4-3
4.12 ASCII
Table 4.1 shows the ASCII (American Standard Code for Information Interchange)
symbols which can be encoded in a 7-bit binary code (DB0 through DB6). When
organized in table form, these 7 bits may be regarded as the symbol address, the
most significant 3 bits determining the column and the last four bits determining the
row.
These symbols include all the decimal numerals, letters, punctuation marks, common
abbreviations and control characters, including non-printed symbols such as Carriage
Return and Line Feed.
The 7-bit symbol code (or address) is called a “character”, and digital communication
with the meter is made with a string of these characters.
When transmitted, each character is preceded by a start bit (BAUD) and followed by
one or two stop bits plus an optional parity bit, making a train of 10 or 11 baud for
each transmitted character. If you are building a system from the UART up (Universal
Asynchronous Receiver/Transmitter), that device must be informed of the number of
data bits, parity, stop bit length, etc., so that it properly decodes the incoming stream
into the bytes that your program can recognize (check the UART or plug-in board
literature for required control signals).
As dictated by FORMAT statements, a symbol may be sent by transmitting just its
table address (one character, plain ASCII) or by HEX-ASCII, which uses two
characters, one for each of the two hex address nibbles (0 through 7 for the column
nibble, 0 through F for the row nibble, shown on top and left-hand side of Table 4.1).
4. Definitions
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