Campbell Measurement and Control Module CR10 Operator's Manual page 92

If the computer/terminal is configured as DCE
equipment (pin 2 is an input for RD), a null
modem cable is required. See the SC32A
manual for details.
6.7.3 COMMUNICATION PROTOCOL/TROUBLE
SHOOTING
The ASCII standard defines an alphabet
consisting of 128 different characters where
each character corresponds to a number, letter,
symbol, or control code.
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 1st (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 minus three volts with
respect to ground, and spacing when the
voltage is more positive than plus three volts.
Most computers use 8-bits (1 byte) for data
communications. The 8th 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 8th bit is set to either a 1
or a 0 to make the parity of the character
correct. The CR10 ignores the 8th bit of a
character that is receives, and transmits the 8th
bit as a binary 0. This method is generally
described as "no parity".
To separate ASCII characters a Start bit is sent
before the 1st bit, and a Stop bit is sent after
FIGURE 6.7-1. Transmitting the ASCII Character 1
SECTION 6. 9-PIN SERIAL INPUT/OUTPUT
the 8th 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 6.7-1 shows how the ASCII character "1"
is transmitted. When transmitted by the CR10
using the SC32A RS232 interface spacing and
marking voltages are positive and negative, as
shown. Signal voltages at the CR10 I/O port
are 5V in the spacing condition, and 0V in the
marking condition.
BAUD RATE
BAUD RATE is the number of bits transmitted
per second. The CR10 can communicate at
300, 1200, 9600, and 76,800 baud. In the
Telecommunications State, the CR10 will set its
baud rate to match the baud rate of the
computer/terminal.
Typically the baud rate of the modem/computer/
terminal is set either with dip 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 simultaneously.
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 received
from such a modem, especially when implemented
by computer software.
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