Spa Controller

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Joined: 30/03/2016
Location: United Kingdom
Posts: 366
Posted: 05:31pm 01 Dec 2019      

  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, 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.