MIDI out / thru circuit questions

Question

I met some questions in my circuit design, I'd like to help from somewhere, the situation is following, 
 I made a MIDI in/out circuit, which works, however, I see I have some changes compared to some other circuits I see.

In the official documentation Midi Org I see two resistors on the MIDI out circuit, both 220 ohm. I'm sure I only use one (but I'm not at home, so I cannot check which one I might miss). But so far I didn't get any problems (but never tested in a 'noisy' environment.

In the same scheme, there are 2 inverters (I think), the triangle, one without an A and one with. I also don't use that. I think it's for keeping a steady 0 or 5 V signal, so I think it's good to add. Now I see in some other circuits (randomly from google search with pics), a 74LS04 , or 74LS14 or others. At home I have some 74LS14's (Schmitt Trigger hex inverter), but I also read those are for processing signals (like MIDI I guess), but can be slow. So I am not sure what to use, 7404 or 7414? I see also a lot of schemes with 7414's so I guess these are ok.

For the thru I see the same ... does one triangle without and one with A means, I need 2 inverters after each other? Probably it is. Since I want to use 3 MIDI In/Out/Thru's, would that mean 2 (per out or thru) * 2 (out + thru) * 3 (# 3 in/out/thrus) = 12 hex inverters (meaning 2 ICs?); see also item 6

In this circuit, DSS306 are used. I cannot find them on aliexpress, but also they are not in the official MIDI org circuit. Are they needed ... or better (and why)?

In this circuit, transistors are used (2N2222) ... yet another solution (?) What is the advantage? (btw, I plan using BC337's since I have those and are similar, at least if there is a clear advantage).

Also it mentions about the hex inverter, which I probably do not need for the MIDI Out (since it comes from the Arduino, not sure if the Arduino handles this), but for the MIDI Thru I want to connect it directly from the MIDI In. Is that assumption correct? (better use it for both MIDI Out and Thru?)

Thanks if you can help me with one or more of the above questions. 
 Thanks all guys.

Mike A · Accepted Answer

Some comments 
 
1. The pair of resistors are really to protect your circuit. If you've left out the one connected to pin4 then a short of pin 4 to ground would be shorting out your power supply. If the other one is ommited then someone puting 5v on pin 5 would blow your driver output. 
 
2. The difference between a 7404 and and 7414 is the second one is a schmitt trigger, which means it switches from on to off and a different voltage to switching from off to one, so that means if there is noise on the line its going to ignore it as once it switched its on (actually off because its an inverter) and the noise would have to be very large to make the input go low enough to switch if back the other way. So you should be using 7414 not 04 in the input circuits. (you dont need to worry about speed their propagation delay is 14 nano seconds which would limit your speed to 70MHz but midi data rate is something like 32KHz so these are 2,000 times faster than you need) 
 
3. The one with the A on it should be an open collector one like 7417, that only has on transitor in its output that pulls the output down to ground, normally 74xx have a transistor pulling the output up for 1 and down for 0 but here you are trying to act like a switch so you shouldn't really be trying to output 5 volts on that pin, so you dont need the pull up transistor. (You could use 7405 which is the open collector version of the 7404 but 7417 would be better as the output is protected up to 15 V so if something bad comes down your cable its less likely to blow the chip) 
 
You need 2 inverters in each path as you need the signal to not be inverted, an inverter changes a 1 coming in to a 0 going out and visa versa, so to keep the signal the same way up you need 2 inverters The ones without the A should be 7414 as its taking the noisy input signal and cleaning it and the one with the A 7405 as its the open collector for the output. 
 
4. Those are radio interference suppressors, they wont make any difference, other than stopping the midi cable perhaps causing interference with someone listening on AM radio next to it. But in reality there is going to be a lot more other radio and sound noise around when you are using this I would imagine anyway. Its just regulatory for commercially produced equipment. 
 
5. That transistor circuit is pretty much what the output circuit of a 7417 looks like. 
 
6. You will want the inverter in the output, otherwise you're going to have to reverse signals between input and output in the Audrino and depending on how much control you have over (I assume a UART in the Audrino) that may be tricky.