[sdiy] MIDI specifications round 2

[Gene Stopp]

Yup yup yup let the serial controller take care of it all.

Keep in mind that a logic signal propigating through an inverter or two will
add delays in the sub-microsecond range, which will never make the slightest
shred of timing difference in the macro-world of MIDI-controlled musical
events. The two inverters in a row are there because we want the inverter's
output structure to drive the line (which is a well-defined output rather
than the unknown electrical characteristics of UART brand X) and to preserve
the polarity of the logic we need two inverters to keep the signal
right-side-up. And often times you will see multiple inverters in a row just
to use up the extra inverters cuz you get six in a package!

Regarding Current Loop transmission, basically what this means is that the
transmitting instrument is sending data to the receiving instrument by
blinking a light at it (the LED inside the optocoupler on the receiving
end). This provides electrical isolation, which is a very safe way to send
low-voltage data between machines that may be plugged into different power
strips running off of different breakers at either end of the stage a
75-year-old theater in downtown Podunk. The sender therefore offers two
wires to the LED at the far end of the cable - the current source and the
current return path. The third pin is to be connected at one end only (the
sending end? I forget) and provides a presumably earthed protection against
excessive EMI.


- Gene


-----Original Message-----
From: Blitzcraig [mailto:blitz at nmt.edu]
Sent: Thursday, April 17, 2003 12:13 PM
To: synth-diy at dropmix.xs4all.nl
Subject: [sdiy] MIDI specifications round 2



A lot of helpful info guys, thanks.

I'm not familiar with UART's, but I'm assuming they're IC's that are used
for Asynchronous transfer's, add start/stop bit, etc.
Looking over my microcontroller a little more, I have notice that it has a
SCI built in.  All I would have to do is set the baud and write to the
register and it should take care of the start/stop bit for me.  I think I
should be able to handle the serial stuff from here.

I'm still a bit confused about the pin's, I'm not sure what the current
loop is/does.  From these schematic/diagrams shown, it looks like my data
should go through two inverters through a 220ohm resistor?  I'm not sure
why you would do that other than to delay the data, why?  ok, and the
other pin is tied high through a 220ohm resistor... this is the twisted
pair that's mentioned right?  The middle pin ties to what, I think this is
confusing because of my lack of current loop knowledge.

thanks for all the help,

-Joe


www.blitzcraig.com