Campbell Scientific CR1000 Measurement and Control System - Manual (Page 204)

Section 7. Installation
204
Marks and Spaces
RS‐232 signal levels are inverted logic compared to TTL. The different
levels are called marks and spaces. When referenced to signal ground,
the valid RS‐232 voltage level for a mark is ‐3 to ‐25, and for a space is
+3 to +25 with ‐3 to + 3 defined as the transition range that contains no
information. A mark is a logic 1 and negative voltage. A space is a logic 0
and positive voltage.
MSB
Most significant bit (the leading bit).
RS‐232C
Refers to the standard used to define the hardware signals and voltage
levels. The CR1000 supports several options of serial logic and voltage
levels including RS‐232 logic at TTL levels and TTL logic at TTL levels.
RX
Receive
SP
Space
Start bit
Is the bit used to indicate the beginning of data.
Is the end of the data bits. The stop bit can be 1, 1.5 or 2.
TX
Transmit
7.8.8.5 CRBasic Programming
To transmit or receive RS-232 or TTL signals, a serial port (see table CR1000
Serial Ports
(p. 202)
) must be opened and configured through CRBasic with the
SerialOpen() instruction. The SerialClose() instruction can be used to close the
serial port. Below is practical advice regarding the use of SerialOpen() and
SerialClose(). Program CRBasic example Receiving an RS-232 String
(p. 210)
shows the use of SerialOpen(). Consult CRBasic Editor Help for more
information.
SerialOpen (COMPort,BaudRate,Format,TXDelay,BufferSize)
• COMPort — Refer to CRBasic Editor Help for a complete list of COM ports
available for use by SerialOpen().