INFICON STM-2 USB Thin Film Rate/Thickness Monitor - Manual (Page 72)

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STM-2 Operating Manual
5.2.1.1 Checksum
 Compute the mod 256 checksum of ADDR, CMD_RSP and DATA fields, before
byte stuffing with escape characters. This is the logical content (payload) of the
command.
 CKSUM1 is the upper (most significant) four bits of the checksum (read as a
nibble, 0 through 15, or right justified) plus hexadecimal 30 (ASCII zero). This
yields an ASCII character from zero (hexadecimal 30) to ? (hexadecimal 3f).
 CKSUM2 is the lower (least significant) four bits of the checksum plus
hexadecimal 30 (ASCII zero). This yields an ASCII character from zero
(hexadecimal 30) to ? (hexadecimal 3f).
NOTE: If the checksum of the command is invalid (in form or value), the packet
is deemed invalid and will be ignored.
CAUTION
Invalid commands (bad checksum, too short, corrupt
data, bad escape sequences) will be ignored by slaves
(and masters). No response will be sent by the slave.