Hardware manual

intervals between bytes (like someone typing characters at a keyboard).
When a file is being sent thru the async serial port, the flow of bytes will likely be at the speed of the port (say
115.2k) which is a constant rate. This flow may frequently start and stop due to flow control. Is this sync or
async? Ignoring the flow control stops, it might seem like sync since it's a steady flow. But it's not because
there is no clock signal and the bytes could have been sent erratically since they are framed by start/stop bits.
Another case is where data bytes (without any start-stop bits) are put into packets with possible erratic spacing
between one packet and the next. This is called sync since the bytes within each packet are transmitted
synchronously.
Synchronous Communication
Did you ever wonder what all the unused pins are for on a 25-pin connector for the serial port? Most of them
are for use in synchronous communication which is seldom implemented in chips for PC's. There are pins for
sync timing signals as well as for a sync reverse channel. The RS-232 spec provides for both sync and async
but PC's use a UART (Universal Asynchronous Receiver/Transmitter) chip such as a 16450, 16550A, or
16650 and can't deal with sync. For sync one needs a USRT chip or the equivalent where the "S" stands for
Synchronous. A USART chip supports both synchronous and asynchronous. Since sync is a niche market, a
sync serial port is likely to be quite expensive.
SCC stands for "Serial Communication Controller" or "Serial Controller Chip". It's likely old terminology and
since it doesn't say "sync" or "async" it might support both.
Besides the sync part of the RS-232, there are various other EIA synchronous standards. For RS-232, 3 pins of
the connector are reserved for clock (or timing) signals. Sometimes it's a modem's task to generate some
timing signals making it impossible to use synchronous communications without a synchronous modem (or
without a device called a "synchronous modem eliminator" which provides the timing signals).
Although few serial ports are sync, synchronous communication does often take place over telephone lines
using modems which use V.42 error correction. This strips off the start/stop bits and puts the data bytes in
packets resulting in synchronous operation over the phone line.
22. Other Sources of Information
22.1 Books
Axleson, Jan: Serial Port Complete, Lakeview Research, Madison, WI, 1998.1.
Black, Uyless D.: Physical Layer Interfaces & Protocols, IEEE Computer Society Press, Los
Alamitos, CA, 1996.
2.
Campbell, Joe: The RS-232 Solution, 2nd ed., Sybex, 1982.3.
Campbell, Joe: C Programmer's Guide to Serial Communications, 2nd ed., Unknown Publisher, 1993.4.
Levine, Donald: POSIX Programmer's Guide, O'Reilly, 1991.5.
Nelson, Mark: Serial Communications Developer's Guide, 2nd ed., Hungry Minds, 2000.6.
Putnam, Byron W.: RS-232 Simplified, Prentice Hall, 1987.7.
Seyer, Martin D.: RS-232 Made Easy, 2nd ed., Prentice Hall, 1991.8.
Stevens, Richard W.: Advanced Programming in the UNIX Environment, (ISBN 0-201-56317-7;
Addison-Wesley)
9.
Tischert, Michael & Bruno Jennrich: PC Intern, Abacus 1996. Chapter 7: Serial Ports10.
Serial HOWTO
Defining Asynchronous vs Synchronous 78