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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 2022-11-02 14:27 +0100
Organization A noiseless patient Spider
Message-ID <tjtr7f$16c7a$1@dont-email.me> (permalink)
References <6b8c6b92-a2ca-4849-ba68-f03060bc41fcn@googlegroups.com> <tjtd7g$145er$1@dont-email.me> <tjti8a$129ep$1@dont-email.me>

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On 02/11/2022 11:54, pozz wrote:
> Il 02/11/2022 10:28, David Brown ha scritto:
>> On 02/11/2022 06:28, Rick C wrote:
> 
>> RS-485 is easy from a PC using appropriate USB devices.  We make heavy 
>> use of FTDI's chips and cables - they handle the drive enables for the 
>> RS-485 automatically so that from the PC side you just read and write 
>> to the serial port.
>>
>> The reception of the last byte from a slave is not finished until the 
>> stop bit has been properly received by the master - that means at 
>> least half-way through the sending of the stop bit.  Then there is a 
>> delay before the data gets sent back to the host PC, a delay through 
>> the kernel and drivers before it reaches the user program, time for 
>> the program to handle that message, time for it to prepare the next 
>> message, delays through the kernel and drivers before it gets to the 
>> USB bus, latency in the USB device that receives the USB message and 
>> then starts transmitting.  There can be no collision unless all that 
>> delay is less than half a bit time.  And no matter how fast your 
>> computer is, you are always going to need at least one full USB 
>> polling cycle for all this, which for USB 2.0 is 0.125 us.  That means 
>> that if you have a baud rate of 16 kbaud or higher, there is no 
>> possibility of a collision.
> 
> However this depends on the speed of slave too, because it could be slow 
> to move *its* direction from TX to RX. If the master starts transmitting 
> after the stop bit from the slave, but *before* it changes *its* 
> direction from TX to RX, the first bytes could be corrupted.

True, and it that is an important point.

> 
> Unfortunately, not all UARTs in MCUs are able to drive automatically the 
> DE (Drive Enable) signal, so it sometimes happens that DE is a normal GPIO.
> If you are lucky, you have the TXC (transmit complete) interrupt that 
> fires *after* stop bit is transmitted, a safe time to move DE signal.

I think the OP has FPGA's for the slave side of the equation, so there 
should not be a delay in switching their drivers off after the last byte 
is sent.  Even if it is a microcontroller and has no hardware control 
for the DE line, pretty much any half-decent microcontroller from this 
century has a TXC interrupt and can react and turn off the driver within 
a few microseconds.  I am assuming he is not trying to do this project 
using PIC's or 8051's !

> 
> In this case interrupt delay is short, but you could have other active 
> interrupts that occasionally could delay the TXC interrupt for some time.
> 

If this kind of thing is a risk, then it's not hard to put a short delay 
on the PC side between receiving a reply and sending out the next 
telegram.  But it's good that you brought it up, so that the OP can 
decide if it /is/ a risk.

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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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