Economical answer to time shared CAL in school
By TONY CRAVEN
WE had a problem with our computers. How do you organise the
use of ten BBC Micros in a sixth form college so as to give everyone
as much time as you possibly can?
Some students knew more than any teacher, some had never seen
a keyboard before.
Nine computers were tape based, one was a disc machine with
a single sided 100k drive. The timetable demanded 40 minute slots.
We had a reasonable software library, something for everyone.
Computer Studies groups were forever writing programs and wanting
them saved until the following day, when they would drop in at
lunchtime with 20 minutes to spare.
And those cheap tape recorders! Would they work reliably? You
tell me!
I had to do something to sort out the fights for who got the
disc machine. Fortunately, while browsing through the User Guide
one day I came across the FX calls which direct data to the RS423
port, that middle DIN socket on the back of the BBC Micro.
Paul Beverley's article in the October edition of The Micro
User about wiring up the RS port helped greatly, and within a
couple of days we had a simple network running where each of the
nine TFS (tape filing system) machines were loading from and saving
to the one DFS micro.
Not very fast, but at 10k in about 10 seconds who is grumbling?
Now we don't use tape recorders at all. Students can save a
program at the very end of a class, confident that it will be
there tomorrow, when they can pop in with that quick amendment.
Classes can book the computer room for 40 minutes knowing that
39 of those minutes can actually be spent using the Computer Assisted
Learning programs.
All for the price of a few five pin Domino DIN plugs, a multi
wafer rotary selector switch and some scrap five core cable that
fell off the back of a telephone engineer's lorry.
Figure I shows the wiring necessary from the DFS micro to any
number of TFS machines. I took the wires through a 12 position,
four way selector switch, as shown in Figure II.
The software is very simple being contained in function keys.
Each disc contains the program given in Listing I.
This program is CHAINed into the disc micro and then each TFS
micro must be forced to listen to its RS port by typing in *FX2,1
followed by Return.
The particular TFS machine is selected on the rotary switch
and f9 on the DFS is pressed.
This configures the function keys on the TFS machine so that
f0 makes it listen to the network and f 1 transmits its current
program listing to the DFS, provided the DFS has been forced to
listen by having its f0 pressed.
This configuration need only be done at the beginning of each
day. After that single function key presses will suffice.
The students have taken to the system very easily. All they
need to do is to place the right disc in the drive, select their
micro number on the rotary switch, press f1 on their micro, f0
on the disc machine and they're off.
Saving is an equally easy process, and we have one disc dedicated
per group. Everybody in that group saves their work one after
the other on the same disc.
I have recently also found that simple Wordwise files can be
sent over this network.
Providing there are no embedded commands in the document, *FX5,2
typed into the transmitting computer in the menu mode, *FX2,1
entered into the receiving computer followed by Escape, and then
option six from the menu of the transmitter will dump the formatted
document from one to the other.
Here the RS port is being selected as the printer for the transmitter
— the receiver simply thinks that the document is being typed
in, very quickly.
The receiver is locked onto its RS port and so has to be unlocked
by pressing the Break key. The text can be recovered by answering
Y to the old/new text prompt that occurs after a Wordwise break.
If you have more than three BBC Micros with one DFS, then rigging
up this network is well worth doing.
The hands-on time is increased dramatically and it is also really
viable to have a fast change over for CAL groups.
I know that Econet can knock this network into a cocked hat,
but this one costs virtually nothing. What's more, it works now.
10*KEYO*FX3,7¦ML.¦MP."*FX2,0"¦M*
FX3,4¦M
20*KEYl*FX2,l¦M
30*KEY9*FX3,7¦ML.¦MP."RUN"¦MP."*
FX2,0"¦M*FX3,4¦M
f0 = transmit current listing
fl = listen to RS423 port
f9 = configure the function keys on the TFS that has been forced
to listen.
Listing I
By the way, ignore any syntax error reports. It works!
Tony Craven is Microelectronics Coordinator at Shena Simon College,
Manchester.
Figure I: Connections between two BBC RS 423 ports
Figure II: Rotary wafer
