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 Forum index » Instruments and Equipment » MIDI Controllers and Interfaces
Hacking a serial midi interface?
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squarewhite



Joined: Jun 06, 2007
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PostPosted: Thu Apr 19, 2018 5:31 am    Post subject: Hacking a serial midi interface? Reply with quote  Mark this post and the followings unread

Hi,

I was just cleaning up my room and discovered that I still have a serial midi interface from Emagic. It was the first interface I ever use when using Opcode back in the 90s. It has served me well.

Fast forward--->

Now recently acquired a Digitakt and love the interface the possibilities of beats making. I might next acquired a tabletop polyphonic synth, maybe a Micromonsta or a preenfm2. So this discovery of this midi interface came as a blessing.. well maybe....

On the interface, it has a midi thru button. I plug my midi out from the Digitakt to the interface midi in and midi out from the interface to a Volca bass. Before doing this, I've already test that I could control the Volca Bass on track 9 of the Digitakt. So that works. But thru the midi interface nothing is transmitted.

So is there any way of hacking this to work as thru device? Maybe I need to power up the unit for it to work? Any ideas or suggestions?

There are 3 outs on this devices... hence if I get this working, it would be a super great bonus! and I can save money for one of the aforementioned synths!!


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ian-s



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PostPosted: Fri Apr 20, 2018 9:14 am    Post subject: Re: Hacking a serial midi interface? Reply with quote  Mark this post and the followings unread

squarewhite wrote:
Maybe I need to power up the unit for it to work?


Probably, I don't think the optocoupler on the input will work without power.
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squarewhite



Joined: Jun 06, 2007
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PostPosted: Sat Apr 21, 2018 2:53 am    Post subject: Reply with quote  Mark this post and the followings unread

I was looking at the cable layout for the mac serial port connectors.. it seems that the connector has only 8 pins. and the pin for power is missing. My guess is that it the power is coming from the TX and RX port.

How and where can i get power into it the optocoupler?

i use this link for information on the serial ports.
https://whitefiles.org/b1_s/1_free_guides/fg1mt/pgs/h10b.htm
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PHOBoS



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PostPosted: Sat Apr 21, 2018 4:18 am    Post subject: Reply with quote  Mark this post and the followings unread

It might work if you supply +5V to pin 9 of the serial computer input (and GND to pin 4).
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squarewhite



Joined: Jun 06, 2007
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PostPosted: Sat Apr 21, 2018 4:39 am    Post subject: Reply with quote  Mark this post and the followings unread

there is no pin 9 on the serial port that I have... this is the diagram related to the port


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PHOBoS



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PostPosted: Sat Apr 21, 2018 4:52 am    Post subject: Reply with quote  Mark this post and the followings unread

ah I see, that's too bad.

Maybe if you open it up and take a photo of both sides of the PCB (if it's not too hard to take out)
I can see where to apply power. Or you might be able to figure it out yourself too.

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squarewhite



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PostPosted: Sat Apr 21, 2018 5:09 am    Post subject: Reply with quote  Mark this post and the followings unread

here are the pics...hopefully it's clear enough..

there's 3 ics, they are ,
Sharp RC900V
SN74HC74N
MM74HC14N

thanks for having a look at it.


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PHOBoS



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PostPosted: Sat Apr 21, 2018 5:39 am    Post subject: Reply with quote  Mark this post and the followings unread

I think I can work with that. It's probably possible to just insert the power on to the PCB (I can see some traces that carry the +5V)
but it must be possible to power it through one of the connectors. Maybe as you mentioned through the TX/RX lines. I'll try to work
it out later today.

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squarewhite



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PostPosted: Sat Apr 21, 2018 5:48 am    Post subject: Reply with quote  Mark this post and the followings unread

i couldn't find any info on the RC 900V chip.

The MM74HC14N is a hex inverter and the SN74HC74N is a trigger flip flop switcher. My very very rough guess is the -14N is for decoding and encoding the midi signals, the -74N is for the thru and not thru. That leaves the RC900V chip..
Yes I remember only when I plug the device to the mac serial port that I get the leds blinking on midi send or receive.

If it's possible to add power, I would like to add a simple circuit just to be able to power it from a 9v adapter.. likely have to use a TL 7805 voltage regulator for this or even better a 5v usb socket. But that would mean cutting a squarish hold for the plug... err.. going for the path of least resistance... Wink.

well let me know if that's possible to get 5v on this! Would be super ace!!!

Thanks!
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Blue Hell
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PostPosted: Sat Apr 21, 2018 6:03 am    Post subject: Reply with quote  Mark this post and the followings unread

PC900V is what I'm reading (not RC) .. which seems to be the opto coupler :: http://acon.en.seekic.com/product/integrated_circuits_ics/pc900v_pc900v_original.html
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squarewhite



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PostPosted: Sat Apr 21, 2018 6:56 am    Post subject: Reply with quote  Mark this post and the followings unread

:facepalm:
time to get my eyes check!

ah ic...ok so I should be able to add power on pin 6 and 0V on pin 5?
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Blue Hell
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PostPosted: Sat Apr 21, 2018 8:40 am    Post subject: Reply with quote  Mark this post and the followings unread

Dunno ..

Since there are no dedicated power pins the power is either obtained from a MIDI input or from the serial port connection. My guess would would be that the four diodes near the LEDs make a bridge rectifier and the electrolytic capacitor would buffer it ..

In that case your suspicion of the power coming from the RX signal would probably be right, and PHOBoS' idea that a connector could be used would be right as well ... would have to trace a couple of connections.

It might also come from the MIDI input, but in that case the thing should already work for what you want to do with it. Also in that case a bridge rectifier would not be needed but just a single diode would do (with the elco).

Anyways .. such are my guesses .. but its hard to track the copper traces from the images .. do you have a multimeter or a continuity tester?

And also .. when my guesses would be right connecting power to the opto coupler would not power the rest of the circuit .. as the bridge rectifier would block it .. I don' t know if any part of that would be needed .. but I'd be guessing that it would not be needed.

[Edit] the above paragraph is nonsense, sorry :-)

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PHOBoS



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PostPosted: Sat Apr 21, 2018 11:52 am    Post subject: Reply with quote  Mark this post and the followings unread

Blue Hell wrote:
My guess would would be that the four diodes near the LEDs make a bridge rectifier and the electrolytic capacitor would buffer it ..

Your guess is correct. Now the question is what are the other sides of the diodes connected to. It's tricky to see with the
large switch covering a part of the PCB. As Blue Hell suggested a multimeter or a continuity tester would be useful.
I circled the pads in yellow where the (AC) voltage enters the bridge rectifier. You can probaby power it from there but
I wonder what else might be connected to it and which serial pins it actually gets the power from.


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squarewhite



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PostPosted: Sun Apr 22, 2018 3:00 am    Post subject: Reply with quote  Mark this post and the followings unread

here are some better pics.. to help see the traces.


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PHOBoS



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PostPosted: Mon Apr 23, 2018 6:49 am    Post subject: Reply with quote  Mark this post and the followings unread

Even with the new photos it's still very hard to see how everything is connected wthout having the physical board in my hands.
It just doesn't really work well from a screen. However, I am pretty sure you can power it from the pads I circled or, probably better,
power it from 2 of the pads I marked black (gnd) and red (+5V). This way the power is blocked by the diodes to whatever it originally
derives the power from. An 78L05 should be sufficient as a regulator.

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