INFICON STM-2XM 2-Channel Rate/Thickness Monitor - Manual (Page 85)

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STM-2XM Operating Manual
5.5.1.1 Command Format
<STX><DATA LENGTH><DATA>...<CKSUM>
STX . . . . . . . . . . . . . . . . . . . . . . . . . Start of text character (hexadecimal 02).
If a different character is received, it is
discarded.
DATA LENGTH . . . . . . . . . . . . . . . . ASCII character from SOH (1 decimal)
through P (80 decimal) and indicates the
number of data characters in the message.
The data length is the count of message
bytes following the data length byte plus one
to designate the checksum termination. The
count of message bytes can be any valid 8 bit
decimal, from 0 (hexadecimal 00) through
255 (hexadecimal FF).
STM-2XM uses this number to determine
where the end of the command is. If a
mismatch between this number and the
actual number of data characters in the
message occurs, one of the following will
ensue:
 If the data length number is low,
STM-2XM will terminate the command
prematurely. The checksum will not
match. It will not respond in any way.
 If the data length number is high,
STM-2XM will be waiting for more
characters than actually contained in the
message.
NOTE: In order to recover from either error,
the host computer must have a
time-out/retry capability built into the
software. There is a maximum of 80
data characters allowed in any
communications message, although
most messages are short. The
length of commands and replies
depend on command specifics. This
does not include any packet
formatting characters.