Sigma – INFICON SQC-122 Thin Film Deposition Controller Communications Manual User Manual
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Sigma
instruments
SQC-122 Communications Protocol
The SQC-122 communicates with a host computer via an ASCII based protocol. The
instrument defaults to 19200 baud, 8 data bits, and no parity. The baud rate can be
changed in the System Menu of the SQC-122, but is always 8 data bits with no parity.
The basic protocol is:
Once a valid command has been transmitted to the SQC-122, a response is returned.
The structure of the packet is identical in both the command and response. In the
response, the first character is a Response Status. These are summarized in the
following table.
Response Letter
Meaning
A
Command understood, normal response
B
Command understood, but instrument reset
C
Invalid command
D
Problem with data in command
E
Instrument in wrong mode for this command
The sync character is an exclamation point ‘!’. Anytime this character is received, the
communications for that packet is reset. Following the sync character is the length
character. This is the number of characters in the packet starting with the length and
counting the 2 CRC characters. This character has a decimal 34 added to it so there
cannot accidentally be a sync character embedded in the packet. The two character
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