Pull resistors on the Isolator interface

[FreshmanNewbie]

I am currently working with the IL4822 IC (datasheet) and have some questions regarding the handling of its bus driver/receiver pins.

• Pins 11 and 12 function as the non-inverting bus driver/receiver.
• Pins 13 and 14 function as the inverting bus driver/receiver.

I have the following queries:

1. On page 23, Figure 31, the diagram suggests connecting pins 11 and 12 together and pins 13 and 14 together. What is the purpose of this connection?
2. The datasheet does not mention any recommendations regarding pull-up or pull-down resistors for these pins. Should any be used, and if so, what would be the appropriate configuration? Why should I place pull resistors if required?

 

[betwixt]

In a classic RS485 configuration the transmit and receive pins are normally connected together because all data is transferred over two wires. It is a half-duplex configuration, data is either sent or received but not at the same time from a single device. The DE and -RE pins control whether the driver or the receiver is active and it is quite normal for the master device to link them together too. They work in opposite polarity so one control signal decided which way the data flows.

Pull-up resistors may not be needed in two wire configuration although I find it advisable to add a pull-up on one wire and a pull-down on the other so if nothing is driving the bus it adopts a defined logic state. When used in RS422 configuration with 4 wires you should add a termination resistor at the receiving end (only) to prevent reflections when high speed is used, particularly when the wiring length is long.

Brian.

 

[FvM]

Using or noting using termination resistors and possibly pull-up and pull-down resistors for RS-485 interfaces as application dependant and no driver specific question.
Differential termination is required for long bus lines or high communication speed. Without pull-up/down resistors, the bus-state is undefined if all transmitters are disabled. Some receivers have problems to detect start of transmission in this case. It's essentially a software problem, but if you have no access to the UART software driver, this can be a big problem.

 

[FreshmanNewbie]

Using or noting using termination resistors and possibly pull-up and pull-down resistors for RS-485 interfaces as application dependant and no driver specific question.
Differential termination is required for long bus lines or high communication speed. Without pull-up/down resistors, the bus-state is undefined if all transmitters are disabled. Some receivers have problems to detect start of transmission in this case. It's essentially a software problem, but if you have no access to the UART software driver, this can be a big problem.

I have added 1k pull-up resistor on pins 11 and 12. 1k Pull down on 13 and 14. Would it cause any issues? Any specific value apart from 1k is recommended?

 

[KlausST]

Hi,

I will never understand ...
why some people are unable to do the simplest research on their own.

It´s a simple RS485 interface, used for decades ... so why not simply look up at wikipedia: https://en.wikipedia.org/wiki/RS-485
It basically answers all questions.

Besides this any internet search for: "RS485 tutorial", "RS485 basics", "how to RS485" or similar shows thousands documents, internet sites, even videos ...

Klaus

 

[FvM]

1k sounds good. Do you also use differential termination, e.g. 120 ohm? In this case, 1k will still guarantee a voltage difference A-B > 200 mV, required to signal "mark" state for idle bus.

 

[KlausST]

Hi,

to add to my post before...

even the manufacturer of these ICs provides:
* application notes
* evaluation boards
* design examples
* schematics
* an application center
* an FAQ section ....

when I would articulate the behaviour of the OP ... it probably would be called rude.

Klaus

 

[FvM]

O.k., e.g.: http://www.nve.com/Downloads/ab6.pdf

I have however seen less reflected questions at Elementary Electronic Questions, such questions are welcome here, you may ignore it silently.

 

[KlausST]

Fair enough. You posted a broken link .. to prove me wrong.
Still..

https://www.nve.com/Downloads/ab6.pdf

does work - at least for me.

.. and the other thousands where the link does work... there are more than plenty of alternatives
It´s not that one needs to spend more than 5 seconds or it takes special effort.

****
You mentioned forum sections:
What are the other forum sections for, if this member posts almost everything in the "Elementary Electronics Questions"?
What do forum rules say about sections?

I remember very well the times when moderators denied membership to a person who asked st**id questions and was unwilling to learn...
There´s nothing against a single unelaborated question ... but "unwilling to learn" is a different story.

Why is this? Times have changed? or different persons are treated differently?
You know I´ve multiple times critizised this double standards before where forum rules apply to some people but not to other people.

You are free to call me a liar if one of my above statements is not true.
And you have the power to silence me .. but from history we should have learned that "silence" usually supports the wrong people.
This is not meant political, it applies to familiy, school, work, sports, forum ...

Klaus