The circuit is given in Fig. 1. To wire up, connect the 74L505 to
address line A3 through A7 and to IORQ. Common the collectors together using a
1K5 resistor to form a ‘wired OR’ gate, and connect this common output to pins
1, 4 and 5 of IC46 which are lifted out of their socket for the purpose. Pin 6
of IC46 is also lifted and connected to the (lifted) pin 2 of IC45. So far no
snags have cropped up in testing, with or without the IVC in either Nascom or
Well, as you see all ended well. Mr. Thornton is happy, I’m happy
because that’s one more problem solved, and my boss is no less happy than usual
in that he knows nothing about this episode, and I have spared him the anguish
and cramp of the wrist he gets when he signs cheques.
Whilst on the subject of Nascom 1’s we have recently heard of a turbo
charged N1 using a Z80B and running at 6MHz. Apparently the main snag of the
conversion was not any defficiency in the design but the crazy cost of 16 150nS
2102 RAMs for the work space and video RAMs.
Centronics Interface Nasty
On the subject of problems solved, one 000-nastie which cropped up
recently which I may have mentioned before but is well worth repeating. This
concerns the well known Centronics parallel printer connection. Now Centronics
when they designed the interface very wisely put all the control signals on one
side of the plug and made the pins on the other side all earths. This is so that
when ribbon cable is used with insulation displacement type connectors, each
signal line is separated from the next by an earth line thereby reducing noise.
Now the Centronics plug has 36 pins, yet there are only thirteen signal lines,
and appropriately enough, thirteen matching earths. This leaves ten unallocated
lines. This means that other manufacturers who have adopted the Centronics plug
convention are free to do as they will with the remaining lines, and believe me,
they do. Herein lies the danger. It is tempting when wiring a Centronics socket
to run a continuous earth across all the pins on one side, pins 19 through 36,
as these are the signal earths as defined, or otherwise are unused. Well watch
out. Epson in their wisdom have made the printer SELECT line pin 36. That’s Ok,
as it has to be earthed anyway to turn the printer on. But Seikosha have made
pin 35 a +5 volt output for some inexplicable reason, so if it’s earthed the
least it will do is blow the fuses in the printer. NEC go even better, pin 31 is
the INPUT PRIME, a sort of reset line, Ok it can be earthed, except it will hold
the printer permanently at reset so it won’t work, further pin 32 is the FAULT
output which is a TTL level open collector output, so earthing it won’t do it a
lot of good. Pin 36 is the INPUT BUSY, a duplicate of pin 11, earth it and the
handshake doesn’t work. Another classic, but this time at the machine end is by
the makers of that fire breathing Welsh creation (Dragon if you haven’t worked
it out). Because they use a 20 way IDS socket on the computer that means there
are only 10 lines along the top edge, so what happens to the BUSY signal?
Simple, they move it to the other edge so it means hand wiring the plug at one
end or the other. Better still, the Dragon shoves +5 volts out of one of the
other pins on the earthy side. Forget that and connect it to earth as you might
expect, and you might get more smoke out of the thing than you bargained for.
Brilliant isn’t it!!!
A SYS ‘feature’.
Now on to more mundain matters. At a reasonable guess there are a couple
of hundred of Richard’s SYS program out there and most of those must belong to
readers of this magazine as otherwise how would you have heard about it? Well
anyway Richard has built a somewhat undesirable feature into the SYSB versions