Campbell Scientific CR23X Micrologger - Manual (Page 85)

SECTION 6. 9-PIN SERIAL INPUT/OUTPUT
6-7
TABLE 6.2-4. DTE Pin Configuration
PIN = 25-pin connector number
ABR = Abbreviation for the function name
O = Signal Out of terminal to another device
I = Signal Into terminal from another device
PIN ABR I/O FUNCTION
2 TD O Transmitted Data: Data
is transmitted from the
terminal on this line.
3 RD I Received Data: Data is
received by the terminal
on this line.
4 RTS O Request to Send: The
terminal raises this line
to ask a receiving device
if the terminal can
transmit data.
5 CTS I Clear to Send: The
receiving device raises
this line to let the
terminal know that the
receiving device is ready
to accept data.
20 DTR O Data Terminal Ready:
The terminal raises this
line to tell the modem to
connect itself to the
telephone line.
6 DSR I Data Set Ready: The
modem raises this line to
tell the terminal that the
modem is connected to
the phone line.
8 DCD I Data Carrier Detect: The
modem raises this line to
tell the terminal that the
modem is receiving a
valid carrier signal from
the phone line.
22 RI I Ring Indicator: The
modem raises this line to
tell the terminal that the
phone is ringing.
7 SG Signal Ground: Voltages
are measured relative to
this point.
6.2.6.3 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 CR23X 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 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 6.2-5 shows how the ASCII character "1"
is transmitted. When transmitted by the CR23X
using the SC32A RS-232 interface spacing and
marking voltages are positive and negative, as
shown. Signal voltages at the CS I/O port are
5V in the spacing condition, and 0V in the
marking condition.