I've seen a driver being used in place of an inverter after coming out of the optocoupler separating the MIDI In from MIDI thru. I was wondering what are the benefits of each approach and why one might use one or the other.
Thanks.
Non-inverting buffers were traditionally used when TTL gates were available. These have an NPN pulldown output driver, but a weaker pullup. MIDI current loops have a fairly large current requirement (typically 10-20 mA). For the best speed and current drive capability when using open collector (or open-drain) or TTL outputs, circuit A (see drawing) is the best choice.
If you have an inverting driver with a strong pull-UP capability available, you can use a circuit as shown in figure B.
(http://www.electrongate.com/images/midithru.jpg)
Hi! Looks like I'm bumping an old topic. I couldn't see figure A or B. I was wondering if you could recommend an IC/circuit to use for the midi thru connection.
I see many designs that use any spare logic element that happens to be available. Despite not really having the output current drive, CMOS 4000 and 74HC series work ok. 74LS are better (more robust).
If I was free to pick a chip just for it, I'd probably use 74LS14, 2 inverters in series to drive pin5 (non-inverting). 74LS14 are Schmitt trigger inverters which will sharpen up the switching speed of the data.
In the past I've used mulitple inverters with success: 74HCT14, 74LS04, 74LS14..
And NAND ICs as well as 74LS00