INFICON SQC-122C Thin Film Deposition Controller Communications Manual - Manual

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SQC-122c Communications Protocol
The SQC-122c communicates with a host computer via a serial ASCII protocol at
19200 baud, 8 data bits, and no parity. The SQC-122c only responds to commands
received. It never initiates communications.
The command protocol sent to the instrument is:
<sync character> <length character> <1 to n data characters> <CRC1><CRC2>
The sync character is always an exclamation point ‘!’. Following the sync character is
the length character. This is the number of characters in the packet (not counting the
sync, length, and CRC characters). The length character has a decimal 34 added to it
so there cannot accidentally be a sync character (!) embedded in the packet.
Following the length character are the command and data characters as detailed later
in this section. After the data come two CRC characters.
The CRC is computed using the following algorithm:
1. The CRC is initialized to 3FFF hex.
2. Each character in the message is examined, bit by bit, and added to the CRC
in the following manner:
a) The character is exclusive OR’d with the CRC.
b) The CRC is shifted right one bit position.
c) If the character’s least significant bit is a 0 then the CRC is exclusive
OR’d with 2001 hex.
d) Steps b and c are repeated for each of the 8 bits in the character.
The CRC contains 14 significant bits. This is split into two characters of 7 bits each,
and then a decimal 34 is added to offset the character outside the range of the Sync
Character.