How does Tx Only Serial Communication Actually Work?

Question

I have a YDLidar X2 device that I found doesn't actually have a Rx line. By design, it has no wire on the connector and doesn't expect any commands to be sent to it. It just continuously transfers out its packets representing the state of its sensor.

I hadn't realized it until today that every project I've ever done with serial was by me sending a command and awaiting a response. It was so intuitive, I never thought how the bits where banging.

So, my question is - does such a device that has no Rx line somehow fill a buffer and pause until bytes are read by the user or is it just pulsing and not caring what's happening on the line?

If it does pause, what gives the flag that it is good to continue?

If it does not pause, how in the world does the receiving device know how to sync up with the bits streaming?

Or, does serial protocol actually use the Tx line alone as a bus to talk back to coordinate the transfer of data from the transferring device? If not, I would think it would create nothing but garbage if it started listening at a random time.

Thanks,

Sean

neuromodulator · Accepted Answer

Because you must know the timing (Actually there are timing algorithms too, but thats more complex). I recently implemented an UART receiver on my FPGA, so if you understand Verilog and have an FPGA I could share it with you. Usually algorithsm work kinda like this.

you run a clock at say 16 times the speed of 1 bit duration. So for instance if you plan to receive a signal of say 2400 bps, you need to sample at a rate of 38400. Here is an example of a signal 8N1 (8 data bit, no parity bit and 1 stop bit).

So you once you detect a falling edge, you will wait enough time to sample right in the middle of the bit1 and then bit2, and so on. If you want the receiver to be more robust you may want to sample multiple times every bit in order to avoid that transients corrupt your data. In the best case scenario a transient would just corrupt a single or very few samples from the bit. So for instance if you sample 5 times every bit, its likely that if a transient occured, it would flip just 1 or very few samples out of the 5 in total. The transmitter and receiver could use different clocks, and there could be a mismatch in their timing, but that shouldn't be an issue if you continuosly resync the clock.

neuromodulator · Answer

When the line is in idle its at 1, when it begins transmitting goes to 0 (start bit). The falling edge indicates the beginning of a byte transmission. So unless the transmitter is sending data at full speed, its straightforward to synch to the transmitter, as the line cant stay high for 10 or more time units, unless its in idle.

dougw · Answer

It is just a fire and forget transmission. The sender does not care or respond to what is happening with the receiver. It depends on the receiver being ready and able to receive anything and everything that gets transmitted, whenever it gets transmitted. Sometimes such devices can be configured for different baud rates or to have a longer delay between bytes to make it easier. GPS modules are like this - if the receiver misses a byte or gets out of sync, the receiver must recover by looking at the data or the timing of the transmissions. Data may be preceded by a synchronizing header that needs to be parsed, but often it is just preceded by a defined break in transmission data.

neuromodulator · Answer

The simple approach for 8n1 is to wait for 10 or more time units that line is high. That will tell you that the line is idle. At that moment you wait for falling edge (start bit) wich will tell you that byte transmission will begin.

dougw · Answer

Serial bytes take a long time (measured in CPU instruction cycles) to receive a whole byte, so there is lots of time to deal with the last byte while the next one is arriving, assuming there is a UART receiving individual bits. But often the low level handler just collects a whole message in a buffer, in real time, while performing higher level parsing of complete messages in the background. Then the parsing routine just needs to be completed before the next complete message is received.

How does Tx Only Serial Communication Actually Work?

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