USB to CAN bus to TTL serial

On Fri, 25 Nov 2022 14:43:36 -0800 (PST), Ricky
<gnuarm.deletethisbit@gmail.com> wrote:

>On Friday, November 25, 2022 at 6:36:55 AM UTC-4, upsid...@downunder.com wrote:
>> On Thu, 24 Nov 2022 12:59:53 -0800 (PST), Ricky 
>> <gnuarm.del...@gmail.com> wrote: 
>> 
>> >On Thursday, November 24, 2022 at 2:36:22 PM UTC-4, upsid...@downunder.com wrote: 
>> 
>> >> To be pedantic, the standard calls this 4-wire RS-485. 
>> > 
>> >Can you refer me to this standard?
>> I read it from the official RS-485 standard document from the 1980's. 
>> Of course it was on paper. This standard also talks about "fail-safe" 
>> termination, in which the quotes are part of the standard :-).
>> > I've not seen this in the many sites I've visited regarding the approach.
>> Did you study the official TIA RS-485 standard document ?
>
>Can you provide a link?  I've seen many sites that quote significant portions of the standard an none mention this. 


Standard organizations make their living by selling individual copies
of their standards. To get a full text copy of a standard you may have
to pay for copy (typically a few hundred dollars). You may have to
figure out which American standard organization has the original
version and pay for it.


>> >> The nice thing is that there are good pauses in the Master Tx pair 
>> >> (when a slave transmits on the other pair). Thus you can run even 
>> >> Modbus without being too accurate with timing. With 2-wire RS-485, the 
>> >> slaves must adhere to the timing specification quite accurately. 
>> > 
>> >Hmmm... I'm not following this, but I doubt Modbus offers any advantages over RS-422/485 that are relevant to me, so I don't think I need to pursue this.
>> Modbus is a software protocol typically using RS-xxx hardware. The 
>> Modbus RTU version is notorious for the timing requirements. It was 
>> created before RS-485 standard was finalized and apparently used some 
>> RS-232 diode tricks, i.e. had to avoid reflections on a serial bus 
>> with impedance missmatches.. Also RS-485 "fail-safe" solved some 
>> problems due to a floating tri-state bus.
>
>What does an RTU do? 

https://en.wikipedia.org/wiki/Remote_terminal_unit

There are three versions of Modbus:

* Modbus RTU: binary version with nasty timing requirements
* Modbus ASCII: with printable hexadecimal characters ending by <CR>
* Nidbus/TCP: binary protocol with extra header over TCP/IP