20 Mar
2007
20 Mar
'07
8:53 p.m.
I am currenly contemplating what kind of IO to put on my coming i8008
system : what is it that makes blinkenlights (i.e. leds and switches )
seem so attractive ?
It must be about the worst possible way to interact with a computer...
Actually, having use both a lights-and-switches panel and a
keypad-and-numeric-display panel I much prefer the former. I am not sure
why, but I find it more natural.
Maybe it's because a lot of older machines have an instruction set that
makes sense in binary -- that bit field is a register select code, that bit
field determines the ALU operation, etc, but said bit fields do not align
with boundaries of hex or octal digits.
So why is it then that almost all early micros had them ?
It may have been that the hardware is a little simpler (encoding a
keypad, if done with MSI or SSI chips takes a fair few chips, ditto
driving a 7 seg display). The PDP11/34 after all had am 8008 processor to
drive the keypard/display, older PDP11s had a lights-and-switches panel
with a handfull of TTL ckips to control them
a 7segment display with keyboard ( as in a H8) is clearly more usable,
and would have cost nothing more. Or were early eproms (for the monitor
program) that expensive ?
If you're thinking of using the 'main' CPU to drive the panel, don't. For
the keypad/display to work, that CPU and most of the rest of the
electronics, has to be working correctly. Remember that these development
boards were often modified with the user's own hardare and it only takes
a bus short or an incorrectly wired eanble line to keep the thing from
running. It's a lot easier to debug the machine when you can send stuff
onto the bus in hardware. Most, if not all, of the lights-and-switches
panels (and, indeed, keypad panels on 'real' machines) were totally
independant of the main CPU.
-tony