MIDI circuit questions

[burningman]

I am housing two circuits under one enclosure and I am trying to clarify ideas on feeding the Rx of both circuits with a MIDI signal.

1. Is it okay and reliable to passively split a MIDI signal?


2. Are the resistor values I have defined in my 6n138 and 6n137 MIDI input circuits good?


3. When it comes to the buffer stage of the thru circuit I was wondering what is the best practice for splitting.
In some instances I have seen the split happen before the first, inverting I/O stage. In other instances I have seen the split occur after the first, I/O stage.


4. IC choice
I have seen various 74x series ICs used in MIDI thru circuits (ie. 74LS, 74HCT, 74AC). Are there compatibility considerations when choosing the  optocoupler and buffer IC for a MIDI thru circuit?

5. Are the connnections/resistors circled in red always compulsory, given that both circuits are in close proximity to the inverter circuit?
I am not clear on their purpose.


Thanks

[slacker]

1. Is it okay and reliable to passively split a MIDI signal?

I think that should work with the 6n138, according to the datasheet that only needs about 1.6ma and MIDI can provide about 10ma so two in parallel should be OK. The recommended current for the 6n137 is at least 7ma so two in parallel exceeds the 10ma.

2. Are the resistor values I have defined in my 6n138 and 6n137 MIDI input circuits good?

They will work fine. You can make R2 bigger on the 6n138 circuit 10k will work just as well, you probably don't need R3.

3. When it comes to the buffer stage of the thru circuit I was wondering what is the best practice for splitting.
In some instances I have seen the split happen before the first, inverting I/O stage. In other instances I have seen the split occur after the first, I/O stage.

There's no particular advantage to either method, except the top one saves you an inverter, this might be a help if you need the extra one for something else.

4. IC choice
I have seen various 74x series ICs used in MIDI thru circuits (ie. 74LS, 74HCT, 74AC). Are there compatibility considerations when choosing the  optocoupler and buffer IC for a MIDI thru circuit?

Any of them will work just as well.

5. Are the connnections/resistors circled in red always compulsory, given that both circuits are in close proximity to the inverter circuit?
I am not clear on their purpose.

Those resistors limit the current the MIDI out of a device can provide and prevent damage in case someone shorts pins 4 and 5 of the MIDI connector together or to ground. If that part of the circuit is hardwired inside your pedal then you don't need them, you just need to make sure R1 and R8 limit the current to less than 20ma so you don't damage the 6n138s.
I'm not sure what that schematic is showing though you might not need all that stuff. The purpose of the opto isolators is to isolate the power supplies of different devices so that you don't get ground loops and other problems, if all that stuff is in one enclosure and both circuits are powered from the same supply then you don't need the second set of isolators and can probably connect the output of the first inverter to both RXs.

[burningman]

Thanks for the response Ian.

Both circuits source the same 5V supply/ground. Is this recommended, or should each have its own regulator, splitting the 9VDC at the power jack?

Regarding the MIDI signal split. Would splitting the signal passively after the optocoupler be advisable? This would place each circuit's Rx in parallel. Can I make this work without having to buffer to both Rx's?

[Digital Larry]

I don't see any reason why you could drive two micros from one optoisolator.  They all want to be part of the same circuit though right?  Part of the rationale for optoisolators is to eliminate ground loops.

[anotherjim]

+1^
No problem driving 2 Rx inputs.

[burningman]

Each circuit functions independently. They both source the same 5VDC from a single regulator and share the same ground with only one connection to chassis.
Should there really be two regulators and two 9Vs inputs powering this, or can the two independent circuits share the same supply without issue?
Thanks.

[Digital Larry]

In my excitement to talk about MIDI (which consumed my mind 30 years ago) I left out the word "NOT".  I don't see why they could NOT be driven by one optoisolator.  If you wanted two, and everything's on the same power supply then I don't see that you need two regulators.  But I'd back up and stick with "you don't need two optoisolators".

[burningman]

Thanks Larry