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Section 6. 9-Pin Serial Input/Output
6-9
An ASCII character is a binary digital code composed of a combination of seven
bits, each bit having a binary state of 1 (one) or 0 (zero). For example, the binary
equivalent for the ASCII character ‘1’ is 0110001 (decimal 49).
ASCII characters are transmitted one bit at a time, starting with the first (least
significant) bit. During data transmission the marking condition is used to denote
the binary state 1, and the spacing condition for the binary state 0. The signal is
considered marking when the voltage is more negative than -3V with respect to
ground, and spacing when the voltage is more positive than +3V.
Most computers use eight bits (one byte) for data communications. The eighth bit
is sometimes used for a type of error checking called parity-checking. Even-
parity binary characters have an even number of 1’s, odd-parity characters have an
odd number of 1’s. When parity checking is used, the eighth bit is set to either a 1
or a 0 to make the parity of the character correct. The CR10X ignores the eighth
bit of a character that is received, and transmits the eighth bit as a binary 0. This
method is generally described as ‘no parity’.
To separate ASCII characters a start bit is sent before the first bit, and a stop bit is
sent after the eighth bit. The start bit is always a space, and the stop bit is always a
mark. Between characters the signal is in the marking condition.
Figure 6-5 shows how the ASCII character ‘1’ is transmitted. When transmitted by
the CR10X using the SC32A RS232 interface, spacing and marking voltages are
positive and negative, as shown. Signal voltages at the CR10X I/O port are 5V in
the spacing condition and 0V in the marking condition.
Figure 6-5 Transmitting the ASCII Character 1
Baud Rate
The baud rate is the number of bits transmitted per second. The CR10X can
communicate at 300, 1200, 9600 and 76,800 baud. In the Telecommunications
State, the CR10X sets its baud rate to match the baud rate of the computer/terminal.
Note that the CR10X does not support telecommunications at 76,800 baud.
Typically the baud rate of the modem/ computer/terminal is either set with
switches or programmed from the keyboard. The instrument’s instruction manual
should explain how to set it.
Duplex
Full duplex means that two devices can communicate in both directions simulta-
neously. Half duplex means that the two devices must send and receive
alternately. Full duplex should always be specified when communicating with
Campbell Scientific peripherals and modems. However, communication between
some Campbell Scientific modems (such as the RF95 RF modem) is carried out in
a half duplex fashion. This can affect the way commands should be sent to and