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Polarization of the MAX485 for the protocol RS485

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kkdelabaca

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max485 pull up

Hello, I'm very confused. I'm designing one board with the protocol RS485 and I need polarize the lines of the MAX485.

**broken link removed**

Obviously, the value of the terminal resistence is 120ohm, but what is the value of the pull-up and the pull-down resistors to polarize the line?

The pull-up resistor is connected to the pin 6 or 7 ? The description of the pin 6 is "Noninverting Receiver Input and Noninverting Driver Output" and the description of the pin 7 is "Inverting Receiver Input and Inverting Driver Output".

Looking for in google I have found a lot of different circuits and a lot of different polarizations.

Thanks!!
 

FvM

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rs485 pullup

Hello,
I think it's very easy. Pin 6 is actually non-inverting, as it can seen from the schematic symbol. That means, it has the same idle level as logic side of the transceiver, which is high. So if you want to set the line state to idle rather than break level, pin 6 must be pullup and pin 7 pulldown.

Regards,
Frank
 

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laktronics

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max485 and rs485 basics

Hi,
Yes as FVM told pin6 is pullup and pin7 pull down. Now regarding how much to pull up, I think it depends on how many nodes you have in the system, since the line driving gate has to sink the total current of all pullups. For the basic working, only terminators are required and that too only one pair. The receiver outputs are guranteed to remain in high state when its inputs are open and therefore pull up on inputs are not required. Similarly as I remember, a switched off node does not load the bus. By the way, what is the basic purpose of this polarisation?
Regards,
Laktronics
 

    kkdelabaca

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kkdelabaca

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max485 recommended pullup

Hi laktronics, the polarisation is necessary because the line are in hi impedance and is necessary put the line in know state.

Thanks, I put one pullup in the pin 6 and one pulldown in the pin7.
 

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resistores no max485

Hello,

I overlooked the pullup strength question. I use to have weak pullup/down, e. g. 10 k at every node, to prevent unconnected devices from detecting a break state, I think that's rather common.

For the end nodes, where the parallel termination, e. g. 120 ohms would be placed, some standards specify strong pullups and pulldowns. PROFIBUS has 390 ohms and replaces 120 ohms AB termination with 150 ohms, to have the nominal impedance with the combination of resistors.

Strong termination has the purpose of suppressing false start bits from signal reflections after disabling the transmitter. Cause PROFIBUS has a strict timing scheme for token rotation, false start bits could effectivly knock out the protocol.

Regards,
Frank
 

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