Re: Shared Communications Bus - RS-422 or RS-485

From	David Brown <david.brown@hesbynett.no>
Newsgroups	comp.arch.embedded
Subject	Re: Shared Communications Bus - RS-422 or RS-485
Date	2022-11-03 16:26 +0100
Organization	A noiseless patient Spider
Message-ID	<tk0mih$1g2fh$1@dont-email.me> (permalink)
References	(2 earlier) <b4a93d7d-3d6f-4b23-86b1-f1066b6b72ebn@googlegroups.com> <tjul46$18dkn$3@dont-email.me> <ec91ddbe-6777-494f-8001-cf57fc15deeen@googlegroups.com> <tk09ev$1f0de$1@dont-email.me> <tk0e27$1f1rv$1@dont-email.me>

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On 03/11/2022 14:00, pozz wrote:
> Il 03/11/2022 12:42, David Brown ha scritto:
>> On 03/11/2022 00:27, Rick C wrote:
>>> On Wednesday, November 2, 2022 at 4:49:16 PM UTC-4, David Brown wrote:
>>>> On 02/11/2022 20:20, Rick C wrote:
>>>>> On Wednesday, November 2, 2022 at 5:28:21 AM UTC-4, David Brown
>>>>> wrote:
>>>>>> On 02/11/2022 06:28, Rick C wrote:
> 
> 
>> You are correct that reception is in the middle of the stop bit 
>> (typically sub-slot 9 of 16).  The first transmitter will be disabled 
>> at the end of the stop bit, and the next transmitter must not enable 
>> its driver until after that point - it must wait at least half a bit 
>> time after reception before starting transmission.  (It can wait 
>> longer without trouble, which is why faster baud rates are less likely 
>> to involve any complications here.)
> 
> Do you mean that RX interrupt triggers in the middle of the stop bit and 
> not at the end? Interesting, but are you sure this is the case for every 
> UART implemented in MCUs?

Of course I'm not sure - there are a /lot/ of MCU manufacturers!

UART receivers usually work in the same way, however.  They have a 
sample clock running at 16 times the baud clock.  The start bit is edge 
triggered to give the start of the character frame.  Then each bit is 
sampled in the middle of its time slot - usually at subbit slots 7, 8, 
and 9 with majority voting.  So the stop bit is recognized by subbit 
slot 9 of the tenth bit (assuming 8-bit, no parity) - the voltage on the 
line after that is irrelevant.  (Even when you have two stop bits, 
receivers never check the second stop bit - it affects transmit timing 
only.)  What purpose would there be in waiting another 7 subbits before 
triggering the interrupt, DMA, or whatever?

> 
> I wouldn't be surprised if the implementation was different for 
> different manufacturers.
> 

I've seen a bit of variation, including 8 subbit clocks per baud clock, 
wider sampling ranges, re-sync of the clock on edges, etc.  And of 
course you don't always get the details of the timings in datasheets 
(and who bothers measuring them?)  But the key principles are the same.

> 
>>> None of this matters to me really.  I'm going to use more wires, and 
>>> do the multi-drop from the PC to the slaves on one pair and use 
>>> RS-422 to multi-point from the slaves to the PC.  Since the slaves 
>>> are controlled by the master, they will never collide.  The master 
>>> can't collide with itself, so I can ignore any issues with this.  I 
>>> will use the bias resistors to assure a valid idle state.  I may need 
>>> to select different devices than the ones I use in the product.  I 
>>> think there are differences in the input load and I want to be sure I 
>>> can chain up to 32 units.
>>>
>>
>> OK.  I have no idea what such a hybrid bus should technically be 
>> called, but I think it should work absolutely fine for the purpose and 
>> seems like a solid solution.  I would not foresee any issues with 32 
>> nodes on such a bus, especially if it is relatively short and you have 
>> terminators at each end.
> 
> In my experience, termination resistors at each end of the line could 
> introduce other troubles if they aren't strictly required (because of 
> signal integrity on long lines at high baud rates).
> 

RS-485 requires them - you want to hold the bus at a stable idle state 
when nothing is driving it.  You also want to have a bit of load so that 
you have some current on the bus, and thereby greater noise immunity.

> The receiver input impedance of all the nodes on the bus are in parallel 
> with the two terminators. If you have many nodes, the equivalent 
> impedance on the bus is much small and the partition with bias resistors 
> could reduce the differential voltage between A and B at idle to less 
> than 200mV.
> 
> If you don't use true fail-safe transceivers, a fault start bit could be 
> seen by these kind of receivers.
> 

Receiver load is very small on modern RS-485 drivers.

Thread

Shared Communications Bus - RS-422 or RS-485 Rick C <gnuarm.deletethisbit@gmail.com> - 2022-11-01 22:28 -0700
  Re: Shared Communications Bus - RS-422 or RS-485 David Brown <david.brown@hesbynett.no> - 2022-11-02 10:28 +0100
    Re: Shared Communications Bus - RS-422 or RS-485 pozz <pozzugno@gmail.com> - 2022-11-02 11:54 +0100
      Re: Shared Communications Bus - RS-422 or RS-485 David Brown <david.brown@hesbynett.no> - 2022-11-02 14:27 +0100
    Re: Shared Communications Bus - RS-422 or RS-485 Rick C <gnuarm.deletethisbit@gmail.com> - 2022-11-02 12:20 -0700
      Re: Shared Communications Bus - RS-422 or RS-485 David Brown <david.brown@hesbynett.no> - 2022-11-02 21:49 +0100
        Re: Shared Communications Bus - RS-422 or RS-485 Rick C <gnuarm.deletethisbit@gmail.com> - 2022-11-02 16:27 -0700
          Re: Shared Communications Bus - RS-422 or RS-485 David Brown <david.brown@hesbynett.no> - 2022-11-03 12:42 +0100
            Re: Shared Communications Bus - RS-422 or RS-485 pozz <pozzugno@gmail.com> - 2022-11-03 14:00 +0100
              Re: Shared Communications Bus - RS-422 or RS-485 David Brown <david.brown@hesbynett.no> - 2022-11-03 16:26 +0100
                Re: Shared Communications Bus - RS-422 or RS-485 pozz <pozzugno@gmail.com> - 2022-11-04 08:45 +0100
                Re: Shared Communications Bus - RS-422 or RS-485 David Brown <david.brown@hesbynett.no> - 2022-11-04 10:49 +0100
                Re: Shared Communications Bus - RS-422 or RS-485 pozz <pozzugno@gmail.com> - 2022-11-04 15:37 +0100
                Re: Shared Communications Bus - RS-422 or RS-485 David Brown <david.brown@hesbynett.no> - 2022-11-04 17:36 +0100
                Re: Shared Communications Bus - RS-422 or RS-485 Rick C <gnuarm.deletethisbit@gmail.com> - 2022-11-04 10:11 -0700
                Re: Shared Communications Bus - RS-422 or RS-485 David Brown <david.brown@hesbynett.no> - 2022-11-05 11:58 +0100
                Re: Shared Communications Bus - RS-422 or RS-485 Rick C <gnuarm.deletethisbit@gmail.com> - 2022-11-05 09:55 -0700
                Re: Shared Communications Bus - RS-422 or RS-485 Stef <me@this.is.invalid> - 2022-11-07 11:00 +0100
                Re: Shared Communications Bus - RS-422 or RS-485 David Brown <david.brown@hesbynett.no> - 2022-11-07 15:46 +0100
                Re: Shared Communications Bus - RS-422 or RS-485 Rick C <gnuarm.deletethisbit@gmail.com> - 2022-11-07 08:05 -0800
                Re: Shared Communications Bus - RS-422 or RS-485 Richard Damon <Richard@Damon-Family.org> - 2022-11-07 19:02 -0500
                Re: Shared Communications Bus - RS-422 or RS-485 Rick C <gnuarm.deletethisbit@gmail.com> - 2022-11-07 17:15 -0800
                Re: Shared Communications Bus - RS-422 or RS-485 Richard Damon <Richard@Damon-Family.org> - 2022-11-08 06:54 -0500
                Re: Shared Communications Bus - RS-422 or RS-485 Rick C <gnuarm.deletethisbit@gmail.com> - 2022-11-08 05:50 -0800
                Re: Shared Communications Bus - RS-422 or RS-485 Clifford Heath <no_spam@please.net> - 2022-11-09 10:45 +1100
                Re: Shared Communications Bus - RS-422 or RS-485 Rick C <gnuarm.deletethisbit@gmail.com> - 2022-11-08 16:46 -0800
                Re: Shared Communications Bus - RS-422 or RS-485 Clifford Heath <no_spam@please.net> - 2022-11-09 22:32 +1100
                Re: Shared Communications Bus - RS-422 or RS-485 Rick C <gnuarm.deletethisbit@gmail.com> - 2022-11-09 04:53 -0800
                Re: Shared Communications Bus - RS-422 or RS-485 Richard Damon <Richard@Damon-Family.org> - 2022-11-09 23:45 -0500
                Re: Shared Communications Bus - RS-422 or RS-485 Rick C <gnuarm.deletethisbit@gmail.com> - 2022-11-09 21:42 -0800
                Re: Shared Communications Bus - RS-422 or RS-485 Rick C <gnuarm.deletethisbit@gmail.com> - 2022-11-04 08:40 -0700
                Re: Shared Communications Bus - RS-422 or RS-485 antispam@math.uni.wroc.pl - 2022-11-04 22:53 +0000
                Re: Shared Communications Bus - RS-422 or RS-485 Rick C <gnuarm.deletethisbit@gmail.com> - 2022-11-04 18:07 -0700
                Re: Shared Communications Bus - RS-422 or RS-485 antispam@math.uni.wroc.pl - 2022-11-05 03:46 +0000
                Re: Shared Communications Bus - RS-422 or RS-485 Rick C <gnuarm.deletethisbit@gmail.com> - 2022-11-05 02:09 -0700
                Re: Shared Communications Bus - RS-422 or RS-485 Stef <me@this.is.invalid> - 2022-11-07 10:40 +0100
                Re: Shared Communications Bus - RS-422 or RS-485 David Brown <david.brown@hesbynett.no> - 2022-11-05 12:47 +0100
                Re: Shared Communications Bus - RS-422 or RS-485 Rick C <gnuarm.deletethisbit@gmail.com> - 2022-11-05 10:23 -0700
                Re: Shared Communications Bus - RS-422 or RS-485 David Brown <david.brown@hesbynett.no> - 2022-11-05 19:57 +0100
                Re: Shared Communications Bus - RS-422 or RS-485 Rick C <gnuarm.deletethisbit@gmail.com> - 2022-11-05 13:42 -0700
                Re: Shared Communications Bus - RS-422 or RS-485 David Brown <david.brown@hesbynett.no> - 2022-11-06 11:55 +0100
                Re: Shared Communications Bus - RS-422 or RS-485 Rick C <gnuarm.deletethisbit@gmail.com> - 2022-11-06 05:56 -0800
                Re: Shared Communications Bus - RS-422 or RS-485 David Brown <david.brown@hesbynett.no> - 2022-11-06 21:53 +0100
                Re: Shared Communications Bus - RS-422 or RS-485 Rick C <gnuarm.deletethisbit@gmail.com> - 2022-11-07 01:58 -0800
                Re: Shared Communications Bus - RS-422 or RS-485 Stef <me@this.is.invalid> - 2022-11-07 11:26 +0100
                Re: Shared Communications Bus - RS-422 or RS-485 Rick C <gnuarm.deletethisbit@gmail.com> - 2022-11-07 02:39 -0800
                Re: Shared Communications Bus - RS-422 or RS-485 Stef <me@this.is.invalid> - 2022-11-07 12:07 +0100
                Re: Shared Communications Bus - RS-422 or RS-485 Rick C <gnuarm.deletethisbit@gmail.com> - 2022-11-07 08:25 -0800
                Re: Shared Communications Bus - RS-422 or RS-485 Stef <me@this.is.invalid> - 2022-11-07 17:57 +0100
                Re: Shared Communications Bus - RS-422 or RS-485 Rick C <gnuarm.deletethisbit@gmail.com> - 2022-11-07 09:50 -0800
                Re: Shared Communications Bus - RS-422 or RS-485 Stef <me@this.is.invalid> - 2022-11-07 21:30 +0100
                Re: Shared Communications Bus - RS-422 or RS-485 Rick C <gnuarm.deletethisbit@gmail.com> - 2022-11-07 14:27 -0800
                Re: Shared Communications Bus - RS-422 or RS-485 Stef <me@this.is.invalid> - 2022-11-08 00:07 +0100
                Re: Shared Communications Bus - RS-422 or RS-485 Rick C <gnuarm.deletethisbit@gmail.com> - 2022-11-07 16:50 -0800
                Re: Shared Communications Bus - RS-422 or RS-485 David Brown <david.brown@hesbynett.no> - 2022-11-08 09:02 +0100
                Re: Shared Communications Bus - RS-422 or RS-485 Richard Damon <Richard@Damon-Family.org> - 2022-11-08 06:58 -0500
                Re: Shared Communications Bus - RS-422 or RS-485 Richard Damon <Richard@Damon-Family.org> - 2022-11-06 18:34 -0500
                Re: Shared Communications Bus - RS-422 or RS-485 Paul Rubin <no.email@nospam.invalid> - 2022-11-06 15:37 -0800
                Re: Shared Communications Bus - RS-422 or RS-485 Richard Damon <Richard@Damon-Family.org> - 2022-11-06 19:18 -0500
                Re: Shared Communications Bus - RS-422 or RS-485 Rick C <gnuarm.deletethisbit@gmail.com> - 2022-11-07 02:07 -0800
                Re: Shared Communications Bus - RS-422 or RS-485 Rick C <gnuarm.deletethisbit@gmail.com> - 2022-11-07 02:03 -0800
                Re: Shared Communications Bus - RS-422 or RS-485 Stef <me@this.is.invalid> - 2022-11-07 11:55 +0100
                Re: Shared Communications Bus - RS-422 or RS-485 Rick C <gnuarm.deletethisbit@gmail.com> - 2022-11-07 08:18 -0800
                Re: Shared Communications Bus - RS-422 or RS-485 Stef <me@this.is.invalid> - 2022-11-07 18:20 +0100
                Re: Shared Communications Bus - RS-422 or RS-485 Rick C <gnuarm.deletethisbit@gmail.com> - 2022-11-07 10:26 -0800
                Re: Shared Communications Bus - RS-422 or RS-485 Stef <me@this.is.invalid> - 2022-11-07 22:04 +0100
                Re: Shared Communications Bus - RS-422 or RS-485 Rick C <gnuarm.deletethisbit@gmail.com> - 2022-11-07 14:54 -0800
                Re: Shared Communications Bus - RS-422 or RS-485 Paul Rubin <no.email@nospam.invalid> - 2022-11-07 15:14 -0800
                Re: Shared Communications Bus - RS-422 or RS-485 Rick C <gnuarm.deletethisbit@gmail.com> - 2022-11-07 16:57 -0800
                Re: Shared Communications Bus - RS-422 or RS-485 Rick C <gnuarm.deletethisbit@gmail.com> - 2022-11-07 07:51 -0800
              Re: Shared Communications Bus - RS-422 or RS-485 Rick C <gnuarm.deletethisbit@gmail.com> - 2022-11-03 08:29 -0700
                Re: Shared Communications Bus - RS-422 or RS-485 Paul Rubin <no.email@nospam.invalid> - 2022-11-03 10:36 -0700
                Re: Shared Communications Bus - RS-422 or RS-485 Rick C <gnuarm.deletethisbit@gmail.com> - 2022-11-03 12:32 -0700
                Re: Shared Communications Bus - RS-422 or RS-485 Paul Rubin <no.email@nospam.invalid> - 2022-11-03 13:08 -0700
                Re: Shared Communications Bus - RS-422 or RS-485 Rick C <gnuarm.deletethisbit@gmail.com> - 2022-11-03 13:40 -0700
                Re: Shared Communications Bus - RS-422 or RS-485 Paul Rubin <no.email@nospam.invalid> - 2022-11-03 16:50 -0700
                Re: Shared Communications Bus - RS-422 or RS-485 Rick C <gnuarm.deletethisbit@gmail.com> - 2022-11-03 21:10 -0700
                Re: Shared Communications Bus - RS-422 or RS-485 David Brown <david.brown@hesbynett.no> - 2022-11-04 11:13 +0100
                Re: Shared Communications Bus - RS-422 or RS-485 Rick C <gnuarm.deletethisbit@gmail.com> - 2022-11-04 08:52 -0700
                Re: Shared Communications Bus - RS-422 or RS-485 Paul Rubin <no.email@nospam.invalid> - 2022-11-04 20:03 -0700
                Re: Shared Communications Bus - RS-422 or RS-485 David Brown <david.brown@hesbynett.no> - 2022-11-05 17:25 +0100
                Re: Shared Communications Bus - RS-422 or RS-485 David Brown <david.brown@hesbynett.no> - 2022-11-04 11:01 +0100
  Re: Shared Communications Bus - RS-422 or RS-485 Paul Rubin <no.email@nospam.invalid> - 2022-11-02 15:00 -0700
    Re: Shared Communications Bus - RS-422 or RS-485 Rick C <gnuarm.deletethisbit@gmail.com> - 2022-11-02 16:31 -0700
      Re: Shared Communications Bus - RS-422 or RS-485 Paul Rubin <no.email@nospam.invalid> - 2022-11-02 17:36 -0700
        Re: Shared Communications Bus - RS-422 or RS-485 Rick C <gnuarm.deletethisbit@gmail.com> - 2022-11-02 21:58 -0700
          Re: Shared Communications Bus - RS-422 or RS-485 Paul Rubin <no.email@nospam.invalid> - 2022-11-03 09:57 -0700
            Re: Shared Communications Bus - RS-422 or RS-485 Rick C <gnuarm.deletethisbit@gmail.com> - 2022-11-03 12:02 -0700
              Re: Shared Communications Bus - RS-422 or RS-485 Paul Rubin <no.email@nospam.invalid> - 2022-11-03 12:53 -0700
                Re: Shared Communications Bus - RS-422 or RS-485 Rick C <gnuarm.deletethisbit@gmail.com> - 2022-11-03 13:33 -0700
  Re: Shared Communications Bus - RS-422 or RS-485 Dave Nadler <drn@nadler.com> - 2022-11-03 15:37 -0400
    Re: Shared Communications Bus - RS-422 or RS-485 Rick C <gnuarm.deletethisbit@gmail.com> - 2022-11-03 13:32 -0700
      Re: Shared Communications Bus - RS-422 or RS-485 Richard Damon <Richard@Damon-Family.org> - 2022-11-04 22:47 -0400
  Re: Shared Communications Bus - RS-422 or RS-485 chris <chris-nospam@tridac.net> - 2022-11-05 00:13 +0000

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