INFICON PSG55x ATM to Medium Vacuum Gauge (RS232C / RS485C) - Manual (Page 4)

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tira59e1 (2010-07) RS232C.cp
Every protocol layer of the communication protocol is represented in a protocol
frame. The maximum length per frame is 64 byte. The data field of the data link
layer consists of
• a command (Cmd)
• a parameter identifier (PID)
• a data field (Data).
The command determines whether the data transmitted are read or write requests.
There are four different command types:
Cmd 1 Read request from the Master
Cmd 2 Read response from the gauge to a read request
Cmd 3 Write request
Cmd 4 Write response from the gauge to a write request
• Cmd 1: Read request from the Master
0 1 2 3 4 5 7 9
MSB LSB MSB LSB
Address Device ID Ack MsgLength Cmd = 1 PID Reserved Crc
• Cmd 2: Response from the gauge to a read request
0 1 2 3 4 5 7 9 9+n
MSB LSB MSB LSB
Address Device ID Ack MsgLength Cmd = 2 PID Reserved Data Crc
• Cmd 3: Write request from a Master
0 1 2 3 4 5 7 9 9+n
MSB LSB MSB LSB
Address Device ID Ack MsgLength Cmd = 3 PID Reserved Data Crc
• Cmd 4: Response from the gauge to a write request
0 1 2 3 4 5 7 9+n
MSB LSB MSB LSB
Address Device ID Ack MsgLength Cmd = 4 PID Reserved Crc
The parameter identifier (PID) addresses a defined parameter of the device.
The data field contains the data of a request. In the case of a write request it con-
tains the data transmitted from the Master to the gauge, in the case of a read
request it contains the data to be transmitted from the gauge to the Master. Data
are transmitted in Big Endian.
The medium access layer contains the following data fields:
• RS485 address (for RS232 the address is 0)
• Device ID (Master 0, PCG5xx 2)
• Header with Ack and Message Length (all APDU data are counted: Cmd, PID,
Reserved, Data)
• 16 bit CRC (example → 10)
Protocol Frame
Command (Cmd)
Parameter Identifier (PID)
Data field (Data)
Medium Access Layer