Serial Flow Control. What's CTS RTS XOFF XON and why should you care?

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Serial Flow Control. What's CTS RTS XOFF XON and why should you care?

Рет қаралды 2,974

9 ай бұрын

How do you control serial besides TX and RX? That's what I try to explain in this video featuring a Raspberry Pi Zero W, a USB to Serial FTDI232 dongle and my very own 6502 Single Board Computer - the 65uino.
Guest starring: An 8 channel logic analyzer.
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Check out the whole series (playlist): • 6502 Computer
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Пікірлер: 16

@AndersNielsenAA 9 ай бұрын

Leave a comment and come say hi on Discord - I promise nobody bites: discord.gg/kmhbxAjQc3 :)

@GregCoonrod 9 ай бұрын

It is so cool to see this working with minimal (if annoying) changes to the Pi! Thanks for taking the time to explore this topic. I will be leaning on your work for my own 6502 SBC.

@AndersNielsenAA 9 ай бұрын

I certainly thought it was worth covering a little bit - it’s easy to overlook when you learn the RX/TX basics and don’t need flow control until much later :)

@derekchristenson5711 6 ай бұрын

Neat! I'm often frustrated, while trying to get a serial connection (a "simple" serial connection) between two computers by how many different possibilities there are for flow control and no flow control, and the problems that arise when two computers both eschew flow control, but they still drop characters, showing that they clearly need it. It's interesting to see it implemented on your 65uino!

@AndersNielsenAA 6 ай бұрын

There's also matter of noise - doesn't take very long wires to make serial data into complete nonsense. But at least 9600 baud is rather forgiving :)

@GregStrike 9 ай бұрын

Hey Anders, GREAT explanation of flow control. Keep it up man!

@ScienceReasonLove 9 ай бұрын

For the kernel boot messages: check the kernel command line. The documentation says that the console=ttyS0 line takes flow control information from the second passed parameter. So to have ttyS0 be configured for RTS/CTS flow control at 9600 baud with no parity you’d have this: console=ttyS0,9600n0r

@AndersNielsenAA 9 ай бұрын

That part is actually in the video :) For the kernel in Raspbian though, that part doesn't seem to be compiled in, so console=ttyAMA0,4800n8r only sets 4800 baud, no parity and 8 bits ... but doesn't make it respect CTS.

@melkiorwiseman5234 9 ай бұрын

Short version of how RS-232C flow control is supposed to work: If XOFF/XON flow control is used, the transmitter is supposed to monitor incoming bytes and if an XOFF byte is received, the transmitter side should pause transmission and wait for an XON byte to be received before continuing sending. If CTS/RTS flow control is used, then the transmitter is supposed to check RTS before sending each byte and wait for RTS to be enabled before transmitting the byte. Unfortunately, the advent of very fast CPUs has caused many programmers even at the system level to just assume that the receiver can handle whatever speed has been selected and so the transmitter programs often do not implement any form of flow control. This is a very common but very poor practice and illustrates how common "lazy programming" has become.

@AndersNielsenAA 9 ай бұрын

Yup. But note the transmitter should check (its) CTS - not RTS. I hope the correct meaning came across in the video even if I misnamed RTS as CTS in the 6502 code.

@melkiorwiseman5234 9 ай бұрын

@@AndersNielsenAA Yeah, I got confused. The receiver requests sending, and the transmitter checks that it's clear to send. I have to remember that.