Problem with Long cables Organ Project

Testing/Troubleshooting
1

I would welcome any comments on this problem,  especially from those who have solved such problems.

I have completed the C code for the 2 keyboards using a Core Module and 4 Din Modules.  This works fine. I have used unscreened 

Din - Din cables for the keyboards.

 

I have finished the basic hardware and software (in C) design for the organ console. This uses a second Core module , 2 Din modules and 2 Dout modules.  This works fine.  However when I add the 3rd Din module  and the 31 pedal switches and a Din -Din cable of 1 metre I get interference and the LEDS previously working behave randomly with some fluttering ie half on.

If I shorten the pedal cable to 0.5metres.  The system works.

However this solution is not practical.  The console to pedalboard dimension is about 0.8 metre.

My guess is that  have loaded the 4 core connections RD0, RD1,RD2,RD3 with too much cable capacitance and the signals have degraded.  The Din and Dout interconnect cables are screened.

I admit that, with hindsight, I could have minimised the Din - Din and the Dout - Dout cable lengths.  However to minimise these cable lengths would mean lengthening the switch and Led wiring.  Also I would need a new layout with a new wooden front panel with a fair amount of woodwork. (A lot of work!)

An alternative would be to mount the 3rd Din module nearer the console.  This would mean pedal switch cable lengths of about 1.8 metres.

Question 1

If I extend the Switch and Led cable lengths, is screening necessary?

Quesion 2

Is screening LED cables more important than screening Switch cables.

I do understand that most Midibox Constructors will not have the need for such long cables.

Here are my total cable lengths.

Core - Din - Din (1.8 metres)

Core - Dout - Dout (1.2 metres)

Best Wishes Robin Fawell

2

Gee, Robin,

  I use din’s to encode an old keyboard (korg poly ensemble S) for my moog modular controller, and the cable distance I use is at least 2 meters.  However, I am only using 2 din’s for this purpose.  I din is in the keyboard case and max cable length from the HI-C key is about a meter, then I use a 25 pin parallel port ribbon cable to hook both it and my pots on the keyboard to the core/din on the first midibox (I don’t use any buttons only pots on the second core)… approx 2 meters of ribbon, but I think it would work with more.  I DID have to run 2 extra lines for power to the din on the keyboard, double the current capacity. Have you tried feedin a bit more current to the kbd/din combo?

hope this helps,

gb

3

Screening is important for such distances to avoid interferences with other signal sources, but in general it won’t fix the problem. It’s more related to signal runtimes, and they depend not only on C, but also on L and R - in other words: on the impedance. This page provides some theory stuff http://www.epanorama.net/documents/wiring/cable_impedance.html, but I must clearly state, that I’m no expert in this area, and cannot say immediately without studiying in my old dusty study books, what could help here. Maybe another cable, maybe a termination resistor, I don’t really know. :-/

But maybe there is somebody else who has more experience in this topic?

P.S.: it would be worth a try, if it works better, when you are not using two seperate branches for DIN and DOUT modules, but one branch for the Ground/+5V/SC/RC signal (connected in parallel to each module), and the DI/DO signal like before - in a chain

Best Regards, Thorsten.

4

Sorry TK and Robin,

  ‘Screening’ ?  is this the same as ‘Shielding’ ? … a ground wire(or multiples of grounding wires) embedded in the cable…?

excuse my multilingual ignorance.

gb

5

‘Screening’ ?  is this the same as ‘Shielding’ ?

More or less. It means a complete shielding ot the hole cable with a foil or braid wire.

First hit in google for ‘screening cable’:

http://www.raychem.com/resources/documents/datasheets/Wire_and_Cable/Elecscrn.pdf

Raphael

6

TK’s suggestion of making  parallel connection of the common signals to Din and Dout makes a lot of sense. If the connections (Ground/+5V/SC/RC signal) are in parallel then the capacitance can be halved (approx) since capacitance is proportional to cable length.

This leaves the non common signals which must be chained ie SI on the Din module and SO on the Dout module.  It is difficult to be certain, but these signals are regenerated on each module and the degeneration of these signals may be less.

For SI and SO there is always the the possible improvement by matching the cable impedance.  See http://www.fairchildsemi.com/an/AN/AN-393.pdf.

Solution (d) from the above might be worth a try. I would try 100pf with 100ohms (the ac termination method).

I will try the parallel connection suggestion above and report on the outcome.

Many thanks to all.

Robin

7

Thanks Thorsten

The suggestion worked.  In my case situation was made worse by having the core in the centre of the console.  The Din and Dout interconnect cables travelled left from the core then travelled right, crossed over the core to the remaining Din/Dout module on the right.  The common signals Ground/+5V/SC/RC cable lengths were thus halved in the revised layout.

The SO and SI cables stayed the same length.  However these signals are regenerated in each Din or Dout module.

This would suggest that screening of the of LED and switch wiring is less important.

Regards Robin