ESP8266 Community Forum

- Thu Dec 30, 2021 11:30 am #93198 Hello,

I'm working on a project to drive a stove which is sending TTL half-duplex messages. So I decided to plug an ESP8266 (D1 mini, but should work also with an NodeMCU)
In order to separate emission from reception to correctly read packages on Rx and send them from Tx, I built this electronic (see attached). But, the signal at the Rx pin stay at +3.3V

I did several trials with other schematic (or only by changing the voltage supply for the NAND HCF4001BE to +5V) and I saw (with an oscilloscope) that the signal goes down to 3.3V but never bellow.
So when signal have a voltage is coming for a "1" at +5V the pin is at +5V and the "0" is at +3.3V instead of 0V. And when signal have a voltage is coming for a "1" at +3.3V the pin is at +3.3V and the "0" is at +3.3V instead of 0V.
So I'm struggling to read "0" and "1" received from the stove.

Does anyone have a clue to solve this ?

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- Fri Dec 31, 2021 6:56 am #93210 So, just so the others can understand what you have and are trying to do:-

1, You have a single wire with 2 way communication?

2, You are using 2 stages of the 4001 simply as a level shifter?

I am not familiar with the transistor circuit you are using, however my first thought is where is the collector load?

If you have a single wire with 2 way communication have a look at THIS

- Sat Jan 01, 2022 8:52 am #93218 Hello @AcmeUK, thanks for the reply/

To answer to your questions :
1. Yes, the stove is using a single wire with 2 way communication. Knowing that the stove is always in "listening" mode and wait request from the remote to reply. So When I send a request I'm sure it's not responding at the same time.
2. I'm using 2 NAND door in order to have better voltage shape (see attached) and yes to have kind of level shifter. It's possible that I don't need it if I found a way to have the voltage dropping to 0V at Rx pin. But currently it's the option I'm using at least to be able to measure the reply from the stove with an oscilloscope. When I plug the Rx pin directly after the NPN transistor, the voltage stay at 3.3V (as explained on my first message).

Thanks for the link.
I will (in next few days) try with a resistance lower than 1.9kohm or 100kohm instead of the 10kohm I'm currently using and reply here what it have change.

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- Wed Jan 05, 2022 8:03 am #93262 I think I'm seeing a similar problem. I'm driving the RX pin with serial data via a level converter. If I scope the serial line when not connected to RX it's a clean serial signal between 3.3v and 0v. If I then attach the line to RX it remains at 3.3v, which is what the RX pin measures after reset. I attempted to force 0v on RX by using a resistor. I got down to 220 ohms and it still read .95v. It's definitely not a weak pullup on that pin. I don't get it. It's supposed to be an input!