Laipac TLP-434 RX output confusion

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I have been playing around with a Laipac TLP-434 RX TX pair the last day or two and been receiving nothing but garbage. I have connected the UART TX from a M32 to the TX data in, and the UART RX to the digital data out of a M8. While doing a sanity check (jumper from the TX data input to RX digital data output and jumping the grounds of the two boards together), I accidently connected the UART RX pin to the linear output of the RX, and the thing works. The RX is on a breadboard (which I expected to be a low level problem) and there is wire and cables running over the work bench, fluorescent lamp and all.
I spent an hour or so this morning reading previous post about TLP-434 and inexpensive ASK TX RX in general, so I was expecting poor performance, but I wasn’t getting anything using the digital output. Checked the datasheet from Laipac for pin outs and compared it against several schematics posted on the web to confirm it was wired correctly, BUT when I wire it wrong, it works.

Any suggestions what is going on?

Has anyone else experienced this? Shouldn’t it work better using the digital output?

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what baud rate are you running at? does it start working if you drop to a low (e.g. 1200 baud) speed?

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I am presently using 2400 bps.
Results are similar at other baud rates, using the Rx’s data output produces garbage,
using the linear output produces a valid signal. But slower is better.

In general, performance is as expected, works for ranges under 50 ft indoors with Tx @ 4.5 – 5 V, with error rate under 10% ( no error correction scheme used yet).

I just don’t understand why I get better (much) results using the ‘linear output’.

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Can you look at the two outputs with a 'scope?

Clearly, there IS a difference. And, without some direct observation, its difficult to suggest what might be happening.

For example, voltage levels might be wrong. Bit times might not be what are expected. And, any number of other things might be going on. But, those are only "might", at this point.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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I may be talking out of my arse here, but for what it's worth, I just Googled for TLP-434, and it seems to me that these transmitter/receiver pairs are not designed to take simple serial data such as might come from a UART. I think you need some encoding scheme which is data-slicer friendly. Manchester coding is one such, I believe. What you are looking for is a coding method that has a nearly equal number of ones and zeros, in the short term and the long term. This is to avoid problems with the DC level of the signal drifting upwards or downwards. As to why it works using the linear output, I've no idea. I can only echo what was said by a previous poster, let's see some 'scope traces.

Anyway, I could be completely wrong, but if I'm right, then I'm sure somebody else will explain the whole business much more clearly than I can.

Four legs good, two legs bad, three legs stable.

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Thanks Jim & John

Hopefully I can check the outputs on a scope later this evening, I’ve looked at the ‘data out’ but not ‘linear out’ yet.

As to needing Manchester encoding, the majority of other post and articles I found on the web confirm this. The consensus is that using these things connected directly to UART is unreliable. Mostly I had the things lying around and wanted to give them a try. I tried a direct connection to UART first because I saw a few claims that they would work (although at reduced reliability) this way.

Manchester encoding is the next step.

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Took a look at outputs with scope…
Sorry, no DSO

Without Tx running, both lines have a 60 Hz square wave, about .4V pp on linear out and 3.4V pp on digital out.

With Tx sending, both look like normal serial, but the digital has a lot of noise during and for about 5 – 10 ms after byte, then settles back to 60 Hz wave.

With Tx running, but not sending, the linear out is unchanged, digital out has random noise, then settles to about like linear out every .5 to 1 sec, some times low, some times high.

I know there must be a bunch of RF noise with all of the wiring around, I’ll have to try with battery and away from the building.

I have had these for at least a year, and they have been packed and unpacked, moved around a lot etc, possibly damaged? (I doubt it though)

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One other thing, you probably need a pre-amble, i.e. a string of 0xAAs or 0x55s followed by some special sequence. There are probably loads of examples on the web. I'd say that the chips are working as expected. Not quite sure about the "running" "sending" distinction, but I would imagine that a steady state, be it high or low, would produce no meaningful ouput.

Four legs good, two legs bad, three legs stable.

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I was doing a project with the 915MHz version of the Laipac TX and RX last year.
I was having the same problem. I contacted the Apps. Dept. at Laipac and was
told that using the linear output was probably a better idea than using the digital
output. The digital output is just a comparitor connected to the linear output and
referenced to an internal bias. The tech. told me that thier largest customer was
using the linear output and they hadn't really looked into the digital problem too much.

Will

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Thanks for the response Will

Glad to get confirmation to my experience with TLP
I tried encoding to increase reliability, but linear output still gave better results.
Have yet to try it away from industrial electrical noise.
I should have emailed Laipac with the question.

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how we can add linear output to it,please explain this segment more.

do you mean that we must add a comparator like as LM311(with a REF voltage like as 2.5V) to the output of RF RX module and give the comparator output to UART input of micro?

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Yaaavar
Linear output is connected directly to micro’s UART RX, no comparator