*** Printing by GAS ***
The time had come when hard copy was an absolute necessity, but how did one
obtain (other than beg, borrow or steal), a mechanical marvel of reasonable
quality? A CREED was ruled out since I could not afford any structural
alteration’s like joist support’s or double glazing.
(I almost bought a QUME daisywheel but they didn’t have the right colour).
Think’s.....try the WATER board, GAS board, ELECTRICITY board, GPO even.
WATER board:– Well er...I don’t er... we’ve got ‘one’ but when it goes wrong
we buy another.
GAS board:-YES, IBM magnetic tape typewriter’s, (GOLFBALL’S to you).
Will you be making any redundant in the near future? YES, come over and have a
look at one.
I was there like a shot and came face to face with an IBM MT-typewriter.
I’m afraid we can’t just sell the printer you would have to take the whole
system. This dampened my enthusiasm because the ‘whole’ system looked
completely un-affordable, (have you ever seen an IBM MT-typewriter?).
It consist’s of a ‘metal’ three draw desk with the printer on top, and alongside
that is a rather large tape control unit. The latter, (by the way), just fit’s
into the boot of a MARINA.
Anyway, to cut a long story short, I managed to acquire this highly sought after
piece of equipment for a princely £50.
*** Now to drive it. ***
I eventually intend to drive the printer from an RS232 serial port using an 8035
micro. This will be programmed to recieve RS232 output at whatever speed is
selected by link’s on the RS232 input socket. (If you are not familiar with
the 8035/48/51 series micro, perhaps an article would be of interest, it’s an
ideal control micro?).
For the time being, however, I wanted hard copy, so decided to use a PIO.
There are 7, 48 volt print magnet’s on an IBM called R1 R2 R2A R5 T1 T2 CK.
I’ve wired these via, drivers to port 5 bit’s 0 to 6 respectively.
There are also a few control magnet’s, I have used CRLF TAB SP LC UC connected
to bit’s 2 to 6 respectively of the same port.
A glance at the circuit will show that bit 7 is used to select either print
magnet’s or control magnet’s. This was done so that a 12 core cable could be
used between the computer and printer.
Two other signal’s are required, a BUSY line and a SHIFT line. These are
produced as shown in the circuit. When the printer is ready, BUSY=1.
The SHIFT line is used to identify which case the printer is in, and also serves
to ‘time’ the case change. Port 4 bit 0 is used for SHIFT, and bit 1 for BUSY.
The mechanical timing of an IBM is fairly complicated, so to get a
correct BUSY line signal, the feedback contact’s must be connected in the
correct sequence as shown.
The contact’s work on the following principle:–
When the IBM is ready, the N/C contact’s are used to set BUSY high.
A character is sent.
When the N/O contact’s have closed, the magnet’s may be de-energised.
And when the N/C contact’s have re-closed the IBM is ready for another character.
Refer to JAN 79 Practical Computing for more detailed information.