RS485 Chip

Nov 16, 2005 4 Replies
R

I have a 75176 RS485 Transceiver chip here. Looking at the application notes, it seems pretty straight forward to hook up, however I'm wondering.



Should I add a Jumper and a termination 120 ohm Resistor Between A and B? Should I install any pull ups or Pull downs on the 485 data signal lines? Does A pull down and B pulls Up?



Just wanting to do this 1 time. I will be using it strickly in half duplex (For ModBus Mode).


Thanks for any pointers. Richard


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D

Don't add any pull up or pull down

There needs to be a termination resistor at the end of the cable run.

Dan

-- Dan Hollands

1120 S Creek Dr Webster NY 14580
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J

Check the TI, National, Maxim and Linear Technology websites for application notes related to RS485, termination and failsafe.

James T. White
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T

You need to terminate the transmission line (across A and B) with the characteristic impedance of the twisted pair transmission line you are using. If that characteristic impedance is unknown, any value from 98 to 125 Ohms will usually do the trick.

The only reason to use pullup and/or pulldown resistors is for ESD protection of the 75176. Values between 10 kOhms and 50 kOhms will add protection to the RS485 receiver. You could also use switching diodes or MOV devices. If you pullUP the non-inverted input, and pullDOWN the inverted input, then the fail-safe value of the logic output will be "TRUE".

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R

Thanks for all the advice. I noticed while looking thru many schematics that each person seems to implement the 485 chips in many different ways. I think I will put the termination, pull up, and pull downs all on there being jumper selectable and that way I can have it either way.

Richard

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