SN65HVD888: SN65HVD888 pull up resistor

Stanley Xu

Part Number: SN65HVD888

Hi engineers,

I'm recommending SN65HVD888 to customers and there are some technical details that need your help to confirm.

As we know, SN65HVD888 equipped with polarity correction function which need pull up resistor to ensure high voltage level when Bus IDLE As directed by A datasheet .

Background：

1.The equipment developed by the customer belongs to 485 slave, the pull up resistor in master side is uncertain, so whether the polarity correction function cant work is uncertain.

2.The communication standard adopted by the customer is MODBUS, I don't know much about MODBUS, he said that in the Modbus protocol, each device can be either a master or a slave.

question:

I want to put pull-up resistor at customer's equipment side(slave node) , maybe it can ensure high voltage level when Bus IDLE and ensure polarity correction function can work?

If it works, there are any risks during it? such as too much pull-up resistor decrease number of nodes that can be connected to the bus.

over 4 years ago

+1 Clemens Ladisch over 4 years ago

Guru 318460 points

A pull-up resistor at the slave node is pointless. If the cross-wire fault is somewhere between the master node and the slave node, then the pull-resistor is on the wrong line. (And if the master node has a pull-down resistor, it would form a voltage divider with this pull-up resistor.)

It does not matter which node you call "master" or "slave"; what matters is that there is one node that does not have polarity correction, and that uses these resistors to impose the meaning of the A/B lines on all other nodes.

+1 Stanley Xu over 4 years ago in reply to Clemens Ladisch

TI__Intellectual 1870 points

Hi Ladisch,

Thank you very much, your answer deepen my understanding of this device, so as long as at least one right node possess pull-up resistor, the wrong sn65hvd888 node in the bus can implement polarity correction automatically, Is my understanding correct？

+1 Clemens Ladisch over 4 years ago in reply to Stanley Xu

Guru 318460 points

Yes.

The polarity correction works only if the cross connection is somewhere between the pull-up resistor and the HVD888.

+1 Hao L over 4 years ago in reply to Clemens Ladisch

TI__Mastermind 25705 points

If you're worried about load reduction, please check out this blog.

+1 Stanley Xu over 4 years ago in reply to Clemens Ladisch

TI__Intellectual 1870 points

thank you Ladisch

+1 Stanley Xu over 4 years ago in reply to Hao L

TI__Intellectual 1870 points

thank you L

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SN65HVD888: SN65HVD888 pull up resistor