Campbell Scientific CR7 Measurement and Control System - Manual (Page 66)

SECTION 6. 9 PIN SERIAL INPUT/OUTPUT
6-4
FIGURE 6.5-1. Transmitting the ASCII Character 1
6.5.3 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 or more. 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 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 eighth bit is sometimes
used for a type of error checking called parity
checking. Even parity binary numbers 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
CR7 ignores the eighth bit of a character that is
receives, 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 data bit, and a Stop bit is sent
after the eighth data 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 6.5-1 shows how the ASCII character "1"
is transmitted. The SC32A interface transmits
spacing and marking voltages which are
positive and negative, as shown. Signal
voltages at the CR7 I/O port are 5 volts in the
spacing condition, and 0 volts in the marking
condition.
BAUD RATE
BAUD RATE is the number of bits transmitted
per second. The CR7 can communicate at 300,
1200, 9600, and 76,800 baud. In the
Telecommunications State, the CR7 will set its
baud rate to match the baud rate of the modem.
The baud rate of the modem or computer is
usually set 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.
To overcome the limitations of half duplex,
some communications links expect a terminal
sending data to also write the data to the
screen. This saves the remote device having to
echo that data back. If, when communicating
with a Campbell Scientific device, characters
are displayed twice (in pairs), it is likely that the