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From: russell@ihuxu.UUCP (Larry Russell)
Newsgroups: net.micro.cbm
Subject: Re: RS232 H/W question
Message-ID: <305@ihuxu.UUCP>
Date: Fri, 4-May-84 09:46:53 EDT
Article-I.D.: ihuxu.305
Posted: Fri May  4 09:46:53 1984
Date-Received: Sat, 5-May-84 01:28:42 EDT
Organization: AT&T Bell Labs, Naperville, IL
Lines: 27


I have just built an RS232 card myself for the C64.  To get the +12V
is easy; if you have a "Programmer's Reference Manual," look at the
schematic to see how they do it:  use a VM08 (or equivalent) full-wave
bridge (Radio Shack) across the 9VAC leads, ground the negative output
of the VM08, and put a 220 microfarad electrolytic capacitor between
+12 (the VM08 + output) and ground.
No voltage regulator is needed as the voltage is not critical and the
ripple is not bad for the amount of current you need.
To get the -12V you must AC couple to the 9VAC leads; connect the AC
inputs of another VM08 to 9VAC through 220 microfarad capacitors.
The positive terminals of both caps are connected to 9VAC; the negative
terminals to the VM08.  Put another 220 mfd capacitor across the VM08
output and this time, of course, ground the VM08 + output.

                                       GND
         +  -        ___________        |   +  -
9VAC------|(---------|~       +|--------|----|(---+
                     | VM08    |                  |
9VAC------|(---------|~       -|------------------+--> -12V
         +  -        |_________|                 

					Larry Russell
					AT&T-BL
					Naperville, IL
					ihnp4!ihuxu