RS485 with SN75176 - TX collision

djordj schrieb:

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.

I think you should look at CAN transceivers instead.

Tilmann

So why not use the J1708 scheme? From Wiki (master of the infoverse, dispenser of all knowledge, first in every Google search) pedia on their J1708 page*:

"The hardware utilized are RS-485 transceivers wired for open collector operation through the use of a pullup and pulldown of the separate data lines. Transmission is accomplished by controlling the driver enable pin of the transceiver. This method allows multiple devices to share the bus without the need for a single master node. Collisions are avoided by monitoring the bus while transmitting the MID to ensure that another node has not simultaneously transmitted a MID with a higher priority."

Assuming that you have your polarities right, and that your driver chips all have the data pin tied in the same direction, you should be fine.

• formatting link

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.

This is exactly the solution we're about to try.

djordj schrieb:

But take care that even if you implement it that way (or with any other dom/rec scheme), the system will still hang if one of the devices stucks with TX active at dominant state.

To maintain bus control by the master, you'd need a four wire bus (RS-422), so that a collision on the master TX lines is impossible (since there are only device receivers connected to it). If a device stucks at TX active, the master still can send commands to all devices, trying to gain control again.

Another approach would be a hardware watchdog at each device, forcing the transmitter off if it's active for a too long time.

Tilmann

That's right, but at least transceivers are saved...

Right again, but for mechanical reasons we can't use a 4-wire bus... don't ask me why......

Or a sort of current sensing on transceiver supply.

Thanks!

Design your slave such that after a reset, the DE is inactive and use a watchdog timer. End of problem.

Meindert

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.

on

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.