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What do these RS 485 modules with flow control actually do?

Triode

Active Member
I see these modules showing up now, they say they have "automatic flow control"

to me that sounds like if the line is busy they would receive, and then buffer any messages on their TX line until it's not busy. But I don't see the electronics on here to do that (it's a hex inverting buffer and a normal RS485 chip like the Max485 or SP485)

None of the pages that list it have much detail on what exactly it does. I can see that it doesn't have the usual enable pins, and instead of DriveIn and Receiver Out it just has TX and RX pins.

What I'm hoping is that you can just put this on a multidrop bus and send and receive like you do with basic serial, but I don't see how that is accomplished with these components.

Any ideas?
Thanks!

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JimB

Super Moderator
Most Helpful Member
My first impression is that it will just support a simple point to point link, hardware handshaking could be a bit iffy.

I cannot see these working reliably on a multi-drop line.

I may be wrong, I sometimes am!

JimB
 

Triode

Active Member
Thanks. That's the impression I get. When I looked up what chip they added in addition to the RS485 driver I figured it may buffer messages if the line was busy, but it's just an inverter. I don't see any components to control timing or buffer messages.
 

ChrisP58

Well-Known Member
Most Helpful Member
This TI document shows what's probably going on. The RS485 transceiver is normally in receive mode. When the local TX line goes active, the inverters detect the TX state and the transceiver changes direction. There is probably a bit of an RC timer to keep it in transmit mode for the duration of the message..

TI does that with a 555 instead of a few inverters.

https://www.ti.com/lit/ug/tidubw6/tidubw6.pdf
 
Last edited:

Triode

Active Member
This TI document shows what's probably going on. The RS485 transceiver is normally in receive mode. When the local TX line goes active, the inverters detect the TX state and the transceiver changes direction. There is probably a bit of an RC timer to keep it in transmit mode for the duration of the message..

TI does that with a 555 instead of a few inverters.

https://www.ti.com/lit/ug/tidubw6/tidubw6.pdf
Oh, so it's just removing the need for the usual pins to enable the driver and disable the receiver by having an incoming signal on the TX pin do that automatically. One would still need all of the usual bus arbitration and timing. I can see how that's useful. I sort of expected it wouldn't be some awesome do all solution. Could be useful in some cases though.
 

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