[comp.sys.mac.hardware] Building a MIDI interface for a Mac II

totty@flute.cs.uiuc.edu (Brian Totty) (07/11/90)

	I am trying to build a MIDI interface for a Mac II.  I have a question
	about feeding an external 1 MHz clock into the SCC on the Mac II.  The
	Mac schematic seems to imply that the external clock must vary positive
	and negative with respect to the Mac's internal ground.

	(1) Is this true?
	(2) If it is true can we attach the serial port connector ground line
	    to the inverting input or to 2.5v, or is this ground line tied to
	    chassis ground?
	(3) If I really have to send in a signal that is positive and negative
	    with respect to chassis ground, what is an easy way of doing this?
	    (The oscillator has a maximum 5v swing)

	I had originally based my circuit on John Hengesbach's MAC--MIDI
	interface, but this was used for the original Macs.  His design fed
	the noninverting 0 to 5v clock in on the HSKi line and the inverting
	5v to 0v clock in as ground.

	Anyone have any insights into these areas?


   /                      Brian Totty              o o
  /__  __  o        406 E. Michigan / Apt. 6        o  
 /  / /   /             Urbana, IL 61801           \_/  "We have corn in
/__/ /   /             totty@cs.uiuc.edu                Massachusetts too!"