12/9/2023 0 Comments Coolterm send data xbee![]() A small block of meta data that preceeds the actual data that holds information about the packet. ![]() If you have different sizes of data to send then you also need to tell the receiver how big the data is. If your packets are always the same size then all you need to know is how many bytes to expect, which can be hard coded into your software. And that is where the next two points above come in: Validating your packet. If it ever sees 0xAA, 0x55 then it knows, without a shadow of a doubt, that this is the start of a packet, or random noise. (Actually this is the opposite of escaping which says the next character is a control character, but the underlying concept is the same). Your receiver sees the first 0xFF and thinks "ah, the next byte that arrives, whatever it is, just be data." It then gets a second 0xFF and knows it is data. So to send 0xFF you would actually send 0xFF, 0xFF. It only says that the next character must be interpreted as data. 0xFF, being the escape character, has no actual value in the data. But what if you want to send the value 0xFF, in your data? How do you handle that? Well, you also escape 0xFF as well. So if you happened to have that sequence in your data you would expand it out to 0xFF, 0xAA, 0xFF, 0x55. A common escape marker is 255 (used in the telnet protocol) or 27 (the ANSI ESC character) although any character can be chosen.įor instance if you choose to have a synchronisation sequence of 0xAA, 0x55, you can then decide that neither of those bytes will ever appear in your data without first being preceeded by character 0xFF. This is where you nominate certain byte values as control bytes and decide that these bytes will never appear in your data without some kind of marker to say they are data bytes. And that is where the concept of escaping comes in. It is often done with a specific sequence of bytes (known as a preamble) that can, through various means, never appear anywhere else.Īnd at that point things start to get a little more complex, for now you need some way of wrestling that kind of thing into a system that can only send bytes of 0 to 255 with no way of distinguishing raw data from control information like this synchronisation sequence. It needs to look out for something unique, something it cannot mistake for anything else. The receiver has to have some way of knowing where the start of a packet is amid the torrent of bytes it is being sent. In order to reliably transfer data there are a few things you need to consider from the point of view of the receiver: A packet starts it's life in your own head. That's not something where you just wave a magic wand and a packet is created. So you have to come up with a reliable method of transferring data in such a way that the receiver knows that it has received valid data, and how to find valid data. While you might just about get away with that on a 100% reliable direct serial connection over a short distance, as soon as you introduce anything with any form of uncertainty to it you are, as you have seen, doomed to failure. ![]() The biggest mistake people always make with serial communications of any form is to just send raw data over the connection.
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