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# ISO15: ISO15 internal pull-up down to calculate maximum number of ISO15 on a bus

Part Number: ISO15

Hello,

We want to use ISO15 for our application and I have to calculate the maximum number of ISO15 on a RS485 bus to be compliant to RS485 standard (375ohm max of common mode load) and also calculate the terminal resistors as it is describe in slyt514.pdf application note.

In the datasheet we can see that there are Rpullup and Rpulldown of 36kohms (page 17) on each A and B input and there is also a current generator on the B input but I cannot deduce the input equivalent resistor of the receiver.

So would it be possible to have the equivalent input resistors of the receiver stage to help me to calculate the maximum number of ISO15 that we can put on the RS485 lines.

Thanks a lot!

Regards.

• Cedric,

The differential input resistance (RID) for the A and B inputs is a minimum of 48k. This is stated under the Receiver Electrical Characteristics on page 7 of the datasheet. Also, have you check out the ISO1410? It's the recommended alternative to the ISO15 and is pin-to-pin compatible.

Respectfully,

Lucas

• In reply to Lucas Schulte9:

Hello Lucas,

Yes I saw résistance between A and B is 48k min but it is not very clear for me as it is also written in the device IO schematic page 17 that on each inputs there is 180kohms resistors....

So can I conclude for my calculations that input receiver is polarized with Rpull_up=36kohms, Rpull_down=36kohms and Rab=48kohms as below (would be very surprising to get such polarization)?

I took a look at ISO1410, it has better isolation but in our case ISO15 isolation is sufficient. Is the ISO15 NRND?

Thanks.

Regards

• In reply to cedric LEBOEUF:

Cedric,

Generally finding out how many transceivers you can have on bus line is determined by its Unit Load parameter.A 375ohm load is a standard unit load which could have 32 transceivers on it. The ISO15 is 1/8 unit load which means it can support 256 other 1/8 load transceivers. This is generally how one accounts for how many transceivers one can have on a bus rather than looking at input resistance.

Respectfully,
Lucas
• In reply to Lucas Schulte9:

Hello Lucas,

Thanks for theses informations.

The problem is when we use this ISO15 with other driver none fail safe, I would have liked to do some theoretical calculations but anyway, I will  try to manage this by tests.

Thanks.

Regards.

• In reply to cedric LEBOEUF:

Hi Cedric,

Hopefully you are well. This thread has been inactive for the past few weeks and will be closed. Do you have additional information from calculations or tests? If so, feel free to reply to this thread or you may create a related thread using the yellow button in the top right corner of this window.

Thank you for choosing TI parts!

Respectfully,
Manuel Chavez