INFICON STM-100/MF Thin Film Deposition Monitor - Manual (Page 58)

Sycon Protocol
The STM-100 / MF does not initiate any messages on its own. It responds
only when "spoken" to. In order for error-free communications to occur, several
safeguards are built in for checking data integrity. The general format for sending
and receiving commands is:
STX (Data Length) (Data(Data..Data)) (Checksum)
The STX character 0x02 (hex 02) indicates a start of message sequence.
The STM-100 / MF uses this as a synchronization point. It just sits in a loop
waiting for the STX character. If a different character is received, it is discarded.
When the STX character is received, the STM looks at the rest of the characters in
the command.
The data length is a character from 'SOH' (1 decimal) through 'LF' (10
decimal) and indicates the number of data characters in the message. The data is
defined in the protocol below. The STM-100 / MF uses this number to determine
where the end of the command is. If there is a mismatch between this number
and the actual number of data characters in the message, two things can happen.
If the data length number is low, the STM-100 / MF will terminate the command
prematurely. The checksum will not match and an error will be recognized by the
STM-100 / MF. It will not respond in any way.
If the data length number is high, the STM-100 / MF will be waiting for more
characters than actually contained in the message. In order to recover from either
kind of error, the host computer must have a time-out / retry capability built into
the software.
The checksum is the sum (modulo 256) of only the data bytes. If this does
not match the actual checksum of the data, an error message is recognized by the
STM-100 / MF. It will not respond to the command. Refer to Figure 5.2 is a
BASIC program that will run on an IBM-PC. It prompts you for a command to send
to the STM-100 / MF and will print the response on the display. It is written in
BASICA program, but will also run under Microsoft GWBASIC.
SECTION 5.2
Message Format
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