ISO1042-Q1: TXD-RXD pull-up resistor values and position

Davide Tosatto

Part Number: ISO1042-Q1
Other Parts Discussed in Thread: ISO1042

Tool/software:

Good morning everyone,

in ISO1042-Q1 datasheet it is written: "The TXD pin is very weakly internally pulled up to VCC1. An external pull-up resistor should be used to make sure that the TXD pin is biased to recessive (high) level to avoid issues on the bus if the microprocessor does not control the pin and the TXD pin floats....". So, do you suggest adding this pull-up resistor? Which value should I choose? Should I place it at the TXD pin near the MCU or the ISO1042?

Moreover, could placing a pull-up resistor on the RXD pin benefit the communication?

Kind regards,

Davide

5 months ago

+1 Andrew J 5 months ago

TI__Mastermind 29185 points

Hello Davide,

An external pull up resistor is only recommended if the output driving the TXD pin will cause the pin to float or if TXD can become tri-stated.

So, do you suggest adding this pull-up resistor?
- Typically, a pull up is not needed. It can help if the TXD pins can float in your application.
Which value should I choose?
- ~1k-ohm is usually sufficient
Should I place it at the TXD pin near the MCU or the ISO1042?
- The placement of the resistor does not matter it can be by either device.
Moreover, could placing a pull-up resistor on the RXD pin benefit the communication?
- RXD is a push-pull output, therefore there is no real benefit to using a pull-up resistor on this pin.

Best,
Andrew

0 Davide Tosatto 4 months ago in reply to Andrew J

Prodigy 100 points

Hello Andrew,

Thank you so much, you have been very clear.

Best regards,

Davide

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ISO1042-Q1: TXD-RXD pull-up resistor values and position