1.1 --- a/README.txt Sat Feb 19 23:59:22 2022 +0100
1.2 +++ b/README.txt Tue Feb 22 00:31:29 2022 +0100
1.3 @@ -41,9 +41,27 @@
1.4 --------------------------------
1.5
1.6 This version provides pads for very short resistors which are rather
1.7 -inconvenient for mounting generally available through-hole mounted resistors,
1.8 -although the pad spacing is appropriate for the capacitors employed in the
1.9 -design. A new version is anticipated with longer footprints for the resistors.
1.10 +inconvenient for generally available through-hole mounted resistors, although
1.11 +the pad spacing is appropriate for the capacitors employed in the design. A
1.12 +new version is anticipated with longer footprints for the resistors.
1.13 +
1.14 +Another flaw with this version is the combination of colour signals without
1.15 +diodes isolating this part of the circuit. Since some of these pins are used
1.16 +for parallel input/output, if the VGA signal resistors are connected, pairs of
1.17 +pins are effectively joined, making the affected parallel signals unusable.
1.18 +This affects PMD0/PMD1 (blue channel), PMD2/PMWR (green channel), PMD5 (if D7
1.19 +is used for the red channel). Thus, any use of the parallel signals precludes
1.20 +use of the VGA outputs unless only one pin of every affected pair is used, as
1.21 +opposed to both pins.
1.22 +
1.23 +Diodes would correct this problem, but another approach could involve a way of
1.24 +switching between VGA output and parallel signal usage since the use of one
1.25 +generally precludes the use of the other: parallel traffic leaking through the
1.26 +VGA connector would not produce a valid signal; VGA output leaking through the
1.27 +parallel connector would not produce valid parallel transactions. A
1.28 +multiplexer controlled by an output signal could be coupled with software to
1.29 +permit switching between the connectors, allowing video output to be switched
1.30 +between a panel and the VGA connector, for example.
1.31
1.32 Design Rules and Production
1.33 ===========================