Secondary Lines:

There are two kinds of lines that are specified in the RS-232 specification. There are the primary channels that are normally utilized, and operate at the normal or higher data rates. But, there is also provision for a secondary channel in order to providing control information. If it is utilized it will usually send data at a much slower rate than the primary channel. As the secondary lines are hardly ever used or even implemented on equipment, manufacturers frequently use these connector pins for other cause. In view of this it is worth verifying that the lines are not being utilized for other cause before considering using them. While the secondary system is in use, the handshaking signals operate in the similar way as for the primary circuit.