RS485 & SN75176 do I need a pullup, pulldown or nothing on RX?

[blueroomelectronics]

As the title says, I've notice most schematics have nothing on the RX (input to micro) line, when the unit enters transmit more does RO (SN75176 pin 1) just float? or is it internally pulled up or down? If it float what prevents glitching on the RX input on the micro?
**broken link removed**

 

[mneary]

RO floats. It's probably a good idea to turn on the internal pullup in your micro controller.

 

[mister_e]

This won't solve the possible problem, as the noise would be coming from the Receiver input.

Maxim datasheet, show that 120 ohm value when you connect 2 or more device together. Figure 21, 22.

Assuming it's a balanced input... it shouldn't cause any problem... but yeah, the noise have to be the same on both pins at same time....i don't think it would harm to use a high value (~10k) for safety sake.

 

[blueroomelectronics]

RO floats. It's probably a good idea to turn on the internal pullup in your micro controller.

The RX pin does not have an internal pullup. Just wondering if it should be pulled up or pulled down during idle.

 

[mister_e]

Depending how you implement things. If your software expect a certain 'Serial format Header' and the format is not respected... it just ignore/discard it.

And... you'll be lucky if any noise meet it

 

[3v0]

There is more then one way to setup a RS485 network. The following is what worked for me. See Circuit Cellar Article figure 1 for a schematic.

Once on each network there should be two ~560R resistors that tie the A and B lines High and Low when the network is idle. I think this answers your question?

The ~120R value is generally used between A and B at the first and last drop (or cable ends) to kill reflections on the wire. Aka termination resistors.

I like the MAX1487 which is an ESD > = +/-15kV protected driver.

 

[blueroomelectronics]

The RS485 driver side is well documented. I was wondering about the RX & RO stuff at the TTL side. The RS485 chips float RO when RE is high so I left the resistor on the PCB anyway, I'll omit it from the BOM if its not required. I just sent off the artwork to the PCB house.

 

[3v0]

After a second read I agree with mister_e.