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An interface for Apple's LocalTalk networking protocol.
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README.md

TashTalk

Elevator Pitch

It's a LocalTalk interface, contained entirely within a single Microchip PIC12F1840 (8 pins, ~$1.50) microcontroller.

It handles all the time-sensitive aspects of LocalTalk, bidirectionally bitbanging the SDLC and FM0 (a.k.a. differential Manchester) based protocol at the data link and physical layers, sending and responding to control frames with CRC calculation and checking, and collision avoidance and retransmission. It can also respond to any number/combination of node IDs for use in bridging applications. It can be interfaced directly to user-mode software on a Raspberry Pi or BeagleBone or full PC, or it can be part of a larger embedded system. It slices, it dices, etc.

Project Status

Stable enough to release a v1.0.0.

Caveats

Because of the PIC12F1840's limited memory and the way its UART is used, the host's UART needs to be able to handle a baud rate of 1 Mbps. Conventional UARTs don't expect to be pushed beyond 115.2 kbps. In addition, the PIC has a 128-byte receiver queue, which is considerably smaller than the largest possible LocalTalk frame (605 bytes). As such, the host needs to respect when the CTS hardware flow control line is deasserted so the queue doesn't overflow and be quick about resuming transmission when CTS is reasserted so the queue doesn't underflow either.

Also, "single-chip" doesn't include the separate and necessary driver/receiver chip for RS-422/485. Sorry. =)

Building Firmware

Building the firmware requires Microchip MPASM, which is included with their development environment, MPLAB. Note that you must use MPLAB X version 5.35 or earlier or MPLAB 8 as later versions of MPLAB X have removed MPASM.

Differences Between v1.x and v2.x

v1.x and v2.x use the same UART protocol, but the RA3 and RA5 pins differ in function between versions. In v1.x, RA3 is used as the LocalTalk input and RA5 is used as the output. In v2.x, RA5 is both input and output and RA3 is repurposed to act as !MCLR (the PIC's reset signal). Pulling RA3/!MCLR low will hold the PIC in reset, tristating its outputs and allowing the associated RS-422 driver IC to be driven by another device.

Projects Using It

  • TashTalkHat by bdurbrow - hat for Raspberry Pi
  • AirTalk by cheesestraws - wireless plug-and-play LocalTalk dongle
  • TashTalk 2 Hat - hat for Raspberry Pi using v2.x firmware
  • Yours? =)