SDI-12 to USB adapter from Aliexpress - Page 1
Forum section: electronics / projects

author: Wetherman
author_rank: Newbie
timestamp: March 24, 2021, 08:42:36 pm
content: We have a Modbus product which I've just revised to change the comms interface to SDI-12. I developed it using the excellent Liudr interface. Then I thought it would be a good idea to buy a bunch of other interfaces (USB adapters), for confidence that we aren't going to get lots of support calls from people who can't get our sensor to work. I'm working through a list and I got to this one. I bought only one.It doesn't work because the Baud rate is too high at 1323 baud. It looks like the manufacturer has taken example code from the microcontroller (STC 8F2K16S2) datasheet page 110 which was written for a main oscillator running an 11.0592MHz crystal, and chosen to use the internal 24MHz oscillator divided by 2. (This is cheaper because a crystal oscillator is not needed.) The protocol for SDI-12 is one stop bit, 7 data bits, one even parity bit, and one stop bit. That is 10 bits per byte. So the speed can be up to 5% out (half a bit-time in 10 bits) and still allow recovery of the data. That works out to be 1143 to 1260baud.Incidentally I was amazed that people are still building products with a 8051 microcontroller (yuk!). Then I saw the price: 3.2 Yen!Can someone advise whether I should "do the right thing" and complain to the seller that it doesn't work and live in hope that the message will get back to the designer. I think this is probably cloud-cuckoo-land thinking, but it seems very wasteful to keep making and selling this thing that doesn't work. There are lots of Aliexpress and a few Ebay "stores" selling it. Maybe I have a one-off duffer and I should buy another.

author: thm_w
author_rank: Super Contributor
timestamp: March 24, 2021, 09:31:25 pm
content: Was the measured baud rate as 1323 via oscope or LA?One would think if its version 3.5 that it worked at some point in its lifetime, maybe with other devices less sensitive to baud rate or with automatic baud detection.From thealidescription:QuoteHave you tried adjusting it? Or are they just confused and talking about the PC side.You can message the seller but the chance of them knowing the seller is low, unless its an official Ali outlet store or similar.

author: MIS42N
author_rank: Frequent Contributor
timestamp: March 24, 2021, 09:57:30 pm
content: Complain. Ask for your money back. You've nothing to lose. Maybe you have a dud and they will send you a good one. Maybe there's information you haven't heard of, and they will point you in the right direction. And maybe nothing happens. If you don't make a noise, there is only one outcome - nothing happens.

author: Wetherman
author_rank: Newbie
timestamp: March 24, 2021, 10:03:34 pm
content: Yes thanks they're talking about the PC settings (USB virtual port). I can change all the port settings (baudrate, number of stop bits, parity etc.) and it makes no difference. The onboard micro buffers all received characters up to the terminating "!" character and then transmits a break and spits out the entire string onto the bus with characters back-to-back (as per spec.).