Genuine THRU circuit needed for this?

[Arjan]

Hi,

This is not directly related to MIDIbox as such but I'm pretty sure this would be the best place to ask the following question.

I have a Roland AX Synth 'keytar' and am thinking of building my CME Widi-X8 wireless MIDI interface into it so I don't have to mess around with a separate wireless interface. Because I will be unable to access the Widi-X8 unit once it's built in I'd like to keep the regular MIDI OUT of the AX Synth fully functional in case of problems with the wireless connection.

In order to do this, do I really need to include an actual MIDI THRU circuit to split the original MIDI OUT signal from the AX to send it both to the original MIDI OUT and to the built-in Widi-X8? I was thinking that perhaps I could simply hardwire the MIDI OUT of the AX from to the MIDI IN of the WIDI-X8 and call it a day. It's going to be just one foot of cable with soldered connections.

The MIDI OUT socket of the AX Synth is probably soldered to the main PCB so it may actually be a bit difficult to split the signal before it gets to the MIDI OUT socket. One consideration is that if I do at some point need to use the original MIDI OUT (for example because the wireless unit is getting or causing interference) I'll be using with a fairly long MIDI cable (15 foot at least, possibly longer). This needs to function reliably whie the Widi-X8 is still connected (it's hardwired after all).

If a direct connection is not a good idea is there anything else I could do to reduce the 'load' that the WIDI-X8 will present to the MIDI OUT from the AX Synth without having to _insert_ a 2x THRU between the PCB MIDI OUT and the actual MIDI OUT socket?

Thank you for your opinions and suggestions.

[Wilba]

Perhaps the easiest solution is to add a switch between the AX's MIDI Out pins and the MIDI cable to the Widi-X8.

As I see it, your use cases are mutually exclusive, you never want to use both MIDI Out socket AND Widi-X8 at the same time. So use a switch with a double pole. Connect the two switch "pole" pins to the AX's MIDI Out socket pins (easy to do, just solder to the PCB pads!), connect the two switch "throw" pins to the cable/MIDI plug going to the Widi-X8. Thus the switch will enable/disable the connection to the Widi-X8, the AX's MIDI Out socket will always be wired, but without a cable connected, it can't affect the Widi-8, since MIDI cables work by a current loop.

[Arjan]

Perhaps the easiest solution is to add a switch between the AX's MIDI Out pins and the MIDI cable to the Widi-X8.

Well, the thing is, I don't really want to drill holes in and install additional switches onto my AX Synth 'Black Sparkle Premium Edition' :geek:

As I see it, your use cases are mutually exclusive, you never want to use both MIDI Out socket AND Widi-X8 at the same time.

True, but since I don't really want to install an additional switch I'm really looking for a way to drive both the original MIDI OUT and the Widi-X8 at the same time all the time even if I use only one of them.

There is an ATT switch on the AX which can be used to select either normal or reduced output level at the LINE output, I think it's double pole but I'm not sure. I could use that perhaps but again this switched is mounted/soldered onto a small PCB which also has the actual LINE OUT sockets on it so I'd have to cut some traces from the PCB to use it for this purpose.

Thanks for your suggestion but I'm still wondering if I might be ok if I simply split the original MIDI OUT. In theory it should be possible but I don't know what would happen in practice, especially if a long MIDI cable is connected. I know it will affect the MIDI signal somewhat, but how much and will it cause a problem?

[Wilba]

Since MIDI works via a current loop, splitting the MIDI Out at the SOCKET probably won't work very well, both receivers would be sharing the current, and the optocouplers would get half the current or one would rob the other or some other weirdness... can't be sure exactly.

It would be best to add a buffer (logic gate), fed from the point driving the MIDI Out... in a MIDIbox "Core" this would be the Tx pin of the PIC, between the 220 ohm resistor and the MIDI Out pin. Then the output of the buffer goes to 220 ohm resistor to the replicated MIDI Out socket. Remember that as the signal is high and low, it is sinking current from the other MIDI Out pin (connected to 5V via 220R), out the cable, through the receiving end, returning back in along the cable to the MIDI Out pin. So you replicate this bit too around the buffer.

I have actually tried this with just a BC557 instead of a buffer (made a MIDI Thru by connecting BC557 to Rx pin of PIC).

So unless you are planning to switch between the two, you do need some kind of buffer... or at least tap directly to the Tx pin or whatever is driving the MIDI and replicate the whole 5V->220R->socket->220R->Tx pin arrangement.

[Arjan]

It would be best to add a buffer (logic gate), fed from the point driving the MIDI Out...

I wonder if I could go from that point (before the actual MIDI output driver circuitry) directly to the corresponding input point on the Widi-X8, bypassing the opto-isolator. Both circuits are going to be inside the AX with just a short bit of wire between them and will be powered by the same set of batteries.

[nILS]

I haven't read the whole thread, but anyways: MIDI splitting is fairly easy (see gm5x5x5) or http://m.bareille.free.fr/midithrubox/midithrubox.gif

[julienvoirin]

if i were you i would :

1 - test Y split cable

2 - take wilba's solution

3 - take nILS's solution

2 & 3 require power (5V)

just to remember : MIDI is ok with 7m cable. i tested with 10m cable and it works in low electrical noise environnement

PS : look at x0b0x schematic, there is a very simple midi thru using some resistors and one transistor (similar to what wilba describes)

Edited May 14, 2011 by julienvoirin

[nILS]

Y-cable bad.

[Arjan]

Thanks for your collective advice. I think I will take Wilba's advice and duplicate the MIDI OUT driver circuitry although I might try to make a direct connection between the point just before that part of the circuit and the point in the Widi-X8 just after the opto-isolator (straight into the UART). Of course I'd have to disconnect the opto-isolator from the UART input in that case.

I actually managed to get a copy of the the schematics of the actual AX Synth MIDI circuits:

There seems to be a bit of extra stuff in there before the actual output circuit as I know it from the MIDI spec. Could it be that Q27 and Q29 are used to reduce power consumption when the AX goes into power saving mode? It does that automatically after x minutes to save batteries.

Anyway, would it be possible to take the signal from IC14E and put that directly into the Widi-X8 UART? I think IC14E uses 3.3V logic instead of 5V so I'll have to check what the Widi-X8 input (post opto-isolator) can accept.

[Arjan]

Perhaps the easiest solution is to add a switch between the AX's MIDI Out pins and the MIDI cable to the Widi-X8.

I just had an epihany. Instead of drilling holes and installing an actual switch, I could use the unused pins 1 and 3 of the AX Synth's MIDI OUT along with a DIN plug acting as a shunt to create a switch:

The beauty of this arrangement is that since the MIDI plug inserted into the AX Synth's MIDI OUT is the actual switch there is no possibility of the switch being in the wrong position: as soon as you remove the shunting plug from the MIDI OUT it becomes a fully functional MIDI OUT port again.

I could use the same arrangement for the MIDI IN, but I'm wondering if I should use this method to control the power to the WIDI-X8 instead so I can turn it off in case it is causing interference. That leads me to the following question:

Would it be possible to only disconnect one of the MIDI connections creating the current loop? Are there any downsides to this?

In that case I could use, say pin 1 of both MIDI ports to switch both the MIDI IN and MIDI OUT connection to the Widi-X8 and I could use pin 3 of both MIDI ports together to control the power to the Widi-X8: connect the +5V from the AX Synth to pin 3 of the OUT port, create a shunt device consisting of 2 MIDI plugs with their pins 3 connected and then go from pin 3 of the IN port to the 5V input terminal of the Widi-X8.

Edited May 22, 2011 by Arjan

[Arjan]

I just had an epihany. Instead of drilling holes and installing an actual switch, I could use the unused pins 1 and 3 of the AX Synth's MIDI OUT along with a DIN plug acting as a shunt to create a switch

Just in case someone Googles this thread, my idea above doesn't work because in their infinite wisdom Roland have decided and managed to source a MIDI chassis part which does have 5 holes but only 3 of them have any metal in them. I didn't know such a thing even existed. The manufacturer's part number is YKF1-5054. Damn you Roland :frantics: