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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