MAX485 - Resistors

[Suraj143]

I have wired 12 MAX485 chips as in the diagram.Each MAX chip has its own microcontroller connected that I have not shown here.The 12 max chips placed closed by within 6 inches distance each other.The RS485 data comes from 1 Meter distance from a nearby panel. I used CAT5E cables.

Want to confirm below
1) Where to place Idle polarity resistors on RS485 bus? And its Values?
2) Do I need to place Impedence matching resistors as well? Where to put & its value?

Thanks

 

[Papabravo]

If your cabling is twisted pair, you should place 120Ω at each end of the cable run. Then check that each of the transmitters can produce acceptable levels in both the "0" state and the "1" state. You also need to check that each receiver can receive the transmitted message.

 

[Ian Rogers]

Idle resistors are normally 120 ~ 150 ohm. I place them on the master PCB, A pulled high and B pulled low.
The 120 ohm at the master across A and B (termination) I only ever do ->3 devices so that does me.. But you will need another 120 ohm at the very end as well (where your no. 6 is.).

 

[tumbleweed]

Idle resistors are normally 120 ~ 150 ohm

That's a lot lower than I normally use, but it depends on the transceivers, number of nodes, etc.

Here are some good references that discuss the failsafe resistor calculations:
RS-485: Passive failsafe for an idle bus

https://www.ti.com/lit/an/slyt324/slyt324.pdf

Failsafe in RS-485 data buses

https://www.ti.com/lit/an/slyt080/slyt080.pdf

Application Note 847 FAILSAFE Biasing of Differential Buses

https://www.ti.com/lit/an/snla031/snla031.pdf

RS-485 failsafe biasing- Old versus new transceivers slyt514

https://www.ti.com/lit/an/slyt514/slyt514.pdf

Some RS485 transceivers state that they don't require a failsafe bias, but you have to watch out...
on a lot of them the internal failsafe circuitry is defeated once you add 120ohm termination resistors.

 

[tumbleweed]

Just for reference, assuming you have both ends terminated in 120 ohms some rough ballpark calcs for the failsafe resistors (assuming 5V and ignoring the transceiver load):
680 ohms -> differential voltage = 200mV (usually the min spec for RS485)
510 ohms -> differential voltage = 280mV (which is about what I normally use)
120 ohms -> differential voltage = 1V (larger voltage, but higher power dissipation)

Actual numbers would have to include the transceiver loading effects for whatever parts you're using, as that will lower the differential voltage.