Midi, serial ports, uart's and the compaq aero

[Fraser, Colin J]

Rene Schmitz Wrote:
> 
> Ahh, thats what I've dreamed of, when I made my (very basic) 
> convertor.
> I intend to add more outputs by adding s&h stages which are 
> selected by 
> other lines of the centronics port. So I can make all the 
> programming in C
> on my
> PC)

and Bjarne Nillson Wrote:
> And "where" on the net can we find this multichannel cv/gate 
> converter? 
> Please!

Nowhere on the net yet - some schematics etc are hand drawn and very rough,
some are in my head, and it's all built on stripboard.
I've been looking for a schematic editor for the PC that I'm happy to use
for ages, but the demos I've tried piss me off too much...
Here's a brief description of my cv convertor:

CPU:
It has a 6502 processor with a 2764 eprom for code storage and a 6116, 2k
Static RAM. A 4MHz xtal oscillator is divided down by a 7 stage divider to
give 1Mhz clock for the uP and 500Khz clock for the serial receive clock.

INPUT:
There is a 6N139 opto isolator feeding a 6850 ACIA serial chip. There is
also 74LS374 latch, plus a 7474 flip flop and a monostable that comprise a
parallel port i/f.

OUTPUT:
There is a DAC (an 8-bit ZN558), followed  by an op-amp buffer with scaling
trimmer to give 1v/oct scaling, followed by a number of 4051 8 way analogue
switches. The outputs of the analogue switches feed a poly capacitor which
'stores' the voltage for each output between refreshes.
Each cap is followed by a TL074 op-amp configured as a voltage follower.
There are latches to select which 4051 and channel to use.
The processor holds a table of the values for each output, and when it's not
doing anything else, it cycles thru a process of deselecting the current
analogue switch channel, writing a value to the DAC, then selecting the next
output channel, and so on. (This might be a lot to do over a PC parallel
port.)
The gate outputs are held on a 74LS374 latch which is updated when a
note-on/note-off message is processed. 
That's enough (with a bit of software added ;-) ) to provide basic
multi-channel cv/gate output. There are a few other bits of circuitry -
another latch drives analogue switches after the first 4 cv outputs that
turns simple glide circuits off or on. The software allows glide to be
off/on/auto. Single and multiple triggering is provided in software too.
12 (count them - 12) 74LS123 monostables provide 24 pulsed gate outputs that
trigger a complete set of 808/909 sound generator circuits. 24 cv channels
provide accent cvs for these. The other 16 cv channels, and 8 gate outputs
are for driving synths.
The drum circuits are built into a 4U rack case with the cv convertor. You
should see the amount of spaghetti hooking all that lot up...

One of these days, I'll get a schematic editor I'm happy with, do a proper
PCB etc and stick all this on a web page somewhere.

As far as building a parallel port cv output unit goes, if you used a line
on the parallel port as an address latch load line, you could direct data to
256 diffferent devices - more than enough to implement the hardware in my
convertor after the processor. I'm not sure tho' if the load on the port
wouldn't be too high as you need to refresh the s/h caps at as high a rate
as possible before the voltage droops. You can't just increase the size of
the caps as then it takes longer to change the voltage on them, and the
ability to update many channels at the same time is reduced.

Any questions, drop me a mail.

Colin f