TeslaRel410/display-board-1408.md

Display Board 1408 — outputs, and what the RIO shows on it

Complete definition of the RIO External Display board (P/N 1408 Rev. 1, DSP_1408 sheet 7), from the schematic, the board photos, the GAL decode, and the firmware disassembly (RIOv4_2.disasm.asm).

Outputs: light only — the board is electrically write-only

No signal leaves the board toward the RIO or anything else. J1 has no readback line, matching the GAL's write-only DSP_SEL* decode, and the PC↔RIO protocol has no display command — only the firmware writes it.

1. Eight multiplexed 7-segment digits (U2–U9, LTS313A, red, common-cathode)

The ICM7228B (U1) does all driving. HEX/B/SHD is strapped high via 2.7 K (R1): hexadecimal decode mode, always enabled — each digit renders 0–F plus a decimal point; no raw-segment access exists as built.

• Segment bus — U1 pins 16, 17, 20, 18, 21, 22, 23, 15 drive SEGA–SEGG, SEGDP, shared by all digits (digit pins A–G, DP = 10, 9, 8, 5, 4, 2, 3, 7). Segments source the multiplexed current (~20 mA peak).
• Digit commons — U1 outputs DSP1–DSP8 (pins 4, 25, 3, 1, 26, 2, 27, 24) each sink one digit's tied-together commons (CC1+CC2, pins 6+1). DSP1→U2 (leftmost) … DSP8→U9 (rightmost).
• Multiplex scanning is internal to the ICM7228; the host only writes digit registers.

From the 68HC11 the digits are bytes at $A000–$A007 (AS0-2 = A0–A2, strobed by DSP_SEL* = E & /R/W-qualified, arriving as the 7228's /WR on J1 pin 7). Data format per write: bits 0–3 = hex value (MD[00:03]→ID0-3), bit 4 = decimal point (MD[04]→ID7; DP lights when the bit is low — the firmware always sets it, so the DP is never used). $A000 = leftmost digit (U2), $A007 = rightmost (U9); the firmware's render buffer ($2038, routine $CB49) fills right-to-left so multi-digit hex numbers read naturally.

2. Twelve passive monitor LEDs on the "IBM printer port" header (J2) — an input, not an output

J2 (26-pin IDC, standard LPT-ribbon mapping to a DB25) is electrically isolated from the display circuit; its DD nets touch nothing but LEDs and 1 K resistors referenced to the PC's own ground (IBM_GND). With a PC parallel port plugged in, the LEDs show: D0–D7 = data bits (J2 pins 3, 5, 7, 9, 11, 13, 15, 17), D8 = /STROBE (pin 1), D9–D11 = /AUTOFD, /INIT, /SELECTIN (pins 2, 6, 8). The LPT status returns are strapped to ground so a PC streams without waiting. It is a built-in parallel-port activity monitor / bench tool, powered entirely by the PC.

3. Nothing else

J1 (14-pin, from the RIO's EXTERNAL DISPLAY port) is all input: MD[00:04] on pins 2, 4, 6, 8, 10; AS0-2 on 1, 3, 5; BF_DSP_SEL* on 7; GND on 9, 11, 13; VCC on 12, 14.

What the RIO firmware displays (v4.2)

All display writes live in $CB49–$CC52 (one buffer-render routine + five canned patterns) and are driven from two contexts: initialization and the built-in test mode. During normal gameplay the display is static.

Normal operation: F0000000 — written at cold boot ($C068) and warm re-init ($C64B), then left alone.

Test mode — entered by typing the keypad sequence A, 9, E on a cockpit keypad ($C1CB tracks the sequence at $20F8); entry sets flag $2421, shows F1000000, and notifies the PC (TestModeChange, cmd 0x8C — see PROTOCOL.md). Within test mode the next keypad digit selects a sub-test; F ends a sub-test (back to F1000000), D exits test mode (back to F0000000).

Display format convention: left pair = test number, right = value, zeros as filler; the decimal points are never used.

Key	Display	Test
(entry)	F1000000	test-mode banner
0	00000000	encoder-counter reset: presets the five 16-bit accumulators to $FAFA and re-reads all HCTL-2016s ($C446)
1	0100hhll	axis test, channel 01 — counter at $A028/$A038
2	0200hhll	axis test, channel 02 — $A020/$A030
3	0300hhll	axis test, channel 03 — $A022/$A032
C	0C00hhll	axis test, channel 0C — $A024/$A034
4	0400hhll	axis test, channel 04 — $A026/$A036
5	050000nn	button test — nn = index of the pressed input (scan at $D004 over the pod input state at $2152), with lamp feedback
6	06000003	lamp test, pattern 3 ($D34A) — right digit is the pattern code
7	07000001	lamp test, pattern 1 ($D370)
8	080000kk	keypad test — kk = keypad status byte $204B, refreshed while scanning ($CC7E)

In the axis tests hhll is the live 16-bit quadrature count in hex (high:low = $201D:$201E, alternating with the raw latched bytes $2026:$2025 as the loop runs) — spin the stick/throttle/pedal encoder and watch the number move. The channel IDs are literally the keypad keys that start them, which is why the fourth axis reads 0C.

Cross-references

• README.md — board family, photos, schematic page map
• gal/README.md — DSP_SEL* decode ($A000-$A00F, write-only) recovered from the GAL fuse map
• rio-firmware/RIOv4_2.disasm.asm — display routines $CB20/$CB36/$CB49 (buffer render), $CB7C (F0), $CBA7 (F1), $CBD2/$CBFD (lamp-test banners), $CC28 (zeros), test dispatcher $C241, entry-sequence tracker $C1CB