure that they fail in the "safe" category - thus the name. Note 
that we are not talking of preventing failure - we are assuming 
failure is inevitable. How do we accomplish this controlled failiie? 
Let's say we are controlling a motor and we are concerned that out 
computer might have a heart attack and die after turning the mctoi on. 
Let's also say that, upon system failure, we want this motor off. We 
might design the switch circuit that turned the motor on such that it 
only turned it on for one second. Our program would then have to keep 
turning it on - at least once a second. If the computer dies, the 
motor gets no more commands. Suppose we're really paranoid about it - 
we could have two switch circuits and wire them such that they both 
have to be on to turn the motor on. If either one fails in the off 
direction, the motor turns off. If either one jams in the on 
direction, the other one controls the motor. How about both switches 
si mi; I tanec ,;sly jamming on? Suppose each switch has a one in one 
tncasj.no cnance of failing on a given day. Two switches would then 
have a one in one million chance of failing simultaneously on a given 
day. Three switches? One in a billion. It gets expensive sometimes, 
and reliability suffers (fail safe designs fail more often), but 
there's little excuse for a failure to do serious damage. 

The 8255 spec sheet promised this week has been canceled due to its 
availability at Radio Shack (it comes with the chip - 12 pages' worth) 
and the local shortage of paper for my own form of yellow journalism. 
The circuit schematic that was to have replaced it has been postponed 
until I can verify that the circuit works. There is also a fair 
amount of local pressure to finish this so we can get the January 
issue out - February is starting to roll off the duplicator, and I'm 
beginning to wonder if they'll ever unchain me from this desk. 

"Next Week" , 



Ralph McElroy 
Publisher