Two-wires RS485 and the driver enable problem

From	pozzugno <pozzugno@gmail.com>
Newsgroups	comp.arch.embedded
Subject	Two-wires RS485 and the driver enable problem
Date	2014-10-13 13:58 +0200
Organization	A noiseless patient Spider
Message-ID	<m1geor$bt4$1@dont-email.me> (permalink)

Show all headers | View raw

I have a multi-drop two-wires RS485 bus.  One node is the master and all 
the others are slaves.  The master is the only node that is authorized 
to initiate a transmission, addressing one slave.  The addressed slave 
usually answers to the master.

The bus is half-duplex, so every node disables the driver.  Only THE 
node that transmits data on the bus enables the driver and disables it 
as soon as it can, just after the last byte.  An interrupt (transmit 
complete) usually triggers when the last byte is totally shifted out, so 
the driver can be disabled immediately.

Of course, other interrupts can be triggered.  What happens when 
interrupt X (whatever) triggers just before the "transmit complete" 
interrupt?  The result is the ISR X is called, postponing the execution 
of "transmit complete" ISR.  The RS485 driver will be disabled with a 
certain amount of delay.  In the worst case, the driver could be 
disabled with a delay that is the sum of the duration of all ISRs that 
could trigger.
[In this scenario, I think of ISRs that can't be interrupted by a higher 
priority interrupt.]

If a node on the bus is very fast and starts transmitting (the master) 
or answering (one slave) immediately after receving the last byte, but 
when the previous transmitting node is executing other ISRs, the final 
result is a corrupted transmission.

What is the solution? I think the only solution is to define, at the 
design time, a minimum interval between the receiving of the last byte 
from one node and the transmission of the first byte.  This interval 
could be in the range of 100 microseconds and should be calibrated on 
the sum of duration of *all* ISRs of *all* nodes on the bus.  It isn't a 
simple calculation.

Moreover, implementing a short "software" delay in the range of some 
microseconds isn't a simple task.  An empty loop on a decreasing 
volatile variable is a solution, but the final delay isn't simple to 
calculate at the design time, and it could depend on the compiler, 
compiler settings, clock frequency and so on.  Use an hw timer only for 
this pause?

How do you solve this problem?

[I know there are some microcontrollers that automatically (at the 
hw-level) toggle an output pin when the last byte is totally shifted 
out, but I'm not using one of the them and they aren't so common.]

Back to comp.arch.embedded | Previous | Next — Next in thread | Find similar | Unroll thread

Thread

Two-wires RS485 and the driver enable problem pozzugno <pozzugno@gmail.com> - 2014-10-13 13:58 +0200
  Re: Two-wires RS485 and the driver enable problem Wouter van Ooijen <wouter@voti.nl> - 2014-10-13 14:29 +0200
    Re: Two-wires RS485 and the driver enable problem pozzugno <pozzugno@gmail.com> - 2014-10-13 14:38 +0200
      Re: Two-wires RS485 and the driver enable problem Wouter van Ooijen <wouter@voti.nl> - 2014-10-13 15:28 +0200
        Re: Two-wires RS485 and the driver enable problem Don Y <this@is.not.me.com> - 2014-10-13 07:21 -0700
          Re: Two-wires RS485 and the driver enable problem Wouter van Ooijen <wouter@voti.nl> - 2014-10-13 16:46 +0200
            Re: Two-wires RS485 and the driver enable problem Don Y <this@is.not.me.com> - 2014-10-13 08:02 -0700
        Re: Two-wires RS485 and the driver enable problem pozzugno <pozzugno@gmail.com> - 2014-10-13 16:59 +0200
          Re: Two-wires RS485 and the driver enable problem Don Y <this@is.not.me.com> - 2014-10-13 08:08 -0700
            Re: Two-wires RS485 and the driver enable problem pozz <pozzugno@gmail.com> - 2014-10-14 07:48 +0200
              Re: Two-wires RS485 and the driver enable problem Don Y <this@is.not.me.com> - 2014-10-13 23:28 -0700
                Re: Two-wires RS485 and the driver enable problem pozzugno <pozzugno@gmail.com> - 2014-10-14 09:20 +0200
                Re: Two-wires RS485 and the driver enable problem Don Y <this@is.not.me.com> - 2014-10-14 06:09 -0700
              Re: Two-wires RS485 and the driver enable problem David Brown <david.brown@hesbynett.no> - 2014-10-14 10:36 +0200
          Re: Two-wires RS485 and the driver enable problem Wouter van Ooijen <wouter@voti.nl> - 2014-10-13 18:14 +0200
      Re: Two-wires RS485 and the driver enable problem Tim Wescott <seemywebsite@myfooter.really> - 2014-10-13 16:30 -0500
  Re: Two-wires RS485 and the driver enable problem David Brown <david.brown@hesbynett.no> - 2014-10-13 14:47 +0200
    Re: Two-wires RS485 and the driver enable problem pozzugno <pozzugno@gmail.com> - 2014-10-13 14:56 +0200
      Re: Two-wires RS485 and the driver enable problem David Brown <david.brown@hesbynett.no> - 2014-10-13 16:13 +0200
      Re: Two-wires RS485 and the driver enable problem Don Y <this@is.not.me.com> - 2014-10-13 08:06 -0700
        Re: Two-wires RS485 and the driver enable problem pozzugno <pozzugno@gmail.com> - 2014-10-14 08:50 +0200
          Re: Two-wires RS485 and the driver enable problem Don Y <this@is.not.me.com> - 2014-10-14 14:34 -0700
  Re: Two-wires RS485 and the driver enable problem Richard Damon <Richard@Damon-Family.org> - 2014-10-13 09:11 -0400
  Re: Two-wires RS485 and the driver enable problem rickman <gnuarm@gmail.com> - 2014-10-13 10:53 -0400
    Re: Two-wires RS485 and the driver enable problem pozzugno <pozzugno@gmail.com> - 2014-10-13 17:03 +0200
  Re: Two-wires RS485 and the driver enable problem Grant Edwards <invalid@invalid.invalid> - 2014-10-13 15:20 +0000
    Re: Two-wires RS485 and the driver enable problem pozzugno <pozzugno@gmail.com> - 2014-10-16 09:01 +0200
      Re: Two-wires RS485 and the driver enable problem Tauno Voipio <tauno.voipio@notused.fi.invalid> - 2014-10-16 10:19 +0300
      Re: Two-wires RS485 and the driver enable problem rickman <gnuarm@gmail.com> - 2014-10-16 05:02 -0400
  Re: Two-wires RS485 and the driver enable problem Don Y <this@is.not.me.com> - 2014-10-13 08:17 -0700
    Re: Two-wires RS485 and the driver enable problem Don Y <this@is.not.me.com> - 2014-10-13 08:19 -0700
    Re: Two-wires RS485 and the driver enable problem pozzugno <pozzugno@gmail.com> - 2014-10-14 09:28 +0200
      Re: Two-wires RS485 and the driver enable problem Don Y <this@is.not.me.com> - 2014-10-14 06:09 -0700
        Re: Two-wires RS485 and the driver enable problem rickman <gnuarm@gmail.com> - 2014-10-14 14:28 -0400
    Re: Two-wires RS485 and the driver enable problem rickman <gnuarm@gmail.com> - 2014-10-14 04:36 -0400
  Re: Two-wires RS485 and the driver enable problem Les Cargill <lcargill99@comcast.com> - 2014-10-13 12:35 -0500
    Re: Two-wires RS485 and the driver enable problem langwadt@fonz.dk - 2014-10-13 15:02 -0700
      Re: Two-wires RS485 and the driver enable problem Don Y <this@is.not.me.com> - 2014-10-13 17:36 -0700
        Re: Two-wires RS485 and the driver enable problem Les Cargill <lcargill99@comcast.com> - 2014-10-13 21:53 -0500
          Re: Two-wires RS485 and the driver enable problem Don Y <this@is.not.me.com> - 2014-10-13 20:24 -0700
  Re: Two-wires RS485 and the driver enable problem upsidedown@downunder.com - 2014-10-13 22:02 +0300
    Re: Two-wires RS485 and the driver enable problem Grant Edwards <invalid@invalid.invalid> - 2014-10-13 19:09 +0000
      Re: Two-wires RS485 and the driver enable problem glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-10-13 20:58 +0000
        Re: Two-wires RS485 and the driver enable problem Grant Edwards <invalid@invalid.invalid> - 2014-10-13 21:08 +0000
          Re: Two-wires RS485 and the driver enable problem langwadt@fonz.dk - 2014-10-13 14:40 -0700
    Re: Two-wires RS485 and the driver enable problem Dave Nadler <drn@nadler.com> - 2014-10-14 09:58 -0700
      Re: Two-wires RS485 and the driver enable problem rickman <gnuarm@gmail.com> - 2014-10-14 14:20 -0400
        Re: Two-wires RS485 and the driver enable problem David Brown <david.brown@hesbynett.no> - 2014-10-14 23:35 +0200
          Re: Two-wires RS485 and the driver enable problem rickman <gnuarm@gmail.com> - 2014-10-15 02:00 -0400
            Re: Two-wires RS485 and the driver enable problem David Brown <david.brown@hesbynett.no> - 2014-10-15 09:47 +0200
              Re: Two-wires RS485 and the driver enable problem rickman <gnuarm@gmail.com> - 2014-10-16 01:30 -0400
                Re: Two-wires RS485 and the driver enable problem David Brown <david.brown@hesbynett.no> - 2014-10-16 20:46 +0200
                Re: Two-wires RS485 and the driver enable problem Dave Nadler <drn@nadler.com> - 2014-10-16 12:00 -0700
            Re: Two-wires RS485 and the driver enable problem upsidedown@downunder.com - 2014-10-15 11:01 +0300
            Re: Two-wires RS485 and the driver enable problem Grant Edwards <invalid@invalid.invalid> - 2014-10-15 14:24 +0000
              Re: Two-wires RS485 and the driver enable problem rickman <gnuarm@gmail.com> - 2014-10-16 01:17 -0400
                Re: Two-wires RS485 and the driver enable problem pozzugno <pozzugno@gmail.com> - 2014-10-16 08:36 +0200
                Re: Two-wires RS485 and the driver enable problem rickman <gnuarm@gmail.com> - 2014-10-16 05:05 -0400
                Re: Two-wires RS485 and the driver enable problem pozzugno <pozzugno@gmail.com> - 2014-10-16 12:54 +0200
                Re: Two-wires RS485 and the driver enable problem Grant Edwards <invalid@invalid.invalid> - 2014-10-16 14:16 +0000
    Re: Two-wires RS485 and the driver enable problem upsidedown@downunder.com - 2014-10-15 09:09 +0300
      Re: Two-wires RS485 and the driver enable problem Grant Edwards <invalid@invalid.invalid> - 2014-10-15 14:26 +0000
  Re: Two-wires RS485 and the driver enable problem glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-10-13 20:55 +0000
  Re: Two-wires RS485 and the driver enable problem Tim Wescott <seemywebsite@myfooter.really> - 2014-10-13 16:37 -0500
    Re: Two-wires RS485 and the driver enable problem pozzugno <pozzugno@gmail.com> - 2014-10-14 09:37 +0200
  Re: Two-wires RS485 and the driver enable problem Arlet Ottens <usenet+5@c-scape.nl> - 2014-10-14 20:44 +0200

csiph-web