There is some disagreement on the frame ground line, some say it should be connected at both ends, some at one end only and some say it should not be connected. In my experience it has not made any difference whether it is connected or not.
The textbook maximum line length for a serial connection is 50 feet. However with good quality cable and a route which avoids interference from mains cables it should be possible for runs of 200 feet to be reliable. I would also always use a shielded cable.
Software flow control uses special characters to control the flow of data when a device wishes to halt the flow it sends the XOFF character, when flow can resume it sends the XON character.
The XOFF character is called DC3 or Device Code 3. It is decimal 19 or control S.
The XON character is called DC1 or Device Code 1. It is decimal 17 or control Q.