Question

I’m trying to create a packet to send over serial using ruby-serialport. This seems like it should be simple, and it works when I just write a string:

packet = "\xFF\x03\x10\x01\x01\xFE"
sp.write(packet)
=>hardware does what it's supposed to, opens the door represented by the 4th hex value

but I obviously need to do it programmatically, and I can’t figure out the right way. Here are just a few of the things I’ve tried:

door = 1    
packet = "\xFF\x03\x10" + door.to_s(16) + "\x01\xFE"
sp.write(packet)
=> can't convert fixnum into string

and

door = 1
packet = "\xFF\x03\x10" + door.to_a.pack('H*') + "\x01\xFE"
sp.write(packet)
=> to_a will be obsolete
can't convert fixnum into string

and

door = 1
sp.write("\xFF\x03\x10")
sp.write(door)
sp.write("\x01\xFE")
=>no response from hardware

Can anyone help me out on how to properly convert a number into the right hex notation for serialport and joining to the other hex strings? Thanks in advance!

Answer 1

You’re really going to get into trouble if you insist on using strings to represent otherwise binary data. What you really need is pack:

packet = [ 0xFF, 0x30, 0x10, door, 0x01, 0xFE ].pack('C*')

This makes it very easy to construct and deconstruct arbitrary binary data. The method supports not just unsigned characters but a variety of other types that are commonly used.

You may even want to construct your own method to read and write this:

def write_packet(*bytes)
  sp.write(bytes.flatten.pack('C*'))
end