Help guys...communication issue, I think?

[kevlar129bp]

Hey Henrik,

Thanks for your reply. I looked at the Analog datasheet and the only thing that jumped out at me was their termination schema. My bus is built exactly like this one: http://datasheets.maximintegrated.co...80-MAX3089.pdf (page 20). Did I miss something in the datasheet, or is the 3081 only intended for point to point comms? If so, it sure sounds like a board rework is in order huh. bummer.

Thanks again,
Chris

[HenrikOlsson]

Hi Chris,

My bus is built exactly like this one: http://datasheets.maximintegrated.co...80-MAX3089.pdf (page 20)

Yes and no..... The actual bus may be built the same but your tranceiver chips (the MAX3081) doesn't have the DE-pin which you see in the schematic on that page and which you need for multidrop networks so yes, the MAX3081 is basically intended for point-to-point.

If the tranceiver has no way to turn of its driver then it will always drive the bus to one state or the other. Then, when another tranceivers driver is hooked across the same pair of wires and IT too tries to drive the bus to one state or the other there will be a conflict. One tries to drive it high while the other tries to drive it low - for example. On the chips with the DE-pin (Driver Enabled) you can disable the driver outputs all together.

/Henrik.

[kevlar129bp]

Thanks Henrik,

Bummer, I was afraid you were going to tell me that I wonder why the 887 board works with both boards on the bus though? That's the part that stumps me. Do you have any insight on that? Just seems that both of them would crash, not just one...

Chris

[Demon]

If you're stuck with those drivers, what if the PIC controls power to the drivers? Do they take time to "boot up"?

Robert

[HenrikOlsson]

Robert,
The chips are tranceivers...if you power them down they won't be able to receive either.
The PIC will never get any queries to send any data because the receiver is powered down, you might as well throw out the tranceiver all together... ;-)

Chris,
Can't say why it appears to work in some situations. Perhaps one driver is slightly "stronger" than the other (don't think so though).

/Henrik.

[kevlar129bp]

Hey guys,

I actually did give the power down thing a thought for a moment, but as Henrik pointed out...1 step forward, two steps back

No problem on the mysterious 887 board operation, as I'm reworking the boards now. I do, however, have one more quick question for ya Henrik... I should be ok to still run the MAX3081 on the 232 to 422 master board, correct? My thought is, I shouldn't need to (or wouldn't want to) enable or disable the Rx and Tx lines at the master... Is my thinking correct on that one?

BTW: The new boards will have the MAX3086 on 'em, of course with the sacrifice of my heartbeat led output Oh well, small price to pay.

Thanks again guys,
Chris

[HenrikOlsson]

Hi Chris,

I should be ok to still run the MAX3081 on the 232 to 422 master board, correct?

Lets stop and think about that for a minute....

Finished thinking?

If ANY one of the drivers on the bus (doesn't matter which one) is enabled it will "occupy" the bus. There can only be ONE enabled driver on the bus at any given time or they will end up fighting each other.

All receivers on all devices shouls be enabled at all times. The transmitter on each device should be enabled when, and only when, that specific device is about to put data onto the bus. If two or more transmitters are enabled at the same time there will be conflicts.

/Henrik.

EDIT: OK, hold on, brainfart on my behalf here. You're running full duplex so please disregard the above. Yes, you can keep the transmitter on the RS232<->RS422 converter enabled all the time. Doh.....