Connecting Hammond Drawbars

[Guest JimCook]

I have seen a design for "encoding" drawbars that I don't think will work with actual Hammond drawbars.  Hammond drawbars are NOT linear slide pots.  They are nine position slide switches.  To change the contact closures to an analog resistance value the design uses eight 1.2k resistors in series with with the drawbar wiper switching between the resistors.

On a set of Hammond drawbars, each drawbar has it's own wiper but the contacts are all bussed together.  The switched resistor scheme above would require cutting the bus at each drawbar and connecting a resistor array at each drawbar.

I don't think I want to do this...

I believe I will hack around with MIOS to create a scanned matrix that will eliminate using analog inputs for the drawbars.  Each bus, 0-8, will be connected to an output and each drawbar wiper will be connected to an input.  The outputs will be turned on one at a time and the inputs examined for each drawbar to see if any shows a contact closure.  If so, then that is the position of drawbar.  

If no input for a drawbar shows a contact closure in a scan, the last value will be retained (the scan might catch a drawbar being moved from one contact to the next).

[borntob4]

"The switched resistor scheme above would require cutting the bus at each drawbar and connecting a resistor array at each drawbar. "

No, you don't have to. You 're referring to the schematic I've send to you, right ? You need only 8 resistors for one drawbarset. The wiper is connected to the AIN-module and depending on its position a voltage level (/ a resistance value) is translated by MIOS to a digital value. It is already a kind of matrix.

If you would own a big drawbarset with the 4x9 drawbars plus 2 drawbars for the pedals and assuming that there are only 9 long busbars you would only need 8 resistors to controll all 38 drawbars. Did I make myself clear ? I bigger schematic would help, but I don't know if I find some time.

Bye the way, making it digital with a matrix would be cool, but in my opinion, it's allready done.

Best

andreas

[Guest JimCook]

It's tough reading Hammond's patent application one minute and then thinking about the drawbars and the resistance ladder the next.   :-[

I was confusing the parallel resistances in the wiring that Hammond relied on to generate multiple harmonics from a single tone generator, with Petkov's serial drawbar resistance ladder.  The voltage drop at each "tap" in the 1.2k resistor chain will be constant no matter how many drawbars are set the to the same position.

Thanks for reminding me that it's voltage not resistance that the AIN is reading.   ::)

My progress so far?  I purchased a set of M3 drawbars and a 44-key Baldwin or Thomas organ keyboard which used virtually the same waterfall key board that the M3 uses.  I cleaned up years of grime on the keyboard and stripped all the wiring I didn't need.  I finished wiring  ribbon cables to the keyboard yesterday and I'm splitting the M3 drawbars into the upper and lower manual (I'm just going to use a single drawbar set for now).  I bought SmashTV's Core and DIN kits and will be soldering them together this weak (Thanks, Smash!).  

I'm just going to use the Core's eight analog inputs (I'm doing my Yamaha controller first and I only need eight inputs, I'll be using the M3's lower manual drawbar set which also has just eight drawbars) for the draw bars.  The DIN's will handle keyboard encoding and a few rotary encoders.  The Core's eight digital outputs I'll use for LEDs and I'm going to get a 2x40 LCD display soon.