RS485 with SN75176 - TX collision

Jim Stewart wrote:
> Tim Wescott wrote:
>> djordj wrote:
>>> On Mar 15, 6:13 pm, Tilmann Reh <usenet2007nos...@autometer.de> wrote:
>>>
>>>> RS-485 is pure single master and not capable of a dom/rec scheme. The
>>>> drivers are push-pull, you can't even predict what the other devices on
>>>> the bus will receive during the collision.
>>> The network is a single-master bus, but I have to face problems
>>> deriving from faulty devices.
>>> What about if a slave get stuck in transmitting mode after its reply
>>> has been sent?
>>> The next time the master get bus control we have two transceivers in
>>> TX mode, and that's a problem.
>>>
>> That's a problem that you face with just about any practical 
>> networking choice.  Given careful design of the slave, it becomes 
>> equivalent to "what if a customer takes a hatchet to the main circuit 
>> board?" -- i.e., a truly fatal problem, but one that isn't likely.
>>
>> Design your slave so that it doesn't get stuck.  A suggestion has 
>> already been mooted for unsticking a hard-stuck slave; consider taking 
>> that, then design the _rest_ of your slave's hardware and software 
>> with a goal of never having it land in that particular safety net.
> 
> The only think I'd add is use a watchdog
> and write your TX driver very carefully.

Yup.

-- 
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com

On Mar 16, 1:39=A0pm, Jim Stewart <jstew...@jkmicro.com> wrote:
> Tim Wescott wrote:
> > djordj wrote:
> >> On Mar 15, 6:13 pm, Tilmann Reh <usenet2007nos...@autometer.de> wrote:
>
> >>> RS-485 is pure single master and not capable of a dom/rec scheme. The
> >>> drivers are push-pull, you can't even predict what the other devices =
on
> >>> the bus will receive during the collision.
> >> The network is a single-master bus, but I have to face problems
> >> deriving from faulty devices.
> >> What about if a slave get stuck in transmitting mode after its reply
> >> has been sent?
> >> The next time the master get bus control we have two transceivers in
> >> TX mode, and that's a problem.
>
> > That's a problem that you face with just about any practical networking
> > choice. =A0Given careful design of the slave, it becomes equivalent to
> > "what if a customer takes a hatchet to the main circuit board?" -- i.e.=
,
> > a truly fatal problem, but one that isn't likely.
>
> > Design your slave so that it doesn't get stuck. =A0A suggestion has
> > already been mooted for unsticking a hard-stuck slave; consider taking
> > that, then design the _rest_ of your slave's hardware and software with
> > a goal of never having it land in that particular safety net.
>
> The only think I'd add is use a watchdog
> and write your TX driver very carefully.

Yes, in fact, you should always choose wisely and do everything
carefully.  Is that ever bad advice???

Rick

Yes a fold back current limiter on the line driver, and a reset
integrator (WDT). the Hi Z would reset the fold back.