[Coco] Coco Monitor Question

mike delyea mdelyea at gmail.com
Fri Sep 7 03:39:41 EDT 2012

Here is an edited version of the original text.

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I picked up an old Amiga 1080 monitor for next to nothing and wanted to use
it for my coco because I had heard it was a good display for the 80 column
mode.  The first difficulty I ran into interfacing the 1080 and the coco
was the lack of an available pinout for the 1080’s RGB connector.  Further
research eventually yielded the information that the 1084 pinout was the
same as the 1080.  After finding the correct pinout (at pinouts.ru) and
making a couple false starts I eventually found the correct information I
needed to make this combination work.  One of the mistakes I made was
getting the HSYNC (horizontal sync) and VSYNC (vertical sync) signals going
to the monitor correctly.  At first I assumed (for various reasons) that I
needed to invert the HSYNC and VSYNC so I made up a little circuit using a
74ls04 hex inverter chip.  This didn’t work and was completely off the mark
as I later found out (these chips are maybe 50 cents each so it wasn’t a
big deal moneywise).  Then as I was glancing through an old Rainbow I came
across the information I needed in Marty Goodman’s column.  The chip I
actually needed was a 74ls02 NOR gate to combine the HSYNC and VSYNC into
one CSYNC (composite sync).  So I made up another little circuit using a
74ls02 and 14 pin socket, an LED, a switch and an external 5 volt power
supply.  I had originally intended to grab 5 volts from the coco internally
but I just wanted to get this thing working and it was easier to use the
external power supply.  If you want to try this yourself be warned – this
only works if you have a 1080 (or 1084) with the 9-pin D-type RGB
connector, not the round 6-pin DIN type connector.  If your monitor has the
DIN connector you lucked out - you won’t have to add any circuitry at all.

Parts list:

14 pin 74ls02 NOR gate (dual input)
14 pin socket
Switch (I used a double pole single throw slider I had laying around)
5 volt power supply (I used 5volts 200 mA)
1k ohm resistor (I didn’t need it and you won’t either if you get the right
A small project board to attach everything to

Note: For your power supply you need 5 volts and keep the milliamps below
400.  Also, you might have to test the lines for positive and negative if
you bought the power supply at a surplus store (like I did).  If you don’t
know which line is positive and which is negative it won’t work and you
might let the magic smoke out.  You can use a mulitmeter to find out which
is which.

The Cable

For the actual cable I cobbled together a 10 pin header with ribbon cable
attached (from an old motherboard serial port connector), a 9 pin D type
connector with ribbon cable attached (again from another old serial port),
and a length of Ethernet cable.  Special care must be taken when connecting
the D end.  The wires are not sequential with the pins!  Pin 1 is wire 1,
pin 2 is wire 3, pin 3 is wire 5, pin 4 is wire 7 and so on.

The Pinouts

Coco (looking at the bottom with the back of the coco facing away from you)

2   4   6   8   10
1   3   5   7   9

1    ground
2    ground
3    Red
4    Green
5    Blue
6    No pin
7    Sound
8    HSYNC
9    VSYNC
10    Not used

Amiga 1080 RGB D-type (looking at the back of the monitor)

1   2   3   4   5
  6   7   8   9

1    ground
2    ground
3    Red
4    Green
5    Blue
6    Not used
7    CSYNC
8    Not used
9    Not used

Note:  Pins 6, 8 and 9 are used with other computers, just not the coco.

Cable Assembly:

I started at the coco end, attaching pins 1, 2, 3, 4, 5, 8 and 9 to the
Ethernet cable.  The Ethernet cable is conveniently colour-coded so make a
note of what wire goes to what colour (eg. pin 1 to blue, pin 2 to
blue/white etc.).  At the Amiga end I attached the corresponding wires to
pins 1, 2, 3, 4 and 5 (making sure I followed the wiring scheme I noted
earlier wherein pin 1 is wire 1 and pin 2 is wire 3 and so on).  When
you’ve finished this, check the connections for continuity using a
Connect +5 to the switch and run it to the LED.  Make sure you have the LED
oriented correctly or it won’t work – usually the long wire is positive.
Also, unless you have an LED with a built in resistor (mine does) you
should put a resistor in front of the LED (a 1k ohm resistor will do in a
pinch).  Connect the negative end of the LED to pin 14 of the 74ls02 and
attach pin 7 to the negative side of the power supply.  Attach wire 8 from
the coco to pin 8 of the 74ls02 and wire 9 from the coco to pin 9 (easy
eh!).  Now attach the wire from pin 7 of the Amiga’s RGB to pin 10 of the
74ls02.  When you turn the switch on the LED should light.  If it doesn’t
light up then check your connections (and didn’t I warn you about the

My circuit board looks a bit messy right now so I’m going to redo it and
pack it into a little project box I picked up for $2.00.  I think I’ll even
add a jack for the power supply.
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