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MIDI thru advice 74 logic
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author MIDI thru advice 74 logic
wahee
hi all

I was wondering if it's possible to drive 2 MIDI thru sockets from a 74HC14 hex inverter

I already have a working circuit driving 5 MIDI thru's from a 6n137 opto

I was hoping to add a further 5 thrus but didn't want to use another 74 IC for the extra 5, so was hoping to send each of the existing 5 remaining 74 logic outputs to 2 sockets each

is this a goer? or simply bad practice

many thanks
guest
im assuming youre running it off 5V and following the suggested implementation of 2 x 220ohm resistors. if so, then for normal operation you should be fine. thats 7mA current draw per midi device, so 14mA per buffer, and 70mA per chip. the HC14 is limited to 25mA per pin, and 50mA per device, but the average current will be less than the max number, as that current is only drawn when 0 is transferred. if someone plugs in a bunch of cables that were miswired, then current draw could be much higher and destroy the device, but that could happen anyway with a single output per stage. if you are making something for a production run, spring for the extra hc14. if youre making it for yourself, it will most likely be fine, and the worst case is you have to swap out that hc14 at some point in the future.
wahee
hi thanks for the reply. Yes 5v clean DC supply. 220ohm resistors fitted. I'm going to be short on PCB board space so was hoping to save a bit. I could use smaller SMD devices but prefer through hole at this stage.
Graham Hinton
wahee wrote:

I already have a working circuit driving 5 MIDI thru's from a 6n137 opto

I was hoping to add a further 5 thrus but didn't want to use another 74 IC for the extra 5, so was hoping to send each of the existing 5 remaining 74 logic outputs to 2 sockets each

is this a goer? or simply bad practice


The 7414 inverts so you need two in series to get back in phase with the 6N137 output. MIDI is a current loop so you don't drive it high for off, it is simpler to use a 7407 or 7417 open collector non-inverting buffer.

http://hinton-instruments.co.uk/reference/midi/promidi/index.htm

Quote:
then for normal operation you should be fine. thats 7mA current draw per midi device


No, MIDI is a 5mA current loop. You forgot to allow for the forward voltage across the LED in the MIDI In optocoupler.
wahee
Many thanks for the reply Graham and that's a superb database of MIDI knowledge you have there on your site.

I always wondered why an inverting HEX was used rather than a non inverting.
What I had done was used one of the 7414 inputs to invert then fed the output from this into the remaining 5 so that them would output corrected. There seems to be a number of other similar circuits on the net, so I just did it that way.

I assumed rightly or wrongly that the logic was there to correct for slow rise or fall times in the incoming signal and perform a sort of buffering to ensure high quality output. I was wondering if it as even strictly necessary for say a small 1 into 3 thru device as I remember seeing some examples that omitted this stage. But like with all things it's better to do things right
Graham Hinton
wahee wrote:

I always wondered why an inverting HEX was used rather than a non inverting.


I'm afraid that examples of good digital logic design are hard to find, but examples of "Mickey Mouse Logic" (MML) proliferate, especially in magazines.

Quote:

I assumed rightly or wrongly that the logic was there to correct for slow rise or fall times in the incoming signal and perform a sort of buffering to ensure high quality output.


When MIDI was first introduced optocouplers were divided between slow and cheap and fast and expensive (a 6N137 cost about £5 in 1984 money). So most MIDI products used the cheap ones and suffered Thru pulse width distortion. If you use a slow opto transistor then following it with a Schmitt trigger is a good idea. The 6N137 is a fast opto gate and does not need that, if you use the recommended 1k2 pullup and closely decouple the power the risetime will be fast enough to drive logic directly.
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