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Acorn Econet Bridge

The Acorn Econet Bridge is a standalone 6502-based device that joins two Econet networks, forwarding traffic between them while isolating local traffic to each side. The bridge firmware lives in an 8 KB ROM at &E000-&FFFF and implements the bridge protocols that allow file servers, printer servers, and station clients on different Econet segments to communicate.

Acorn Econet Bridge glossary

Annotated Disassemblies

Analyses

• Acorn Econet Bridge — architecture overview

  Top-down tour of the whole firmware assembled from the annotated disassembly. Covers the hardware, the boot sequence, steady-state operation, the bridge-announce/query protocol, forwarding via the four-way handshake, and self-test. Serves as the entry point that ties the other writeups together.

• Anti-aliasing in the Econet Bridge's RAM test

  A close reading of the thirteen-instruction routine at &E00B that sizes the Bridge's RAM. The INC $00 instructions between each write and read are a layered defence against three distinct failure modes.

• The self-test LED, driven by a repurposed ADLC pin

  How the Bridge's single front-panel LED is driven by CR3 bit 7 on ADLC B, with exactly two writes in the whole ROM, making it a pure function of which init path was taken.

• Escape-to-main: the Bridge's cooperative error-recovery idiom

  Four routines share a PLA/PLA/JMP main_loop abnormal exit that drops the caller's return address and collapses any failed operation back to the main dispatcher. Trades per-site clarity for global simplicity and a meaningful ROM-space saving.

• One frame, two broadcasts: reset-time announcement

  The Bridge announces itself on both Econet sides at power-on by building a single frame template and transmitting it twice, patching a single byte of payload between the two transmissions. A small idiom with a clean separation of concerns between builder and transmitter.

• The Econet Bridge has no station address

  Architectural writeup on why the Bridge operates without a station number on either of its connected networks. Evidence from the board's two 74LS244 network-number buffers, the firmware's network-keyed routing tables, and the Installation Guide. Covers the &18 firmware marker in outbound announcements and the practical implications for anyone reading the disassembly.

• Bridging the four-way handshake

  How the Bridge forwards Econet's four-stage scout/ACK/data/ACK transactions across two segments. Documents the receive-and-stage pattern that makes rx_a_forward's puzzling A-B-A transmit tail resolve into a clean protocol implementation, and the role of escape-to-main in making mid-handshake failures safe without any per-transaction state.

• Event-driven re-announcement: why a solo Bridge goes silent

  A full-ROM audit of every write to announce_flag shows that bridge re-announcement is purely reactive: only receiving a BridgeReset from another bridge triggers a burst, and BridgeReply frames deliberately don't cascade. A bridge with no peers emits two frames in its lifetime and then stays silent forever — by design, not by bug.

References

Sources consulted

• Econet Installation Guide (0482,009 Issue 1, 27 September 1988) (PDF)
  Acorn's official Econet installation guide. Chapter 3 covers the Bridge. A local copy is kept in docs/.
• Ian Stocks's reverse-engineered Acorn Econet Bridge schematic (PDF) [requires Stardot login]
  Schematic reverse-engineered from an original Acorn board, shared in the Stardot forum thread below.
• J.G. Harston's mdfs.net Econet Bridge archive
  Directory listing for the Acorn Econet Bridge on mdfs.net.
• J.G. Harston's Bridge disassembly (BBC BASIC embedded-assembler source)
  A parallel disassembly of the same binary variant using BBC BASIC's inline assembler, with memory-layout comments and protocol-level terminology (BridgeReset / BridgeReply / WhatNet / IsNet / BridgeQuery port). Useful as a second reading. JGH is a fellow reverse-engineer, and his comments have been paraphrased rather than quoted and weighed against the evidence, like ours.
• Motorola MC68B54P ADLC datasheet (PDF)
  Datasheet for the Advanced Data Link Controller used on each Econet port of the Bridge. Authoritative reference for CR1-CR4 and SR1/SR2 bit meanings. The canonical source has dropped off the web; this is the copy kept in the repository's `docs/` folder, served as raw PDF from GitHub.

Further reading

• The Replica Acorn Econet Bridge project
  Chris Oddy's modern reproduction of the Acorn Econet Bridge, with useful photographs and build notes.
• Replica Econet Bridge Schematic (PDF)
  Chris Oddy's schematic for the replica board, closely matching the original Acorn design.
• Rick Murray's notes on the Acorn Econet Bridge
  Overview of the Acorn Econet Bridge hardware, firmware behaviour, and configuration.
• Beebmaster: The Acorn Bridge
  Beebmaster's page on the Acorn Bridge, with photographs, configuration notes, and operational details.

Discussion

• Discuss this disassembly on the Stardot Forums thread: Full disassembly/reassembly of the Acorn Econet Bridge ROM
• Stardot Forums: Econet Bridge schematic thread
  Discussion of Ian Stocks's reverse-engineered schematic and Bridge hardware details.
• Stardot Forums: discussion of the Acorn Econet Bridge ROM
  Thread discussing the Bridge ROM's behaviour and internals.