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Single-mode Durango computer

Since making a relatively versatile video output from off-the-shelf components adds quite a bit of complexity, some people may desire building a limited Durango configuration.

Single video resolution

The original, full-spec Durango·X sports two video modes:

  • Colour mode: 128 x 128 pixels, 16-colour (4 bpp)
  • HIRES mode: 256 x 256 pixels, monochrome (1 bpp)

Components with designators 1xx are used for colour mode only, whereas designators 2xx are for HIRES mode. Thus, a simple approach would be not fitting either of these component groups according to the desired (fixed) video mode.

However, since the RAM addresses are multiplexed depending on the selected video mode (thru enabling either U104/U105 or U204/205, accordingly), if the unsupported video mode is selected via software, the CPU will lose access to RAM, immediately crashing the computer. Thus, some extra connections must be fitted in order to ignore the selected resolution, allowing normal operation of the computer (within the hardwired video mode, of course).

This hack mainly affects components with designators like 3xx-- check below for differences between v1 and v2.

Tip

When a single-resolution configuration is fitted, either U104/U105 or U204/205 may be replaced by 74HC157 instead of the specified '257. Any of these types will do fine.

Mods for HIRES-only Durango·R

  • Do NOT fit U104/U105/U321 neither any other 1xx designated component (you may keep J105 for convenience, though)
  • (v1 only) Do NOT fit Q306
  • (v2 only) Do NOT fit Q906/Q307/R336/R337/R922
  • Make sure R824/R825 ARE fitted (value is not critical)
  • Cut trace to pin 10 on U428 (back side) or remove its pin from the socket
  • Bridge pins 1 and 16 on U321 empty footprint
  • Bridge pins 4 and 3 on U321 empty footprint
  • Bridge pins 7 and 6 on U321 empty footprint
  • Bridge pins 9 and 10 on U321 empty footprint
  • Bridge pins 12 and 13 on U321 empty footprint
  • (v2 only) Bridge emitter and collector (pins 1 & 3) on Q307 empty footprint -- you may bridge ALL of its pins as well, since R336 is not fitted anyway.

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), a newly designed option has been included. The aforementioned PVM monitor do support Component Video as well, thus will remain compatible.

Mods for colour-only Durango·S

  • Do NOT fit U204/U205/U321 neither any other 2xx designated component
  • (v2 only) Do NOT fit Q307/R336/R337
  • Cut trace to pin 10 on U428 (back side)
  • Bridge pins 1 and 15 on U321 empty footprint
  • Bridge pins 4 and 2 on U321 empty footprint
  • Bridge pins 7 and 5 on U321 empty footprint
  • Bridge pins 9 and 11 on U321 empty footprint
  • Bridge pins 12 and 14 on U321 empty footprint

Plus, if you do not intend to use the (deprecated) greyscale mode:

  • Do NOT fit Q306 (v1) or Q906 (v2)
  • Bridge emitter and collector (pins 1 & 3) on Q306 (v1) or Q906 (v2) empty footprint

Removing software-switchable modes

Tip

Since the only component redundant on this mod is U428, it's rarely worth it, thus NOT recommended.

This mod removes the video mode register (U428) which controls:

  • Video resolution (HIRES/colour)
  • Inverse video mode
  • Selected screen area (0 to 3, the last one being the standard)
  • RGB/Greyscale mode (deprecated)
  • Emilio's LED (also useable as a general purpose output)

The empty footprint must be wired appropriately:

  • Bridge pin 10 to +5 V (pin 16) for HIRES or GND (pin 8) for colour. This is not needed on single-mode Durangos.
  • Bridge pin 7 to +5 V (pin 16) for Inverse video or GND (pin 8) for normal video.
  • Bridge pins 5 and 12 to +5 V (pin 16) for the standard screen (3, at $6000-$7FFF).
  • Bridge pin 15 the opposite way as pin 10 (+5 V for colour, GND for HIRES). This is not needed on single-mode Durangos, neither for greyscale/sync-on-green/component video versions.

Since Emilio's LED is no longer available, the associated components (R32 and J4) aren't needed.

In the rare case you'd want to use a different screen memory from the standard 3, wire up **pins and ** as follows:

Pin 12 Pin 5 Screen Addresses
GND GND 0 $0000-$1F00*
GND +5V 1 $2000-$3FFF
+5V GND 2 $4000-$5FFF
+5V +5V 3 $6000-$7FFF

*) Note that the beginning of RAM is used by zero page, stack and system variables -- make sure your software does NOT access that area for display purposes! The $1000-$1FFF range seems quite safe in this case.

Removing extra video features

This mod removes the ability to read back the status of the video mode register (upper nybble only) and, most noticeably, the status of the video blanking signals (both horizontal and vertical). Deleted components are U530 and, in case of v1 only, U529; if so, the empty U529 footprint must be wired appropriately:

  • Bridge pin 12 to 15.

Note

If you have no software-switchable modes (i. e., U428 is NOT fitted), don't bother rewiring the empty U529 footprint (in case of v1 anyway)

Warning

In v1 boards with this mod (but software-switchable modes still installed), any attempt of reading the video mode register will corrupt its value!