Edwards SCU-800 - Transmission Data Format; Frame Control (Checksum)

SCU-800 Control Unit for Turbomolecular Pump
MT-49E-101-G
Page 70
5
SERIAL COMMUNICATION
PROTOCOL
Always assign less than 254 characters (n< 254) to the parameter so that the message is less than
256 characters.
Next, the preceding SIM->PC character is "Ack", the instructed query command is executed and the
SIM returns the following response (1st block).
PC->SIM
Ack or Nak
SIM->PC
Stx
0
0
1
Sp
CHR
C
1
C
253
Etb
LRC
Then "Ack" is sent by the PC->SIM character in reaction to the response (1st block) from the SIM, the
SIM returns the following response (2nd block).
PC->SIM
Ack or Nak
SIM->PC
Stx
0
0
2
C
254
C
n
Etx
LRC
The PC transmits "Ack" or "Nak"; then transmits the next command if necessary.
5.3.5 Transmission data format
Data value is always 16 bits signed hexadecimal value coded ASCII text.
Example: 12090 on a decimal basis equals to 2F3A on a hexadecimal basis.
5.3.6 Frame control (checksum)
The transmission frame is controlled by the odd number parity check. First initialize LRC as FFhex.
Next calculate LRC by EXCLUSIVE-OR (XOR) of all the frame bytes containing "Stx", "Etb", "Etx" and
LRC and transmit the result as LRC.
Examples:
Character string for calculation before calculating LRC.
ASCII
Stx
0
0
1
#
Etx
LRC
HEX
02
30
30
31
23
03
FF
Calculation of LRC:
02
hex
XOR 30
hex
XOR 30
hex
XOR 31
hex
XOR 23
hex
XOR 03
hex
XOR FF
hex
= EC
hex
Character string for transmission after calculating LRC.