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I'm relatively new to behaviors of serial ports such as RS-232, RS-422, and RS-485.

I'm wondering how do you initiate data transmission between a system and a device connected to each other by one of the 3 serial ports? Is this done thru software, or can be done thru commands inside a command line prompt, if so what's this command? How do we do this in Ubuntu and Windows OS?

For example, on a BACNet network, how does one initiate data transfer between 2 devices connected by serial cables

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  • RS-232, RS-422, and RS-485 are only electrical specifications. They do not encompass the message protocol. So there is no answer to your question. The answers would depend on the specific devices involved. When you see a CAT5e cable with RJ45 plugs, do you assume that's it's for Ethernet (electrical spec) and HTTP/TCP/IP (protocols)? You simply cannot make such assumptions with "serial" hardware.
    – sawdust
    Jun 5, 2015 at 19:02
  • Your edit doesn't clarify your question enough. Wikipedia states that BACnet protocol can use "Point-To-Point over RS-232, [and] Master-Slave/Token-Passing over RS-485"
    – sawdust
    Jun 5, 2015 at 19:17

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Usually you use a terminal emulation program to interface with the ports. PuTTY is one of these programs, it is available for all major operating systems.

However, you cannot just wire the pins of RS232/485/422 together since the electrical voltage used for signals are different between the standards. You can connect two devices with RS232 ports using a "null modem cable", but will need some converter to connect a RS232 port to a RS485 port.

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