RS485 with SN75176 - TX collision
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From: Tim Wescott on 16 Mar 2010 13:52 Jim Stewart wrote: > Tim Wescott wrote: >> djordj wrote: >>> On Mar 15, 6:13 pm, Tilmann Reh <usenet2007nos...(a)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
From: rickman on 17 Mar 2010 02:57 On Mar 16, 1:39 pm, Jim Stewart <jstew...(a)jkmicro.com> wrote: > Tim Wescott wrote: > > djordj wrote: > >> On Mar 15, 6:13 pm, Tilmann Reh <usenet2007nos...(a)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. Yes, in fact, you should always choose wisely and do everything carefully. Is that ever bad advice??? Rick
From: jacko on 17 Mar 2010 11:22
Yes a fold back current limiter on the line driver, and a reset integrator (WDT). the Hi Z would reset the fold back. |