Vaisala GMP231 - Manual (Page 58)

User's Guide _______________________________________________________________________
56 ___________________________________________________________________ M211501EN-C
Timing
The minimum time delay that the master must wait between the invoke
and response messages depends on the operation. If the operation
includes a write to the non-volatile memory, it takes more time than other
operations.
Table 16 Timing
Operation Minimum delay
Normal delay between invoke and response 10 ms
Delay when write to non-volatile memory 300 ms
Status Byte
The status byte gives the master device information about the slave
device’s state, and information about the communication status with the
ACK/NACK bit.
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 17 Status Byte
Bit# Description
0
ACK/NACK
1 Critical error
2 Error
3 Warning
4 Status
5
6
7
Checksum
GMP231 uses a CRC16-CCITT checksum in its protocol frame. CCITT
(x.25) polynomial is X
16
+X
12
+X
5
+1 = 11021h. Initial value is 0000h and
the final value is XORed with FFFFh. 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.