Hardware manual

remote computer had a flow path to the hard-disk without bottlenecks, the same flow also went to a terminal
which issued flow control "stops" with disastrous results for the branch of the flow going to a file on the
hard-disk. The flow to the hard-disk passed thru my modem and since the overflow happened at the modem,
bytes intended for the hard-disk were lost.
4.9 Serial Driver Module
The device driver for the serial port is the software that operates the serial port. It is now provided as a serial
module. From kernel 2.2 on, this module will normally get loaded automatically if it's needed. In earlier
kernels, you had to have kerneld running in order to do auto-load modules on demand. Otherwise the serial
module needed to be explicitly listed in /etc/modules. Before modules became popular with Linux, the serial
driver was usually built into the kernel (and sometimes still is). If it's built-in don't let the serial module load
or else you will have two serial drivers running at the same time. With 2 drivers there are all sorts of errors
including a possible "I/O error" when attempting to open a serial port. Use "lsmod" to see if the module is
loaded.
When the serial module is loaded it displays a message on the screen about the existing serial ports (often
showing a wrong IRQ). But once the module is used by setserial to tell the device driver the (hopefully)
correct IRQ then you should see a second display similar to the first but with the correct IRQ, etc. See Serial
Module See What is Setserial for more info on setserial.
4.10 The Serial Port is Now Obsolete on PCs
The serial port is today (2011) sort of obsolete on PCs (and often called a "legacy" device) but it is still in use
on some older computers and is used in imbedded systems, for communication with routers and point-of-sale
equipment, etc. The serial port is slow and after about 2005 most new PC's no longer had them. Most laptops
and Macs discontinued them even earlier. During the era when some new PC's came with serial ports and
others didn't, the PC's that didn't have serial ports were euphemistically called "legacy-free". However, while
PC's today no longer have serial ports, some do have them built into a "legacy" modem (which plugs into a
telephone line). Such a serial port only works with the modem and can't be used for any other device. The
reason they have such a "built in" serial port is that analog modems are designed to only work thru a serial
port.
The physical serial port on the back of a PC (including the chip its connected to inside the PC), must pass data
between the computer and an external cable. Thus it has two interfaces: the serial-port-to cable and the
serial-port-to-computer-bus. Both of these interfaces are slow. First we'll consider the interface via external
cable to the outside world.
4.11 RS-232 Cable Is Low Speed & Short Distance
The conventional RS-232 serial port is inherently low speed and is severely limited in distance. Ads often read
"high speed" but it can only work at "high speed" over very short distances such as to a modem located right
next to the computer. Compared to a network card, even this "high speed" is actually low speed. All of the
RS-232 serial cable wires use a common ground return wire so that twisted-pair technology (needed for high
speeds) can't be used without additional hardware. More modern interfaces for serial ports exist but they are
not standard on PC's like the RS-232 is. See Successors to RS-232. Some multiport serial cards support them.
It is somewhat tragic that the RS-232 standard from 1969 did not use twisted pair technology which could
operate about a hundred times faster. Twisted pairs have been used in telephone cables since the late 1800's.
Serial HOWTO
4.9 Serial Driver Module 18