Keyboard and link port

The keypad scanner and link-port drivers provide the calculator’s local input and wired data-transfer paths. The keyboard path turns matrix scans into cooked key codes, while the link path sends bytes through the legacy two-wire port or the hardware-assisted interface.

Deep dives: Keypad and ON-key hardware covers the electrical matrix, scanner timing, debounce, repeat, ON interrupts, and wake. Two-wire link port hardware covers port 0x00, electrical encoding, raw byte handshakes, timeouts, and background detection. Link / data transfer covers silent-link packets and variable send/receive.

Keyboard

The matrix keypad is read through port 0x01: software writes an active-low group mask and reads active-low key lines. Standard hardware timer 1 scans it through ram:03B4. The separate ON circuit reports its level and interrupt state through ports 0x03 and 0x04. [confirmed]

• _GetCSC = 4018, body ram:04B2, atomically reads and clears the one-byte kbdScanCode mailbox. It returns raw scan events and does not block. [confirmed]
• _GetKey = 4972, body 06:491E, blocks, processes hooks and APD state, applies 2nd and ALPHA, and returns a cooked TIKeyCode. [confirmed]
• _KeyToString at 01:6D10 maps a cooked key code to an editor token or string. [confirmed]

Scan codes such as skEnter identify a matrix position. Cooked key codes such as kEnter = 5 incorporate OS modifier and context policy. _GetCSC returns the former; _GetKey returns the latter. The complete matrix, scan-code formula, diagonal-arrow exception, five-sample release filter, repeat timing, modifier state, and 46.7 ms ON debounce are reconstructed in Keypad and ON-key hardware.

Key → token translation [confirmed]

_KeyToString (01:6D10) turns a key code into a TI-BASIC token for the editor. It’s not a single flat table — it combines:

• range arithmetic: contiguous key ranges map to token ranges by a fixed offset (e.g. key 0x1F→'P'-based, 0x59→'a' for lowercase) — letters/digits;
• per-mode lookup tables on another page, reached via cross_page_jump (the 2nd/ALPHA-mode and function-key token tables);
• special key codes 0xFB/0xFC/0xFE/0xFF are not tokens — they’re the menu / context-switch return codes the main event loop branches on (see Boot, contexts & errors), so _KeyToString routes them out via cross_page_jump rather than translating.

So the input path is: keypad → ISR → kbdScanCode → _GetKey (cooked kXxx + modifiers) → _KeyToString → token → parser (Tokenizer & TI-BASIC).

Link port

The 2.5 mm I/O link uses two open-collector lines. Port 0x00 drives and samples them directly; ports 0x08–0x0D provide the TI-84 Plus hardware-assisted byte path. [standard] for the electrical interface; [confirmed] for the ROM port use.

_SendAByte = 4EE5, body 3C:420D, sends legacy bits least-significant first. It writes 1 for bit 0 and 2 for bit 1, waits for a both-low acknowledgement, releases its line, and waits for idle. _RecAByteIO = 4F03, body 3C:443F, performs the inverse handshake. Both paths use bounded waits that enter the link-error machinery on timeout. [confirmed]

Installed error callback 3C:6136 reaches 3C:618D for applicable transfer states. Its raw branch marks the link busy, selects nominal 6 MHz, drives both lines low for a 7,077,785-base-T-state loop, releases them, and clears busy; its USB branch skips port 0x00. The documented Flash opcode wait raises the loop to 8,191,881 T-states. This is the OS’s transport-specific abort cleanup. [confirmed] for the ROM role and base count; [standard] for the wait-state-adjusted count.

Two-wire link port hardware reconstructs the port read/write inversion, four transitions, receiver rotation, errors, and timer-driven activity check. USB ASIC and link assist covers the assist FIFO selected by the same byte routines.

Variable-transfer packet framing

A TI link packet is a 4-byte header (machine-ID, command-ID, length-lo, length-hi) optionally followed by data[len] and a 16-bit LE checksum; commands include 0x06 VAR, 0x09 CTS, 0x15 DATA, 0x56 ACK, 0x5A NAK, 0x92 EOT. Link transfer covers the framing, silent-link send/receive engine (link_xfer_op, _SendVarCmd), checksum and acknowledgement handling, and 16-byte Flash-batched receive path. [confirmed]