Lua TCP communication - tcp

Question

I have a proprietary client application that sends and receives TCP data packets to|from the network device like this:
Sent: [14 bytes]
01 69 80 10 01 0E 0F 00 00 00 1C 0D 64 82 .i..........d.
Received: [42 bytes] [+00:000]
01 69 80 10 01 2A 00 D0 DC CC 0C BB C0 40 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 .i...*.......#..................
00 00 00 00 00 00 1C 0D F6 BE ..........
or see the picture
I need to make same requests with Lua. I've found some working examples (for ex) for such communications, but I can't understand what string should I give as an argument for
tcp:send("string");
Should I give it a string of hex? I.e.
'01698010010E0F0000001C0D6482'
Or first convert hex to ACSII? If so, then how (zeroes doesn't convert to symbols)?

Answer 1

You should give it the string you want to send. If you write "016980..." it's a string containing decimal values 48 (ascii digit 0), 49 (ascii digit 1), 54 (ascii digit 6), and so on. Which is not what you want to send. You want to send decimal values 1, 105 (hex 69), 128 (hex 80) and so on.
Luckily, Lua strings can hold any bytes (unlike e.g. Python strings). So you just have to write a string with those bytes. You can write any byte into a string using \x and then a 2-digit hex code. So you could write your call like this:
tcp:send("\x01\x69\x80\x10\x01\x0E\x0F\x00\x00\x00\x1C\x0D\x64\x82")
If you are using a Lua version older than 5.2, \x is not available, but you can still use \ and a 3-digit decimal code.