Conference napalm::commusic_v1

212.0. "Op Amps in MIDI spec" by BUFALO::RAVAN () Mon Jan 13 1986 12:16

My thanks to Paul for the MIDI spec, but now that I'm trying to build a
MIDI interface from scratch, I need some hardware help.  The problem is
that the schematic is, well, loose.  If I built stuff like this everyday,
like the synth engineers probably do, I could build an interface from the
spec, but I don't, so I can't.  (help)

About the circuit diagram, the spec says:

"See Figure 1.  5ma current loop type.  Logical 0 is curent ON.  One output
shal drive one and only one input.  The receiver shall be opto-isolated and
require less than 5ma to turn on.  Sharp PC-900 and HP 6N138 optoisolators
have been found acceptable.  Other high-spped optoisolators may be satisfactory.
Rise and fall times should be less than 2 microseconds."

Figure 1 is the circuit diagram.  Note 2 on the diagram states:

"2.  Gates "A" are IC or transistor."

The problem here is simply that the two gates marked "A" have no part numbers
since they can be either ICs or transistors.  I would have liked Note 2 to call
out some suggested IC and transistor part numbers.  Given that this is a 5ma
current loop circuit, can anyone suggest appropriate parts for the amplifiers
that are (I assume) supposed to do the current sinking and sourcing?  If it
makes a difference, both 'gates-marked-"A"' are shown with their outputs
driving 220 ohm resistors. I think I would prefer ICs simply because I have
never built anything with transistors from scratch and I don't remember enough
transistor basics to trust myself. But if it's easier to use a simple
transistor circuit, enlighten me. I'm sure I'd be OK if I knew what current
loop circuits looked like in general and what their design parameters were, but
again, I don't. 

Another thing about op amps, what value feedback resistor do I need?

One last question:  How can I test my current loop interface without hooking
it to either the computer or one of my synths?

One final thing about the circuit diagram, both 'gates-marked-"A"' are shown
as inverting gates.

Thanks,
Jim

On your op amps gain is set using the input and feeback resistor.

gain= r feedback/r input

I don't have a Linear Data book handy to look up a device that could 
source or sink 5mA but I think that it will not be difficult to find 
one.

As far as testing goes, you could build a terminator that would 
reproduce the load impedance of your computer and synth and try it that 
way to see if anything burns up. However if you are reluctant to hook up 
to your PC you may have trouble driving the interface to determine the 
effect on your load.

dave

	Don't forget that if you choose an open-collector part like the LS05
you'll have to pull up the output with a pullup resistor.

CdH

	I stand corrected. It won't be pulled up to +5V due to the drop
across the diode, but it will be pulled up.

CdH

    Is there any possibility someone could send me a copy of their MIDI
    spec. ?????
                                                                  
    Thanks in advance
    
    Mike Rolla
    MLO 5-4/u28

    I thought someone else would ask for one, so I have kept
    a copy at work.  I'll put one in the mail.
    
    -jim

    
    
         Thanks alot Jim, I appreciate it.
    
    Mike