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The basic principles are:
This bitstream represents the byte F4h, using 8N1 (8 bits/character, No parity bit, and 1 stop bit.)
DCE stands for Data Communications Equipment. Usually this implies a modem, but for simple stuff, this usually translates to "anything that plugs into a PC."
DTE stands for Data Terminal Equipment, which is usually a PC.
All signals are shown on the diagrams above; however if hardware handshaking isn't being used, only three signals need to be connected: RXD, TXD, and GND. To connect a DCE to a DTE device, simply wire
To connect two PCs (DTEs) or two DCEs together, a null modem must be used. In the simple case above, all that's required is to swap the location of RXD and TXD on one end of the cable.
The signals are described in terms of a PC (DTE) and a modem (DCE.) As mentioned before, in the simple case an embedded system will only use RXD, TXD, and GND, unless hardware handshaking is used.