This is only a preview of the February 2015 issue of Silicon Chip. You can view 36 of the 104 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Articles in this series:
Items relevant to "6-Digit Retro Nixie Clock Mk.2, Pt.1":
Items relevant to "What’s In A Spark? – Measuring The Energy":
Items relevant to "Spark Energy Meter For Ignition Checks, Pt.1":
Purchase a printed copy of this issue for $10.00. |
CGA-to-VGA
Video Converter
. . . for legacy computer systems
Do you have an old Amiga, Commodore 128, Microbee, Apple
or Tandy CoCo 3 computer that you would like to fire up again?
Sure, it will be a nostalgia trip but you may not have a suitable
CGA monitor – they were obsolete years ago! This CGA-to-VGA
Video Converter from Microbee Technology will allow you to use
any recent model LCD or CRT monitor that has a VGA input.
T
INKERING WITH old computers,
otherwise known as “retro-computing”, has become quite popular
over the last few years for various
reasons. Firing up these old machines
allows you to visit a time when home
84 Silicon Chip
computers first became affordable and
widely available, back in the late 1970s
and early 1980s.
For some, this means revisiting the
start of their career in electronics and
computing. For others, it was the start
of a love of gaming. For other groups,
it was a chance to tinker with the
hardware of these machines – getting
the soldering iron out and adding
memory chips, interfacing to external
hardware, controlling relays, reading
siliconchip.com.au
By Ewan Wordsworth
Director, Microbee Technology Pty Ltd
Left: the CGA-To-VGA Video Converter
is built into a standard ABS case and
is based on a commercial video scaler
board (designated the GBS-8200). It’s
interfaced to the computer via an RGB
Intensity Board (at the lefthand end)
which you assemble yourself.
Right: the unit works with virtually
any PC that has a CGA video output,
including the Apple IIGS as shown
here.
analog signals, decoding and listening in on radio teletype and weather
facsimile transmissions and so on.
The list of hardware projects was
endless. But now, if you want to fire
up one of these old machines and
obtain a full colour display, it isn’t so
easy unless you have a working colour
monitor for your old machine stashed
away in a cupboard somewhere.
Just plugging in to a VGA monitor
will not work or it may only “half
work”. The complete solution is the
Microbee CGA-to-VGA Video Converter. This kit is based around a common
commercial video scaler board, the
GBS-8200 v4 which is readily available
via the internet. It is widely used to
convert arcade machines to use VGA
CRT or LCD monitors.
GBS-8200 drawbacks
The GBS-8200 scaler board takes
analog RGB signals with scan rates
of 15kHz (CGA) or 21kHz (EGA) and
scales the video to suit a VGA monitor with a scanning rate of 31kHz. But
siliconchip.com.au
while the GBS-8200 board is good
on its own, it does have a number of
drawbacks.
Firstly, the scaler board requires
“clean” horizontal and vertical sync
signals. If these are not clean, there is
likely to be display jumping and poor
picture sharpness. Also, the analog
input to the GBS-8200 board does not
cater for a true CGA colour output.
The CGA interface standard provides
digital (TTL level) RGB signals, plus
an INTENSITY signal, giving eight
colours with two levels of brightness;
ie, 16 colours in total.
To fully implement the CGA colour
set, the INTENSITY level needs to be
used to scale the RGB signals to create
an analog output. Once these items are
taken care of, the rest of the work is
done by the GBS-8200 scaler board.
In this case, the drawbacks are over-
come by adding a custom input board
from Microbee – the RGB + Intensity
-to-Analog Adapter, to give it its full
description. From here on, we will
refer to it as the RGB Intensity Board.
While this project is designed primarily for use with Microbee Premium
series computers, it can also be used
with a number of other older computers including the IBM PC (and its
numerous clones), Apple IIGS, Commodore 128, Amiga and Tandy CoCo
3. Other computers that have a 15kHz
scan rate and either analog or digital
RGB output signals should work with
this circuit as well.
Circuit details
Now refer to Fig.1 which shows
the circuit details of the Microbee
RGB Intensity Board. It employs two
MAX4619 analog multiplexers (IC1 &
February 2015 85
Parts List
Short-Form Kit
1 Microbee double-sided PCB
with plated through holes,
Part No. 21-01101-01
1 SPST 90° PCB-mount mini
toggle switch
4 SMD 1206 inductors, 600Ω <at>
100MHz (L1-L4)
3 2-way pin headers, 0.1-inch
pitch (JP1,JP2,JP3)
1 PCB-mount 90° female DB9
socket
1 24-pin DIL socket
1 M3 x 6mm screw & nut
Semiconductors
2 MAX4619 CMOS analog
switches (IC1 & IC2)
1 PAL22V10 PAL IC, programmed
by Microbee (IC3)
1 74HC14 hex Schmitt trigger
inverter (IC4)
1 7805 3-terminal regulator
(REG1)
1 1N4004 silicon diode (D1)
Capacitors
2 100µF 16V electrolytic
5 100nF MMC
6 33pF MMC
Resistors (0.25W, 5%)
3 4.7kΩ
3 330Ω
3 680Ω
1 270Ω
2 470Ω
1 82Ω
Full Kit
1 short form kit (as listed above)
1 modified GBS-8200 video scaler
board & cables
1 drilled and routed ABS case,
200 x 120 x 40mm
1 set of mounting hardware &
rubber feet
1 2m-long DB9/M to DB9/M cable
Power Supply (not supplied): 7.512V DC <at> 1A or 5V DC regulated
<at> 1A (see text)
Where To Buy The Kits
Both the short-form kit and full
kit are available from Microbee
Technology Pty Ltd – see www.
microbeetechnology.com.au for
the details.
IC2), together with a PAL (Programmable Array Logic) device (IC3). IC3
contains the logic that provides the
86 Silicon Chip
Top & above: the CGA-To-VGA Video Converter also works with old Microbee &
Tandy CoCo 3 computers, as well as the IBM PC, Amiga & Commodore 128.
digital-to-analog conversion with
the correct colour map for the CGA
standard.
Switch S1 selects between the
analog and digital RGB modes by
switching the two analog multiplexers to either pass through the analog
signal or divert the digital RGB signals
through IC3. The PAL (IC3) then produces two red (R1 & R2), two green (G1
& G2) and two blue (B1 & B2) outputs
at pins 18-23.
These pairs of outputs are then
summed via 330Ω and 680Ω resistors
to give the correct analog voltages. The
load that the GBS-8200 board presents
for each of R, G & B signals is 75Ω and
the aforementioned summing resistors
provide a video signal of 0.7V peak
and drive the 75-ohm loads via IC2.
The horizontal and vertical sync
signals from the CGA input socket (J2)
are fed through RC low-pass networks,
both consisting of a 470Ω resistor and
a 33pF capacitor, before being fed to
Schmitt trigger stages IC4a & IC4b
siliconchip.com.au
14
13
100nF
ANALOG/DIGITAL
SELECT
4.7k
12
OUT
K
IN
A
1
+
2
GND
100 µF
100nF
POWER
IN
D1 1N4004
REG1 7805CT
+5V
100 µF
16V
–
J3
16V
S1
IC4f
+5V
100nF
100nF
4
BLUE
15
GRN
Z1
Z
Z0
Y1
Y
IC1
MAX4619
14
RED
X
Y0
X1
X0
C
B
6
A
EN
3
3
5
5
1
1
2
2
13
13
12
12
9
9
10
10
11
11
GND
16
Vcc
Z1
Z
Z0
Y1
Y0
X1
Y
X
X0
33pF
BLUE
L2
15
33pF
GRN
IC2
MAX4619
L3
14
33pF
RED
C
B
A
EN
VGA OUT
8
6
7
GND
100nF
8
L1
4
OUTPUT TO GBS-8200
SCALAR BOARD
16
Vcc
6
8
5
CSYNC
4
33pF
3
2
CGA IN
1
INTENSITY
6
RED 2
22
7
7
GRN 3
33
8
BLUE 4
8
44
9
9
5
55
470Ω
J2
IC4a
1
6
7
2 HSYNC
8
33pF
9
HSPOL
VSYNC 10
470Ω
+5V
33pF
3
IC4b
VSPOL 11
4
13
7
Vcc
J1
I1
I2
R1
I3
R2
I4
G1
I5
G2
I6
I7
IC3
PAL22V10
(RGB-VGA)
I8
B1
B2
C128F
I9
HSPOL
330Ω
680Ω
22
21
680Ω
20
19
18
I10
CSYNC
I11
L4
330Ω
330Ω
82Ω
680Ω
17
16
JP3
15
9
14
IC4d
270Ω
8
5
I12
GND
12
2x 4.7k
JP1
23
C128FIX
1
1
6
1
24
11
IC4c
IC4e
6
10
IC4 = 74HC14AN
L1-L4: 600 Ω <at> 100MHz
HSPOL
7805
VSPOL
JP2
VSPOL
1N4004
A
SC
20 1 5
RGB INTENSITY BOARD
K
GND
IN
GND
OUT
(MICROBEE TECHNOLOGY)
Fig.1: the circuit for the add-on RGB Intensity PCB. IC1 & IC2 are MAX4619 analog multiplexers which switch the RGB
signal lines, while IC3 is a PAL (Programmable Array Logic) device which performs digital-to-analog conversion to
provide the correct colour map for the CGA standard.
(74HC14AN) to square them up and
feed them to the PAL (IC3).
IC3 then combines the squared up
HSYNC & VSYNC signals to produce
a composite sync output which is fed
to Schmitt trigger stage IC4d. Sync
signals for CGA are normally positivegoing but some monitors require
siliconchip.com.au
negative-going sync signals. Jumpers
JP1 & JP2 cater for this.
Finally, the reconstituted RGB and
combined sync signals are passed
through individual LC low-pass filters
which each consist of a surface mount
inductor (L1-L4) and a 33pF capacitor.
Each of these SMD inductors has an
impedance of 600Ω at 100MHz.
Jumper JP3 (C128FIX) corrects the
colour output for a Commodore 128
computer – see the accompanying
panel for details.
Power for the RGB Intensity Board
comes from the GBS-8200 scaler board
and this is fed in via reverse polarity
February 2015 87
1
270Ω
330Ω
680Ω
330Ω
680Ω
330Ω
680Ω
4.7k
4.7k
1
IC2 MAX4619
6
5
J1
J3
100nF
82Ω
9
IC1 MAX4619
1
1
REG1
7805CT
100nF
C128FIX
100nF
100 µF
16V
IC4 74HC14AN
J2
VS-POL HS-POL
4.7k
IC3 PAL22V10 (RGB-VGA)
470Ω 1
33pF
33pF
470Ω
100nF
S1
100 µF
16V
D1
Power In
4004
100nF
L2
L1
L3
L4
33pF x 4
1
Fig.2: follow this parts layout diagram to build the RGB Intensity Board. Its
J1 output is connected to the GBS-8200 scaler board via a 5-way cable fitted
with a header socket (see photo at right), while the power supply inputs are
connected to this board via a 2-way cable.
protection diode D1, A 100µF electrolytic capacitor then filters the output
from D1 which is then fed to 7805
3-terminal regulator REG1 to derive a
5V supply rail.
Construction
Construction is straightforward, with
all parts mounted on a double-sided
plated-through PCB measuring 100 x
50mm. Fig.2 shows the layout.
Start with the resistors & capacitors,
then install inductors L1-L4. These inductors are supplied as surface mount
parts on a strip of 8mm-wide tape and
it’s just a matter of peeling the tape off
the backing to remove them. To install
them, first melt a small amount of
solder onto one pad at the component
location. That done, hold the inductor
with tweezers, then reheat the solder
and slide the inductor into place. You
can then solder the other end of the
device to its pad.
Next, the front-panel DB9 connector and the switch can be installed,
followed by the 24-pin DIL socket for
IC3. Take care to ensure that the socket
is orientated correctly, ie, notched end
towards the top edge of the PCB.
If you decide to power both the
GBS-8200 and the adapter board from
5V DC, then regulator REG1 should be
omitted. In that case, it will be necessary to install a link between REG1’s
vacant input and output pads on the
PCB. D1 must also be replaced with
a link but watch the supply polarity.
Alternatively, if you don’t have a
regulated 5V DC supply, then a supply
Commodore 128: The C128FIX Jumper Option
The Commodore 128 has an 80-column mode that outputs RGB+I digital
video on a standard CGA 9-pin D-connector. The colour set is almost identical to
the normal CGA colour set, with the exception of dark yellow which appears on
Commodore monitors as brown.
For the purist who wants to represent this colour correctly, the C128FIX jumper
should be fitted. Logic inside the PAL (IC3) then pulls pin 17 of this IC low when
ever this colour combination is detected. This pin in turn pulls the green level lower
via an 82Ω resistor, creating a brown colour instead of yellow at the RGB output.
Table 1: Resistor Colour Codes
o
o
o
o
o
o
o
No.
3
3
2
3
1
1
88 Silicon Chip
Value
4.7kΩ
680Ω
470Ω
330Ω
270Ω
82Ω
4-Band Code (1%)
yellow violet red brown
blue grey brown brown
yellow violet brown brown
orange orange brown brown
red violet brown brown
grey red black brown
of 7.5-12V DC is recommended and
REG1 (and D1) must be installed to
provide 5V for the adaptor board. It’s
just a matter of bending REG1’s leads
down through 90° exactly 6mm from
its body before fitting it in place. Its
metal tab is then secured to the PCB
using an M3 x 5mm machine screw
and nut, after which its leads are soldered and trimmed.
Be sure to fit diode D1 with the cor-
Table 2: Capacitor Codes
Value µF Value IEC Code EIA Code
100nF 0.1µF
100n
104
33pF
NA
33p
33
5-Band Code (1%)
yellow violet black brown brown
blue grey black black brown
yellow violet black black brown
orange orange black black brown
red violet black black brown
grey red black gold brown
siliconchip.com.au
rect polarity, ie, banded end towards
the 100µF capacitor.
The two MAX4619 ICs (IC1 & IC2)
can now be fitted (watch their orientation) and the power supply cable
soldered to the J3 position (red lead to
positive, black to negative). That done,
solder the RGB input cable (supplied
with the GBS-8200 board) to the J1
position with the black (GND) wire
at the pin 1 end. The accompanying
photos show the wiring details.
Note that only five wires are needed,
ie, for pin 1 and pins 5-8. The supplied
cable also has a yellow wire on pin 3
and this should be removed.
If you are fitting the boards into the
supplied case, you can trim the RGB cable to around 100mm. As always, check
your work before applying power. In
particular, look for shorts and poor
solder joints and check the orientation
of all polarised components.
Getting it going
This view shows how the two PCBs are mounted inside the case, with the RGB
Intensity Board at left. Note that the VGA & component video inputs at the front
of the scalar board are not used and are “blanked off” by the front panel.
As stated, the digital mode converts
a true CGA digital input (RGB + Intensity) to the proper CGA colour map.
This is the mode that’s used for the
Microbee Premium, Premium Plus &
256TC models, along with regular IBM
PCs and numerous other computers.
The analog mode allows the unit
to be used with computers that have
The completed unit can be powered using a 7.5-12V DC 1A plugpack or a
well-regulated 5V DC supply (see text). It’s just the shot for getting that old
“retro” computer going with a recent-model VGA LCD (or CRT) monitor.
siliconchip.com.au
February 2015 89
The completed unit is simple to hook up – all you have to do is connect your computer to the CGA input, connect the
VGA output on the rear panel to a suitable monitor and connect a power supply. Note that it’s necessary to install the
C128FIX jumper on the RGB Intensity Board to get the correct colours from a Commodore 128 computer (see panel).
true analog outputs, such as the Commodore Amiga (the converter has
been tested with the Amiga & works
brilliantly!).
Normally, the unit works with positive TTL level HSYNC & VSYNC as
the timing signals. If you strike sync
problems with an odd-ball system, try
installing jumpers on the VS-POL and
HS-POL headers. The unit will also
work with a composite sync signal.
Final assembly
Once you have the unit working
Modifications To The GBS-8200 Board
While developing this project, we detected a fault in the signal output from the
GBS-8200 scaler board under certain conditions. Intermittently, and mostly when
the board was cold, there would be “snow” on the video output.
Apparently, this is a common fault with the GBS-8200 and appears to be a
result of omitting damping resistors in the SDRAM interface and poor calibration
of the SDRAM timing. As a result, Microbee has modified the GBS-8200 scaler
boards supplied with their kits for optimal output.
Finally, we recommend setting the VGA monitor to a resolution of 1024 x 768
pixels and setting the sharpness close to maximum.
90 Silicon Chip
(it’s just a matter of hooking it up to
a computer and monitor and trying it
out), you can mount the boards in the
case which is supplied pre-drilled and
routed. The two boards mount on M3
x 6mm tapped Nylon spacers and are
secured using M3 x 16mm screws and
nuts. In addition, two “side-mount”
Nylon stand-offs are used to support
the rear of the GBS-8200 scaler board
(see photos).
These side-mount stand-offs are
necessary because the rear mounting
holes in the GBS-8200 PCB are unusable due to the case design.
Once the PCBs are in place, the top
of the case can be fitted and the front
and rear panels snapped into place to
lock the case together.
That’s it – the CGA-to-VGA Video
SC
Converter is complete.
siliconchip.com.au
|