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Durango computer buliding options

Designed in a modular way, several configurations are available for the Durango range of home computers. Thus, you may simplify the building process or simply make the computer that suits your actual needs.

Video modes

The main difference between the Durango home computer and many other retro-inspired SBCs is the built-in display output thru a suitable TV or monitor. Display characteristics make a great difference in circuitry, thus it's worth having the option of limiting the available video modes:

Durango·R

This is the simplest build, where the display output is monochromatic 256 x 256 pixels (a.k.a. HIRES mode). For writing code and any text-based applications (including some games) this is the preferred mode, but for graphical games is usually less attractive, due to the lack of colour.

If this option is chosen, you don't need any component with 1xx designators (e.g. U127). Note that any single video mode Durango build must replace components with 3xxdesignators by suitable jumper wires; check here for details

Durango·S

Somewhat simpler than the fully-featured version, the display output is 4-bit-per-pixel (16-colour) 128 x 128 pixels, allowing a nice, colourful presentation much suited for games. The limited text area (16 x 16 characters) makes it less convenient for code or lots of text on screen, though.

If this option is chosen, you don't need any component with 2xx designators (e.g. U227). Also, this is the only configuration that needs components with 8xx designators -- in fact, just a couple of pull-up resistors. Note that any single video mode Durango build must replace components with 3xxdesignators by suitable jumper wires; check here for details

Note

Even if R824 and R825 are needed for this configuration only, they're harmless in any case and could be fitted for all options with no ill effect.

Durango·X

Tip

This is the full-featured, recommended option.

This option combines the functionality of both Durango·R and Durango·S options. 3xx-designated components are also needed to allow software-switching between modes.

Monitor output

Several options are available, depending on you preferred display device. There are big changes from v1 to v2, for improved compatibility with modern TV sets.

Warning

Due to the slightly non-standard video output, some TVs and monitors may not be compatible with Durango's signal.

SCART

Tip

This is the originally intended port, and it's the recommended option as it provides the best image quality.

SCART connections have been pretty much standard for European TV sets since the early 80's, and they provide both a composite video input (and output, if suitable) and RGB signals for optimum picture quality. A fast blanking signals allow automatic switching between both modes. This is particularly well fitted to the Durango·X video circuitry, as colour mode signals go directly (thru a crude DAC) to the RGB inputs, while the monochrome (HIRES) video goes into composite video, after being mixed with the sync signals.

Since some SCART-equipped devices (e.g. VCRs and combos) do NOT support RGB inputs, another crude Luminance DAC is added to the aforementioned mixer which adds a greyscale version of the colour picture, so the device does at least display the image. There's the option in colour mode to switch off the fast blanking signal, thus making the TV ignore the RGB signals and displaying a grayscale mode image instead.

Note that, in colour (RGB) mode, sync signals are expected to be present at the SCART composite input in any case.

Note

The greyscale mode is deprecated, although all SCART-equipped Durangos up to 2.x do support it. Check the Composite output DAC section below for details.

Sync-on-green (v1 only)

Originally intended for the Sony PVM range of monitors, quite popular within the retro-computing community, as they sport RGB inputs but NOT a SCART connection. Tapping off the separate sync from Durango would be cumbersome, thus this option allows easy connection to these monitors. As a side effect, monochrome (HIRES) mode will display as green-on-black, for a further retro experience!

Bug

Despite what the schematics stated, R111 and R113 must be fitted (maybe with values down to 330 ohm) and pin 11 of J105 must be grounded (easily done from the nearby pins 9 or 13) for best colour accuracy.

Note

This option has been replaced since v2.0 for Component video, which is more popular within modern TVs and the PVM monitors support as well. In any case, greyscale mode is not supported on either type of connection.

Component Video (v2.x only)

Tip

If you are building Durango·R (HIRES mode only), since it generates no colour signal at all, it makes no sense to select the Component Video option. If SCART is not available, use the second video output option for a suitable monochrome signal thru an RCA jack. You may delete C5 and R30 as these provide the SCART output only.

Since SCART is pretty much phased out in Europe (and was never a thing in other continents, anyway), this option has been included. The aforementioned PVM monitor do support Component Video as well, thus will remain compatible.

Tip

You might want to add J706 for the audio output, even if you don't fit R731 and C709 (which enable the second video output from the third RCA jack on it)

Note

RGB is still the native video output of a Durango computer, thus colours may differ a bit thru Component video, but this is to be expected.

Warning

It is known that many modern TVs are not compatible with the 240p/288p signal supplied by Durango-X thru the Component Video input. In such cases, if a suitable YPbPr-to-HDMI converter is not available (we're researching on that), you may always use the composite video input, although in greyscale mode. Note that this will perform fine in HIRES mode.

Tip

If you can hook a PlayStation 2 thru Component video input and properly play a PS1 game on it, your TV is most likely compatible with Durango-X.

Composite output DAC

In the original, standard form (SCART-equipped v1), display info in colour mode is sent via the RGB lines, while the sync pulses are sent thru the composite video output present on the SCART (pin 19) and the second video output as well (if fitted). Since not every SCART-equipped device does support the RGB mode (as enabled by the fast blanking line at pin 16), a crude DAC (Digital-to-Analog Converter) is made up from R107 thru R110, providing a backup signal which will display an acceptable greyscale version of the image, and not a blank screen.

As an afterthought, a greyscale mode was added to v1 (bit 3 of the video mode register at $DF80, write-only) by temporarily negating the fast blanking signal, thus allowing the composite output to be displayed on the TV, ignoring the signals on the RGB lines. For that matter, the values of R107 thru R110 were chosen to get an acceptably linear greyscale.

However, the SCART connector is hard to find on modern TVs, so an alternative connection was needed. Component Video is widely supported nowadays, and it's relatively simple to obtain from the native RGB signals, thus this became a built-in option in v2, and an external Component Video converter was designed for improved compatibility on v1 units. But these converters just generate the needed Pb and Pr signals from the original RGB outputs, while the needed Y (luminance) signal is taken directly from the composite output, again from the crude DAC and intended for greyscale. On the other hand, proper luminance values are expected on this signal when in Component Video mode for accurate colour display, thus a new set of valued for R107 thru R110 must be used. Check at the end of palette considerations for suitable replacement values.

Note

Since the greyscale mode is seldom used and will still work with the Luminance DAC values (even if somewhat less linear), these new values are always recommended in any case.

v2+ silkscreen uses Luminance values as standard. Greyscale mode is still operative (when using the SCART option) but deprecated.

PAL encoder

Since RGB signals are natively generated in Durango·S/·X and they give the highest picture quality, initially no provision to add chrominance (colour) information on the composite video output (both at the SCART and the optional second video output). But again, it might be desired for compatibility reasons.

v1 boards provide space for a PAL chrominance encoder (components with 6xx designators) based around the AD724 IC (unfortunately, only available as a less hobbyist-friendly Surface-Mounted Device). But being an afterthough and, specially, due to the non-standard sync frequencies from the Durango v1 video output, performance might be poor or not display any colour at all, depending on the particular TV set used. Thus, this option is NOT recommended.

Second video output

Essentially the same signal as the one available at the composite signal on the SCART (pin 19), but for impedance matching reasons it's obtained thru a separate RC network (R31 and C9 on v1, R731 and C709 on v2+). If these components are fitted, this signal is available at the yellow (leftmost) RCA jack on J6. The remaining RCA jacks (red & white) provide monoaural audio output for convenience, just in parallel with the SCART audio signal.

Tip

If both composite signals are to be used simultaneously, it's recommended to use a lower value for R15 (down to 120 ohm). v2 boards silkscreen already state this updated value and, in any case, using it won't do any harm, just a somewhat increased power consumption.

Piezo buzzer for audio

The Durango-generated audio (usually a square wave from U12, but also any arbitrary signal thru the AUDIO IN line supplied by suitable cartidges) goes into a simple mixer around Q3, then it takes two routes:

  • Line-level audio at SCART J105 (and J6 if fitted), thru a low-pass filter and attenuator made around R4, R5 and C2.
  • Internal buzzer BZ1 thru an attenuation resistor R23 and a disabling jumper JP2.

If the buzzer is not needed, these three components don't need to be fitted. Also, R23 can be modified for volume control.

Warning

R23 may dissipate quite a lot of heat, thus a half-watt rated resistor is highly recommended. For maximum loudness it may be replaced by a jumper wire, but that might overload Q3.

In any case, the use of the buzzer does increase power consumption considerably.