Vaisala CARBOCAP GMP231, User Manual

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The status byte has bits for signaling Status, Warning, Error and Critical error statuses. These
bits are cleared when the status word is read. Bits are set when the corresponding status or
error state changes in the slave device.
Table 48 Status Byte
Bit# Description
0 ACK/NACK
1 Critical error
2 Error
3 Warning
4 Status
5
6
7
5.1.6 Checksum
GMP231 uses a CRC16-CCITT checksum in its protocol frame. CCITT (x.25) polynomial is
X
16
+X
12
+X
5
+1 = 11021
hex
. Initial value is FFFF
hex
and the final value is XORed with FFFF
hex
. The
checksum uses a bit reversed algorithm.
When the invoke message is received, the checksum is calculated using bytes from the
command byte to the last byte of the data field. The checksum needs to be valid before data
can be accepted. If the checksum is not valid, the message is rejected and the internal state of
the GMP231 is set to Idle.
When the response message is transmitted, the checksum is calculated using bytes from the
status byte to the last byte of the data field.
5.1.7 Status word
The status word is used to monitor the status of the GMP231 device. It can be read in the same
way as any other slave parameter. The status word is used as a 32-bit long bit field, where each
bit represents a state of some error or other essential status. The status word is sent least
significant bit (LSB) first.
The status word content is updated every two seconds. Changes in the status word bits can be
monitored by reading the status byte.
Table 49 (page 53) lists the content of the status word. The status bits are categorized
according to the severity of the status:
• Critical errors are fatal to the operation of the unit. It may not be able to respond to
communication at all, and does not measure correctly.
GMP231 User Guide M211501EN-E
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