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117Appendix A Communications with a Computer
A
Communications with a Computer
Application
The Model 8718B can be connected to a computer for real time
transmission of data via the meter’s RS232 connector. A limited
number of the meter’s functions can also be controlled remotely
via the RS232 bus.
Checksum and Modulo Description
Modulo 256 is the remainder of any number X divided by 256. As
an example, if the checksum is 1234, Modulo 256 of 1234 is 210.
The two ASCII digits to be transmitted as the checksum would
be D and 2 (the decimal-hex conversion of 210). An equivalent
calculation is to convert the sum to Hex and transmit the two
lowest digits. The Hex of 1234 is 4D2, therefore the digits to
transmit are ASCII D and 2.
Computer to Model 8718B Message Format
The first character in any message is the “>”. This allows the
Model 8718B to synchronize to the computer. Upon detection
of the “>” character, the 8718B will:
•
Set itself to a listen mode, thereby eliminating the
possibility of a bus contention problem.
•
Reset its character counter to allow storage of
succeeding characters in an internal buffer.
The next characters(s) is selected from the functions listed in
Command Character Function table.