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TashTalk: Single-Chip LocalTalk Interface


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I'd like one for dev purposes, but who knows when I'd get around to that. How does the hardware currently present itself to the system? Does it just dump serial packets to a /dev devices? I can work with that.


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How does the hardware currently present itself to the system?
The microcontroller connects to the RPi's UART pins. It requires a UART that supports hardware flow control, so you have to do some configuration to make sure that's what's connected to the UART pins on the expansion connector (which may differ depending on which RPi you're using). Details of the protocol are here: https://github.com/lampmerchant/tashtalk/blob/main/documentation/protocol.md - let me know if there's anything I should add there.


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So I've written a bit of firmware code that will kill two birds with one stone...

With this latest change, the chip will come up in a bootloader which allows it to accept a firmware download over the UART, and has a new command that will reset the chip and thus make it fall into the bootloader. This way, should a flaw in the code be discovered, this will allow users without chip programmers/PIC debugging tools to update their firmware.

The bootloader must be exited explicitly with a command byte (the recommended string-of-zeroes used to return the firmware to a known state will take care of this) and from this comes the added bonus: until the bootloader is exited and the mainline code is started, the driver pins are tristated and of course incoming LocalTalk frames aren't processed. This makes the chip suitable for use cases like @bdurbrow 's, where it's desirable to be able to switch a single RS-422 port in and out of LocalTalk mode and allow other components in the target board to use the drivers/receivers.

I'll post it to GitHub once I've tested it - my time is especially short lately...


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On further testing and reflection, I don't like this change. It introduces too many states into the UART protocol and it worries me about getting into a bad state due to the wrong byte at the wrong time. Will have to think about this.


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Finally got a chance to finish the other board layouts that needed to go out in the JLC order... so 15 TashTalkHat PCBs are now in production. I expect the PCBs to arrive before all the parts do, though... (eBay's slow boat from China is, um, slow).

I hope ya'all like blue soldermask, because that's what I ordered. ;)


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FWIW, I just went through the thread; and assuming that I didn't miss anybody; 11 of 15 hats are currently spoken for.


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Good job on building this ! I hope it will give birth to more interesting software.

I will pass for now, too much stuff on the todolist. Will follow though !


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@bdurbrow am I? I'd like two. I messaged you here a while back but got no response
Odd... I did respond to that; I don't know why you didn't get the response (forum glitch???); and yes, you are on my spreadsheet. Also, in answer to your other question from that - I'm not using JLC's assembly services; I'm doing the soldering myself. JLC is the manufacturer of the bare PCBs, though.

Here's who I've got listed so far:

68KMLA userQuantity Requested
Scott Squires1

That accounts for 11 of the initial 15 boards; so there's 4 more available as of this morning if anybody else wants one.



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Hmm, there may have been a forum glitch, because I didn't get a response from you either... if there's any doubt, I'm always in the IRC channel.


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Production status for anybody who's interested:

The package from DigiKey that FedEx misplaced finally turned up and now is in my hands. That's got the PICs, 8-pin IC sockets, the RS422/485 drivers, and bypass caps.

The TVS diodes from China via eBay arrived earlier than expected (yesterday, instead of the early December), and are now sitting next to the PICs in the box with all the other parts that are here so far.

0.100 male headers; single and double row. I have a bunch of these headers on hand. I've ordered more to re-stock with though.

Jumpers for the 0.100 headers: I have a bunch on hand; but more are on the way.

Mini-DIN 8 sockets: I already had these on hand for my MacESP8266+ project. As that one's a longer range project time-wise, I'll just use what I've got now on the hat and order more for the MacESP8266+.

Parts not here yet:

0.100 female header sockets, both standard and stacking. I don't have any on hand; but the ones I ordered are showing as being in California; so they should be here soon.

The PCB itself: JLC is showing that they have been finished; and are awaiting pickup by DHL.


I am hoping to begin soldering boards together sometime next week; exactly when depends on DHL and the USPS.


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Hurrah! I'm really impressed with the speed with which you've generated this hat. Millinery obviously your thing ;-)
The secret is I'm using software I wrote myself. It's still got quite a few rough edges on it (undo is... somewhat unreliable) but it lets me lay out a simple board very quickly.