INFICON LDS3000 Modular Leak Detector Interface description - Manual (Page 28)

28 LD Protocol
5 LD Protocol
5.1 Communication Parameters
5.2 Command format
5.2.1 Telegram structure
Master sends
Slave answers
Data format
Baudrate
19.200, 8 data bits, 1 stop bit, no parity
ENQ LEN ADR CmdH CmdL DATA (n bytes) CRC
0 1 2 3 4 5 5 + n
STX LEN StwH StwL CmdH CmdL DATA (n bytes) CRC
0 1 2 3 4 5 6 6 + n
Command Meaning
ENQ
0x05 Start of master request
STX 0x02 Start of slave response
LEN
Number of
telegram bytes
without ENQ(STX)/LEN, however with CRC
max. 253, so the total slave telegram length is max.
255
ADR Slave address
Slave address = 1: non-addressed bus. Address byte
is ignored.
Stw H/L Status word Info from slave to master ( 5.3 )
Cmd H/L Command
Bit 15 ... 13: Command-specifier Read/Write etc. (see
table “Cmd H/L: Command: Command-specifier”)
Bit 12: free
Bit 11 ... 0: Command (
5.3 )
DATA
Data belonging
to master
request
(Slave reply to
write command
is sent without
data)
0 <= n <= 248
If I/O module (7-byte additional header) is used, then
limit maximum data length to 241.
CRC Checksum
Calculate CRC for all bytes (except CRC byte)
Polynomial: 0x98,
Name: DOWCRC,
Maxim/Dallas, X^8+X^5+X^4+1
Info:
CRC calculation see document "CRC_calculation.c"
(C souce code)