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Double Hi-Res graphics (DHGR) was a nice addition to the Apple ][ line as it allowed for higher resolution 560x192 graphics with 16 colors in comparison to the traditional 280x192 with 6 colors. It first appeared on the Apple //e that came out in 1983. To use the DHGR mode, you needed an 80-column card which fit into the AUX slot and provided the extra RAM needed to support the mode. However, there was a jumper on the 80-column card which determined if the DHGR mode was available and it depended on whether you were using an early revision Apple //e or not.

What was the change to the //e motherboard which allowed for DHGR to work? Was it part of the original design or was it a post-production change? Was it not on the original revisions of the motherboard due to bugs or simply because it wasn't designed in?

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    Note that it's 560x192 B&W, or 140x192 with 16 colors. Contrast with plain hi-res at 280x192 B&W (+half-pixel shifts), or 140x192 with 6 colors. See 1000bit.it/support/manuali/apple/technotes/aiie/tn.aiie.03.html
    – fadden
    Jun 29, 2016 at 5:15
  • Wasn't there also a modification for a regular pre-1982 Apple ][ that supported a form of double hi-res? I've not seen the physical board, but I recall you had to replace chips on the motherboard... I'm not remembering the manufacturer's name, but they advertised in magazines in the US and the UK in the early 1980s.
    – Ralph H
    Aug 1, 2016 at 17:43

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Appendix G of Sather's excellent Understanding the Apple IIe has a summary of the changes:

The primary operational improvement of Revision B is the addition of the DOUBLE-RES graphics capability. This was achieved by rewiring 1/3 of the C5 LS10 NAND gate so that pulling pin 55 of the auxiliary slot low forces TEXT processing at the timing HAL instead of disabling motherboard ROM as it did in Revision A. [...] The nomenclature of pin 55 was appropriately changed from ENFIRM to FRCTXT'.

Changes in the timing HAL and MMU were required to make the new FRCTXT' wiring work. The Revision B timing HAL treats pin 12 as GR+2' whereas the Revision A timing HAL treats pin 12 as GR+2. The MMU was changed so that ROMEN1' and ROMEN2' are gated by INHIBIT', a necessity since the ROM inhibiting function had been performed by the NAND gate which now performs the forced text function. [...]

Wiring and component changes were made to the video summing amplifier in Revision B, and these would be necessary to realign COLOR REFERENCE to the PICTURE signal if video timing in the HAL were changed.

Another area of change in Revision B was in the functions of motherboard configuration jumper pads. Two new jumpers were added (X3 and X7), and the functions of XI and X2 were changed.

According to Apple II History the feature was designed after Revision A release:

Early Apple IIe motherboards were labelled as “Revision A”. Engineers determined soon after its introduction that if the same use of parallel memory was applied to the hi-res graphics display as was done with the text display, they could create higher density graphics. These graphics, which they called “double hi-res”, also had the capability of displaying a wider range of colors, similar to those available with the original Apple II lo-res graphics. The IIe motherboards with the necessary modifications to display these double hi-res graphics were labelled “Revision B”, and a softswitch was assigned to turn on and off the new graphics mode.

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