instructions to control the remote printer. With this new
code, an operator could cause the remote printer to
print 52 characters and could also control where on the
paper the characters were printed.
If you wonder why this matters, look at your
computer’s keyboard and note the Ctrl key. It is used to
alter the meaning of the rest of the keyboard in much
the same way as was done by Baudot’s apparatus,
which is a direct ancestor of your keyboard. Also, when
we transmit something we will use the ASCII code. We
will see that there are a lot of atavistic printer control
codes such as CR (Carriage Return), LF (Line Feed), and
BEL (for bell, as in dingaling). We might wonder why we
need such ‘characters’ in our attempts to send data
between a PC and a microcontroller since neither has a
carriage, a roller, or a dingalinger. Now you know.
Baud rate refers to the number of unique symbols that
can be transmitted per second — the actual physical ability
of the system to change states each second. There is often
some confusion in the use of Bd (Baud) and bps (bits per
second). The bps refers to the amount of information that
can be transmitted each second. If each physical state
change represents a bit of information, then Bd = bps.
While this often is not the case, in our use where one
state change represents one bit of data, we will use them
■ FIGURE 4. Morse’s eponymous code.
Baudot’s invention evolved into the teletype machine
— an electro-mechanical typewriter that could act as both
a transmitter and receiver of text messages over long
In early computers, a teletype machine (Figure 6) was
used to enter characters that were punched into cards or
paper tape for loading programs into computers. [During
WWII, the Colossus computer at Bletchley Park was used
to crack encrypted German teletype messages.]
Eventually, direct connections were developed to allow
the teletype to function much like a PC keyboard.
Figure 7 shows Dennis Ritchie (inventor of the C
programming language) standing next to Ken
Thompson (inventor of Unix), designing the original
Unix operating system at Bell Labs on a PDP- 11 using
a teletype machine to ‘talk’ to the computer.
The RS-232 standard was written to allow teletype
machines — which were referred to as DTE (Data Terminal
Equipment) — to link to a modem — referred to as DCE
(Data Communication Equipment) — that could then
transmit the binary data from a teletype machine over a
phone line to a distant computer. There were several
iterations of this standard, but by 1969 RS-232C became
the standard that would eventually be adopted (sort of) by
Microsoft for the PC serial port. The ‘sort of’ is necessary
since the PC isn’t exactly 100% pure, but close enough
that the PC serial port often became known as the RS-232
The port has nine pins. Data is transmitted on the TxD
pin and received on the RxD pin. Six additional pins are
used to control the communications between the PC and
the modem. They are the DCD, DSR, DTR, RTS, CTS, and
RI, which we’ll learn a lot more about later. The serial port
was originally intended to be used with modems, but
designers found that it could also be used to
communicate with other peripheral devices such as mice,
drawing pads, oscilloscopes, etc., thus leading to our more
generic use for communicating with microcontrollers.
So now you know that we call the communication
speed ‘baud rate’ after Emile Baudot; that a serial
connector has a ring indicator pin because the original
connector was meant to attach to a modem and use a
RS-232 and Modems
The telegraph and teletype machines used a
binary (on or off) signal to transmit and receive data.
However, the world was wired not for binary signals,
but for telephones that sent analog signals (voice —
300 to 3,400 Hz). During the 1950s, folks figured
out how to allow binary signals to be sent over these
plain old telephone lines by acoustic frequency
MOdulating and DEModulating of the signal.
Thus, the name modem.
August 2011 69
■ FIGURE 5. Emile Baudot pictured on a French phone card.