Physical layer RS232 (and RS422,RS423, RS485) are technically NOT an interface standard, except that they specify the physical level standard. They defines wires, voltages and pins. For example, the RLC-1 RS232 TTL Level Converter simply changes the voltage levels (and wires and pins) and does not change the actual data being transmitted. And asynchronous serial at a given baudrate is only *one possible* transport layer. Others are Bi-Sync, Manchester, HDLC/ NRZI etc. Most people use "RS232" to mean async serial over the RS232 physical layer. So, it is not really RS-232 that they are discussing, [and which is typically associated with RS-232] it is asynchronous "teletype" protocol; you will find it described in UART datasheets. In the same way, RS485 "multidrop" is commonly used to refer to a async protocol with peripheral addressing. But the RS standards define only the physical layer.
Direction Simplex simply means that the data flows in one direction only. From the sender to the receiver and nothing back. Half Duplex means that data flows both ways, but only one way at a time. Station 1 sends to 2 then 2 sends back after 1 is finished. Full Duplex means that data flows both ways at the same time.
Clock source Async means asynchronous or "not synchronous" meaning that there is no clock single provided along with the data signal. sync means that the data is synchronized to a clock, which is provided by the sender. The clock may be provided along with the data on the same physical wire, or on a separate line.
Standard "RS 232" (meaning async serial over the RS232 physical layer) is considered async, although the start bits technically provide a byte clock, because the bit clock is not provided and must be developed by the receiver at the same rate as the sender. This is the baud rate which must be defined on both sides before communications can succeed. There are "autobaud" or baud rate detection systems which enable the receiver to guess at the sending baud rate by examining the data bits. The standard modem "AT" command set was developed to facilitate that system as "A" and "T" happen to have bit patterns that are very regular; "A" is "100 0001" and so has a 1 bit at the start and end and "T" is "101 0100" which has a pattern with every other bit set. It's important to realize that async serial at a specified baud rate can be used to transmit data over any of the physical layer standards listed above.
Manchester encoding +is a synchronous system which provides a clock along with the data while still using only one wire, and it can be transmitted via any of the physical layer standards listed here.
Any of RS232,RS422, RS423 or RS 485 can be simplex, or half or full duplex and at the same time can be synchronous or asynchronous, however, in most cases RS232 will be used to refer to full duplex asynchronous over an RS232 physical interface. +
There are also many serial busses.
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<A HREF="http://techref.massmind.org/techref/io/serials.htm"> Serial Interfaces</A>
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