Campbell CR800 Series - Introduction; Self-Heating and Resolution; Serial I;O: Capturing Serial Data

Section 7. Installation
279
7.7.16.7 Self-Heating and Resolution
Programming the CR800 to make a PRT measurement requires a judgment call.
To maximize measurement resolution, the excitation voltage must be maximized.
However, to minimize self-heating of the PRT element, excitation voltage must be
minimized. Keeping the voltage drop across the PRT to ≤25 mV addresses both
concerns since self-heating is normally less than 0.001°C in still air. To
maximize the measurement resolution, optimize the excitation voltage (Vx) such
that the voltage drop across the PRT spans, but does not exceed, the voltage input
range.
7.7.17 Serial I/O: Capturing Serial Data
The CR800 communicates with smart sensors that deliver measurement data
through serial data protocols.
Read More See Comms and Data Retrieval
(p. 426) for background on
CR800 serial communications.
7.7.17.1 Introduction
Serial denotes transmission of bits (1s and 0s) sequentially, or "serially." A byte
is a packet of sequential bits. RS-232 and TTL standards use bytes containing
eight bits each. Consider an instrument that transmits the byte "11001010" to the
CR800. The instrument does this by translating "11001010" into a series of
higher and lower voltages, which it transmits to the CR800. The CR800 receives
and reconstructs these voltage levels as "11001010." Because an RS-232 or TTL
standard is adhered to by both the instrument and the CR800, the byte
successfully passes between them.
If the byte is displayed on a terminal as it was received, it will appear as an ASCII
/ ANSI character or control code. Table ASCII / ANSI Equivalents
(p. 279) shows a
sample of ASCII / ANSI character and code equivalents.
ASCII / ANSI Equivalents
Byte
Received
ASCII
Character
Displayed
Decimal
ASCII
Code
Hex
ASCII
Code
00110010 2 50 32
1100010 b 98 62
00101011 + 43 2b
00001101 cr 13 d
00000001 ☺ 1 1