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Forum Index : Microcontroller and PC projects : Spa Controller

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Tinine
Senior Member

Joined: 30/03/2016
Location: United Kingdom
Posts: 268
Posted: 05:31pm 01 Dec 2019
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  Turbo46 said  If I am understanding the above circuit correctly, the driver IC is only enabled when the Serial Out (Tx) pin is low and disabled when it is high (plus a short delay caused by the R/C circuit). As it says "a driver is enabled for only an average of half the time required to transmit a word".

If the data was 'FF' Hex, the driver would be only enabled for the START bit and disabled for the rest of the word except for a short pulse at the beginning or the first '1' bit. The RS485 A/B signal lines would be tri-state for every '1' bit transmitted.

Surely this arrangement would be highly susceptible to noise? If it works at all I can't see how it would work reliably - especially over long distances. The board shown in Phil23's first photo is a little more complex.

Not explained above is that the two Schmidt trigger gates in series with the Tx and Rx lines will add a little propagation delay to the signals so that little (if any) of the START and other '0' bits are lost.

Please correct me if I am wrong.

Bill


Bill, the way I read it; the TX is only gating itself with the enable.

Not totally sure why the delay is required, TBH...I will read again later.
 
Tinine
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Joined: 30/03/2016
Location: United Kingdom
Posts: 268
Posted: 05:38pm 01 Dec 2019
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FWIW:I always start my transmissions with AA (10101010).

Regards,

Craig
 
Phil23
Guru

Joined: 27/03/2016
Location: Australia
Posts: 1588
Posted: 10:36am 02 Dec 2019
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Ended up finding 3 of the Auto switching modules in one of my parts drawers.

Good thing is that I can directly swap one out for the HC-11 that is currently on Com1 of the heat pump controller, and not need to worry about changes to the board to accommodate the DE pin.

Did also find a good data source on the module, including schematic here...

https://protosupplies.com/product/scm-ttl-to-rs-485-interface-module/

Cheers.
 
Turbo46
Senior Member

Joined: 24/12/2017
Location: Australia
Posts: 286
Posted: 06:57pm 02 Dec 2019
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Thanks for that Phil23. The circuit is similar to that posted by Tinine but the 27uS delay has been extended out to over 5mS! It looks like a good device, well protected  and documented.

Considering that I bought the MAX485 chip locally for $5.95 the price is good too.

I have used one of these devices on my laptop to talk to RS485 devices using DOS MMBasic. It must use a similar automatic direction control.





Bill
Life wasn't meant to be easy.
 
Phil23
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Joined: 27/03/2016
Location: Australia
Posts: 1588
Posted: 07:59pm 03 Dec 2019
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Have the same USB one too.

Found it very useful for monitoring the bus when I was setting up the coms between the MM & the Modbus meters.
 
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