Flow Control
Flow Control is necessary to regulate data flow between 2 devices with dramatically different data transmission speeds (such as a dot matrix serial printer and an RS-232 interface connection). Flow Control is needed to regulate data flow, to ensure that the two devices can communicate with minimal loss of data.
RS-232 communication uses one of two basic approaches to enforce Flow Control.
Software approach: XON/XOFF
XON (0x11) and XOFF (0x13) are defined as special control codes for use during communication. The 2 codes are transmitted along with data characters. The operating scheme is straightforward:
When either of the devices receives XOFF, it stops transmitting data until XON is received. The problem with this approach is that the data itself cannot contain these 2 codes.
Hardware approach: RTS/CTS
RTS (Request To Send) and CTS (Clear To Send) are separate signals, sent on separate wires, used for hardware Flow Control. RTS is an output signal that enables/disables data transmission for the other device. CTS is an input signal allowing the other device to enable/disable data transmission. The drawback to this approach is that 2 more wires are needed, but it also provides the capability to send binary data.
DTE (Data Terminal Equipment)
DTE stands for Data Terminal Equipment, as defined by the RS-232 specification. Examples of DTE are computers, printers, and terminals.
DCE (Data Communication Equipment)
DCE stands for Data Communication Equipment, as defined by the RS-232 specification. The basic function of a DCE is to convert data from one interface, such as a digital signal, to another interface, such as an analog signal. One example of DCE is a modem.
Link
http://www.google.dk/search?hl=da&lr=&oi=defmore&q=define:DCE