Interface (Greaseweazle v4.1)

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Interface (Greaseweazle v4.1)

This page describes the Greaseweazle v4.1 as Tapewyrm uses it: the microcontroller, the 34-pin lines that carry QIC-117, the limits of the USB link, and what the Tapewyrm firmware adds to the stock Greaseweazle firmware. Tapewyrm supports this one board and needs no hardware changes to it.

Board

Item Value Notes
Microcontroller Artery AT32F403A (at32f403acgu7), Cortex-M4 From the board definition, firmware/boards/greaseweazle_v4_at32f403a.json.
Clock 216 MHz f_cpu in the board definition.
SRAM 224 KB Most of it is the flux ring buffer for captures.
USB USB-C, Full-Speed (12 Mbit/s) See USB link limits.
Floppy outputs Buffered, 40 mA Strong enough for drive-side pull-ups.
Power 5 V header; USB 5 V isolation jumper The 5 V header cannot run a tape drive's motor.
Recovery Hardware DFU header Straps the AT32's ROM bootloader, so a broken flash is always recoverable. See tw update and tw dfu.

34-pin lines used for QIC-117

A floppy-tape drive multiplexes two channels over the floppy cable. The command channel (QIC-117) reuses the floppy control lines; the data channel is ordinary MFM on the read-data line. The Greaseweazle already drives and senses every line involved in the same direction a floppy drive needs, so no rewiring is required.

Pin numbers and directions are from QIC-117 Rev J Table I-b (qic117j.pdf). "In" and "out" are from the drive's point of view, as the standard gives them.

Pin Signal Drive I/O QIC-117 use Used by Tapewyrm
8 Index Out Cue pulses (ready, report bit presented, waiting for argument); once per segment during Logical Forward Yes: idle cue and segment boundary
10 Drive Select 0 In Hardware select Through Greaseweazle SELECT
12 Drive Select 1 In Hardware select Through Greaseweazle SELECT
14 Drive Select 2 In Hardware select Through Greaseweazle SELECT
6 Drive Select 3 In Hardware select Not used
16 Motor On In Motor-on wake on some drives insight profile only
18 Direction In Not used by QIC-117 Not used
20 Step In Command and argument pulse trains Yes: every command
22 Write Data In Data channel, writing Never driven
24 Write Enable In Data channel, writing Never asserted
26 Track Zero Out Serial report bits, LSB first Yes: every report
28 Write Protect Out Cartridge write protect Read by tw drive scope
30 Read Data Out MFM flux during motion Yes: captures
32 Side Select In Not used by QIC-117 Not used
34 Diskette Change Out Not used by QIC-117 Not used

Odd pins are ground. QIC-117 Rev J §1.2 defines the three signals that make up the command interface (Step, Track Zero and Index); the QIC-117 page describes how they are used.

Note

Currently, Tapewyrm has no write capabilities, and does not assert Write Enable in software. As an act of caution, you should always verify the Greaseweazle's Write Enable header.

Note

Share the USB bus as little as you can during a capture. The throughput figures above were measured with nothing else busy on the bus; a hub or a busy device on the same controller reduces the margin.

The AT32F403A's USB controller is Full-Speed only. The firmware reports this at runtime, and tw info shows it.

Limit Value Consequence
Endpoint packet size 64 bytes A command response is sent as one packet.
Command response 64 bytes, including the 2-byte header A longer response is truncated, not split. Every Tapewyrm verb is sized to fit, or streams its data like a flux read.
Bulk throughput About 1.2 MB/s theoretical Enough for every standard data rate captured so far.

The 64-byte response limit was found on the bench: a 253-byte SCOPE response arrived cut off at 64 bytes and desynchronised the link. SCOPE now caps its edge log at 10 entries (2 + 3 + 8 + 10 × 5 = 63 bytes).

Measured capture throughput during Logical Forward, from bench captures:

Data rate Average per capture set Highest single track
500 kbit/s 392–396 kB/s 412 kB/s
1 Mbit/s 610–637 kB/s 676 kB/s

The worst 1 Mbit/s track used about 56% of the theoretical ceiling. Two things do not fit through the link: a 2 Mbit/s drive mode (about 1.3 MB/s by scaling), and flux during high-speed physical motion. The firmware ends a capture cleanly when its buffer overflows.

What the Tapewyrm firmware adds

The Tapewyrm firmware is the Greaseweazle v1.6 firmware, vendored in-tree, with QIC verbs added to its command loop. It reuses Greaseweazle's flux read engine for captures and its pin primitives for the command channel. It is a hard fork that diverges from Greaseweazle on purpose, so Greaseweazle's gw command is not supported with it; use tw. To use the board as a Greaseweazle again, flash stock firmware back.

Verb Number Does
CMD_QIC_INFO 0x80 Device information and QIC capability gate: protocol version, capabilities, SRAM, sample clock.
CMD_QIC_SET_TIMING 0x81 Sets the QIC-117 pulse and report cadence from the drive profile. Idle only.
CMD_QIC_PULSES 0x82 Sends N Step pulses verbatim, then optionally clocks k report bits off Track Zero.
CMD_QIC_WAIT_READY 0x83 Polls the Index cue until the drive is ready or a timeout expires.
CMD_QIC_CAPTURE 0x84 Sends a motion command, then arms a free-running flux capture.
CMD_QIC_SCOPE 0x87 Bench logic probe: optional Step pulses, then an edge log of the drive's output lines.
CMD_QIC_BUILD_INFO 0x88 Firmware git commit and dirty flag, for tw info.

The firmware knows nothing about what a command means. It sends the number of pulses the host asks for, which keeps every QIC-117 decision on the host, where it can be logged and changed without reflashing.

During a capture the firmware adds markers to Greaseweazle's flux stream: a session start, a SEGMENT marker at each hardware Index edge, events, and an end marker with counts and a checksum. The stream is saved verbatim as a TWRF file (TWS-1). The wire protocol is in Device protocol, and the firmware internals are in Firmware.

See also