[Coco] Using Extron DVS 304 Video Scaler for CoCo3 RGB on VGA Monitor

Frank Thompson frank.thompson.iv at gmail.com
Thu May 28 17:26:03 EDT 2020

I purchased a used Extron DVS 304 video scaler to convert the 15kHz RGB
signal from the CoCo3 to a standard VGA signal.

*The Hardware:*
The DVS 304 goes for roughly 30-50 dollars on e-bay.  There is a more
modern version of the DVS 304 that has a DVI output port on the back.
Based on the older e-mail threads on this unit from the CoCoList, this one
WON'T work.  I purchased the non-DVI unit and you'll find that most of the
used ones for sale are the non-DVI unit.  The DVI port will be clearly
visible on the back of the unit in place of a DB15 (the size used for VGA

*The Cable:*
I constructed my cable by sacrificing an old VGA cable.  You will also need
a 10 conductor flat ribbon cable and the 10 pin IDC connector that connects
to the bottom of the CoCo3.  I soldered the ends of the flat ribbon cable
to the appropriate wires in the sacrificial VGA cable (using heat shrink
tube as necessary) per the following:

VGA Pin                Function                       CoCo RGB Pin
1                            Red Signal                   3
2                            Green Signal                4
3                            Blue Signal                   5
4                            Monitor ID Bit 2             N/C
5                            Ground                          2
6                            Red Signal Ground        2
7                            Green Signal Ground     2
8                            Blue Signal Ground        2
9                            +5V DC                          N/C
10                          Sync Ground                  2
11                          Monitor ID Bit 0              N/C
12                          Monitor ID Bit 1              N/C
13                          Horizontal Sync              8
14                          Vertical Sync                  9
15                          Monitor ID Bit 3              N/C
shield                    RF Shield                        1

So lots of circuits from for the VGA standard need to share the two grounds
on the CoCo3.  I lumped all the circuit grounds to one pin, and the
shielding to the other.  If anyone has better advice for this, please chime
in as I don't know if this is the best way to do this.  It does give me a
great picture, but perhaps has something to do with some of the fiddling
required later (does combining the sync grounds and R/G/B grounds create

*Plugging things in:*
Connect the CoCo3 RGB/VGA cable hybrid to the back of the unit (to the top
DB15 port)
Connect your monitor to the bottom DB15 port
Connect the power cable to the Extron to power up (no power switch on this)

*Configuring the Extron:*
My experience is that you need to be running the default factory firmware
on this unit.  All attempts as flashing any updated firmware in the unit
resulted in a garbled unviewable image for me.  Extron support was awesome
through this process and sent me a few different versions of firmware to
try out.  The one thing you can always do with this unit is to perform a
reset that defaults the unit back to the original as-shipped firmware
(version 1.27 in my case).

Bottom line, I was only able to get this to work with firmware version 1.27
(the factory reset default version).

Upon initial powerup, I received a garbled unreadable screen.  Here is the
sequence I used after lots of trying everything to get it to work:

1.  Press MENU button twice brings up INPUT CONFIG
2.  Press NEXT button twice to select INPUT 4
3.  Rotate the ADJUST knob on the front of the unit CCW to select YUVp.

At this point you should have a crisp readable black and white image on the

4.  Rotate the ADJUST knob CW to select RGB SCALED


Something about selecting that other video signal snapped my DVS into shape
and allowed it to then display the RGB signal.  Note the original config
for input 4 upon my original powerup was SCALED RGB, but it for whatever
reason couldn't "lock on" to the signal (used in the loose sense of the
term).  But after selecting YUVp, it then was able to interpret the signal
and is good to go.

The Extron user manual has lots more settings for the output signal.  I'm
running 1024x768 @ 60 Hz, but you can select down to 640x480 and up 1080p
(which isn't possible on my old VGA monitor) resolutions and select
different refresh rates.

Note:  If your unit has a newer firmware version, you can perform the
factory "Mode 1" reset (hold reset pin switch on back of unit while
plugging in) to get back to firmware 1.27.  However, on the next power
cycle, the unit reverts back to whatever had been previously flashed into
it.  Extron was able to send me the original 1.27 firmware version, and now
on power cycle, it boots just to 1.27, which is what I needed.

Extron support is awesome even for this long discontinued product.  They
can provide user manual and firmware updates directly.  If appropriate, I
can try uploading to the CoCo Archive (or send them to somebody who can) if
there is interest/need.

Thanks all!  Sorry for the horribly long message.  I hope this helps anyone
who tries to use this same hardware.  I think given the cost (especially if
you can score one for even cheaper on e-bay) it is worth it.

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