🚀 go-pugleaf

A few people have asked how the line level converter circuit works on
the Mini Board.  I thought this might be important to discuss, since
the circuit is a bit obscure and may be incompatible with a few
existing computers.

Serial line voltages are normally +12 volts for logic low and -12
volts for logic high.  This is annoying because TTL/CMOS logic levels
are of course +5 volts for high and 0 volts for low.

In the past, people normally used a bipolar power supply to generate
the serial line levels; nowadays most people would use a single chip
solution like the MAX232 series of chips (which include an on-chip
charge pump and voltage inverter to generate +/- 10 volts from a 5
volt supply).  However, in a board as small as the Mini Board, even
one extra chip is one chip too many.

So, the Mini Board uses the following hack.

For its serial line logic high, the Mini Board "loops back" the logic
high (remember, this is a -12 volts) that the computer it is connected
to generates!  This is using a 1K resistor, which shouldn't cause much
of a voltage drop.

A PNP transistor pulls this line (Mini Board serial out) to +5 volts
to generate the serial line logic low.


     --->>--MB serial in--->>--+   +5V
	        	       |    |
HOST            	       /     \
                	   1K  \      >|
COMPUTER        	       /       |-----/\/\----<<-- Mini Board
	        	       \      /|		  TTL transmit
	        	       |     /
     ---<<--MB serial out--<<--+-----


For input, the Mini Board uses a spare CMOS inverter coupled with two
resistors.

The upshot of this solution is (1) the Mini Board can't transmit while
the host computer is transmitting, because the incoming -12v it needs
would be modulated by the host transmission; and (2) there might be
some computers that don't accept +5v as serial line low.

Regarding compatibility: I have personally tested the Mini Board on a
variety of IBM and clone PCs, a variety of Macintosh models,
DECstation 5000 computer.  I know that there is at least one type of
Sun SPARC that it works on.

So it does work on most machines.  Still, there is bound to be the
model that it won't work on.  A reader believes that it probably won't
work on Sun 3 workstations.  For these cases, it wouldn't be hard to
jumper the board and wire a voltage higher than +5 to the collector of
the transistor.  This should allow the board to work with these other
machines.

	- Fred


Acknowledgment: this circuit was the brainchild of Randy Sargent.