TEROS 12 and SDI-12

[Pesien]

I´m working on a library to comunicate with SDI-12 sensors using a GPIO pin, but I have a problem with the pulses_get function because it seems to have a limit, so I can only read a max of 200 bits or 20 bytes for the SDI-12 sensor.

I posted it here: https://forum.pycom.io/topic/4829/does-pulses_get-function-have-a-200-bit-limitation-when-reading

Apart from that, It works.

[tuftec]

Try this link.
http://www.sdi-12.org/current_specification/SDI-12_version-1_4-Jan-10-2019.pdf

The h/W interface cct is towards the end of the document.

[tttadam]

@thinginnovations Nope, I don't have any SDI 12 interface hardver, I hoped that I can use it directly with pycom without anything in the middle.

[tttadam]

@Pesien I think the github link in your forum thread is written in C.
how should I use it with micropyton board?

[tttadam]

@robert-hh Yes I already saw that, but on my sensor there is only 3 wire (gnd, 5v and data)
And looks like in these example code there is 2 data wire.

[robert-hh]

@tttadam The sensor in that link is a 3 wire device, but a RS485 adapter is used to connect it to the Pycom board. You do not have to do that, but that choice combines level shifting and connecting RX and TX to the sensor.
In any case, you need some kind of electronic adaptation, at least a level shifter and program logic to switch off the Pycom board's TX when you expect data from the sensor. You could try to set the UART output to OPEN_DRAIN, after you have init'ed the UART.

[tttadam]

@robert-hh said in TEROS 12 and SDI-12:

RS485

Can you recommend me a rs485 breakout board for pycom?
All I can find has an usb port on the end.

[robert-hh]

@tttadam search for "RS485 TTL breakout board". Most of the are made for 5V at the UART side, but some too for 3.3V.

[tttadam]

@robert-hh said in TEROS 12 and SDI-12:

@tttadam search for "RS485 TTL breakout board". Most of the are made for 5V at the UART side, but some too for 3.3V.

What I found is this sorry for the hungarian, but I need it ASAP, so I turned to local stores.
I also found this image on online. I think they are the same. But its still not clear how should I use it because it has 4 wire on both side.

Thank you for the help.

[robert-hh]

@tttadam It's not clear to me if that is easier to use than simply a one transistor level shifter. And so I would not use that choice.

Besides that:
Receiver Output -> Pycom RX
The board operates at 5V, so you have to add at least a resistor of 4.7k to 10k between receiver Output and Pycom RX to protect the Pycom board.
Driver Input -> Pycom TX
Receiver Enable and Driver Enable can be switched together to a GPIO which is used to switch between receive and transmit.
Data A and B are the differential output/inputs. And that makes the trouble, since the sensor has not differential data interface. Thats why in the post I linked there is a 1.8V reference, to create a virtual differential signal.
5V and Ground are obvious.

As said, I would prefer to connect RX and TX at the pycom device and connect it via a level shifter to the Sensor data line. You may need a pull-up resistor at the sensor to 5v, and in the software you will receive the commands that you have sent in addition to the response. In any case, it is not just plug and run.

The obvious advantage of the RS485 version is it's ability to drive long wires.

[tttadam]

@robert-hh Alright, my knowledge is not so strong in this area, so I accept your recommendtation, no RS485 and level shifter it is. Thank you for the advice.
So level shifters, I found this one.
This is what was in your mind? If so, how would you wire it?

[tvetter1976]

Regarding Level shifter. Take a look at this one:
https://learn.sparkfun.com/tutorials/single-supply-logic-level-converter-hookup-guide/all
and 3.3V RS485 Converter:
https://eckstein-shop.de/RS485-Board-33V

[robert-hh]

@tttadam That one is OK. You need just one channel. LV to 3.3V, HV to 5V A1 to both RX and TX of the pycom device. B1 to the data line of the sensor.
You have still the task to make TX high impedance when you receive. So you can try:
a) to set up the UART to the desired mode, and then
b) init the PIN for TX as OPEN_DRAIN.
if that does not work, connect a diode (e.g. 1N4148 or 1N5711) between A1 of the level shifter to TX, such that the Cathode (the side with the bar) is at TX. That prevents the TX output from driving the signal when it is high. It still can sink the signal line.

[Pesien]

@tttadam That link is the pulses_get function, not the library.
I'll post the library on github once it works.

[robert-hh]

@Pesien I made a PR for inverted UART signals. It is here:
https://github.com/pycom/pycom-micropython-sigfox/pull/310

[tttadam]

Okay, so I facing with this topic again. (I put it into the drawer for some time, but it's on my desk now again)
So I am using a level shifter.
Also if I am right I need to use an external power source to power the teros 12, I use a 1.5A usb brick.
But the wiring is still not clear to me.
This is what I come up with. I don't want to fry it so first I would like to ask the community.
Thanks,

[robert-hh]

@tttadam A diagram of the wiring with labels about what is what would be more helpful than a picture of a few wires.

[robert-hh]

@tttadam So, what you need is not only a level shifter. If you compare the interfaces you see quite a number of differences:
SDI-12:
a) Level 0 and 5 V
b) Halfduplex
c) Inverted
d) Load must be between 1 and 2 kOhm

WiPy:
a) Level 0 and 3.3 V
b) Full Duplex
c) Straight
d) Load > 200 Ohm

For a), a level shifter might work
for b) That has a hardware and software part. If you do the hardware right, if serves also for a) and c). The software part is easy. You just have to cope with the fact, that you will also receive what you have sent.
for c), you need to invert, unless you change the WiPy firmware
for d) minor problem.
So, lets look for a solution to that. What comes into my mind is a circuit with two transistors and a few resistors.

Instead of the 2n7000 you can also use a small signal npn transistor (e.g. 2N3904, BC547), instead of the BS250 a small signal pnp transistor (e.g.2N3906, BC557). That circuit should shift the levels, invert the signals and provide a proper impedance for SDI-12. The ration of R3 and R5 has to be chosen, such that the SDI line is low when the TX line is high.

[robert-hh]

@tttadam Another variant with two optocouplers, which has the advantage that is completely isolates the SDI-12 wire from the WiPy. Given that these wires can be long, that may be useful.

There are also devices with two couplers in one package, like the ILD74, but the CNY17 is more common. Since it is operated in a digital mode, the actual device is less critical.

[tvetter1976]

You can still to use the serial output from the sensor and directily read the measurement using the UART. You don't need sdi12 at all. The only thing you need to is to switch on/off the senor. Then the sensor will send its measurement. The logic is not invereted. Why is this not an option for you? Also the digital output voltage output from Terros 12 is only 3.6V and not 5V.
From the manual:
http://manuals.decagon.com/Integrator Guide/18224 TEROS 11-12 Integrator Guide.pdf
"DDI SERIAL INTRODUCTION
The DDI serial protocol is the method used by the METER data loggers for collecting data from the sensor. This protocol uses the data line configured to transmit data from the sensor to the receiver only (simplex). Typically, the receive side is a microprocessor UART or a general-purpose I/O pin using a bitbang method to receive data. Sensor measurements are triggered by applying power to the sensor"