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RS485 Chip

R

Richard

Jan 1, 1970
0
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
 
J

James T. White

Jan 1, 1970
0
Richard said:
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).

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

tlbs

Jan 1, 1970
0
Richard said:
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).

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".
 
R

Richard

Jan 1, 1970
0
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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