The Readout module renders text onscreen for scale factors and indications. When requested by the display sequencer, it begins to query the bus for character data one location at a time, and renders each character as a raster one cycle after.
The raster layout is generated by a resistive DAC driven from the logic signals, and covers 16 fields of 8 characters each. As shown below, low fields are rendered at the edges of the screen and high fields towards the inside; the high field in each position is used for “extra” data. Fields 04, 05, and 06 (also high fields 14, 15, 16) are merged together into larger 24-character fields for use by the Special module.
(TODO: bus cycle timing diagram)
There are three total revisions:
- ASY-001-00 on PCB-001-00 — didn’t work.
- ASY-001-01 on PCB-001-00 — several fixes, mostly forgotten connections, to get the board working.
- ASY-001-02 on PCB-001-02 — totally new revision on a new PCB. Changes:
- Components to SMT
- Operating mode logic reworked to fix some janky clocking issues
- Added 8 more fields
- Reassigned bit 7 of the bus from “intensity” to a 7th character code bit. Added an intensity output on the font ROM, so individual codepoints can be intensified.
- Redesigned analog output section. Earlier system designs specified differential outputs with an elevated (2-6V) common mode; current design specifies outputs centered near zero. Remove mode conversion long-tailed pair, direct output from opamps.
- Redesigned blanking output section. Earlier system designs specified a simple open-collector blanking output. Current design specifies a multi-level signal selecting an intensity mode (with each corresponding to a different front panel pot) terminated into 100 ohms.
- Replace DAC opamps with much faster units for higher quality video.