Campbell Scientific CR1000KD CR1000 Measurement and Control System - Page 201

Section 7. Installation 7.8.8.1 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. Imagine that an instrument transmits the byte "11001010" to the CR1000. The instrument does this by translating "11001010" into a series of higher and lower voltages, which it transmits to the CR1000. The CR1000 receives and reconstructs these voltage levels as "11001010." Because an RS-232 or TTL standard is adhered to by both the instrument and the CR1000, 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. 201) shows a sample of ASCII / ANSI character and code equivalents. Table 30. ASCII / ANSI Equivalents Byte Received ASCII Character Displayed Decimal ASCII Code 00110010 2 50 1100010 b 98 00101011 + 43 00001101 cr 13 00000001 ☺ 1 Hex ASCII Code 32 62 2b d 1 Read More! See the appendix ASCII / ANSI Table (p. 553) for a complete list of ASCII / ANSI codes and their binary and hex equivalents. The face value of the byte, however, is not what is usually of interest. The manufacturer of the instrument must specify what information in the byte is of interest. For instance, two bytes may be received, one for character 2, the other for character b. The pair of characters together, "2b", is the hexadecimal code for "+", "+" being the information of interest. Or, perhaps, the leading bit, the MSB (Most Significant Bit), on each of two bytes is dropped, the remaining bits combined, and the resulting "super byte" translated from the remaining bits into a decimal value. The variety of protocols is limited only by the number of instruments on the market. For one in-depth example of how bits may be translated into usable information, see the appendix FP2 Data Format (p. 557). Note ASCII / ANSI control character ff-form feed (binary 00001100) causes a terminal screen to clear. This can be frustrating for a developer who prefers to see information on a screen, rather than a blank screen. Some third party terminal emulator programs, such as Procomm, are useful tools in serial I/O development since they handle this and other idiosyncrasies of serial communication. When a standardized serial protocol is supported by the CR1000, such as PakBus® or Modbus, translation of bytes is relatively easy and transparent. However, when bytes require specialized translation, specialized code is required in the user-entered CRBasic program, and development time can extend into several hours or days. 201