Badger Meter SRD/SRI Valve Positioners - Manual (Page 90)

Output Pressure Scaling Low
This is a 4-byte real, expressed in the units defined in the Units page . It specifies the minimum value for scaling of the output
pressure, which is displayed in the Device Data Screen
Options Page
This is a single-byte parameter with each bit representing different options that may have been added to this particular
SRD991 . Currently there are five bits defined as follows:
7 6 5 4 3 2 1 0
NOT USED NOT USED NOT USED
Extern Analog
Sensor
Extern Binary
Output
Extern Binary
Input
Intern Press
Sensor
Extern Posit
Xmit
Check Sum
If the 5 bytes of the Device ID are not correct, an improper CRC will be generated and sent as part of the message . On
arrival, the CRC will be detected as invalid and the command will be ignored - that is, no response will be sent at all from
the positioner!
Calculation of the CRC for non-secure commands is simply to apply the CRC algorithm to all bytes of the message . For secure
(and enhanced mode) transmissions, the 5 bytes of the Device ID are used to seed the CRC calculation . It is as if those bytes
were actually sent, but they are not for timing reasons . This second, more elaborate, CRC is labeled the PCRC in the new
command definitions .
Presume the current byte being added to the CRC is 0x5A and presume the two byte CRC for the current command is 0x12,
0x34 up to this point . Then the steps taken in the software calculation are as follows:
• The current byte is exclusive or'd with the most significant byte of the CRC (crcmsb)
crcmsb = 0x12 ^ 0x5A = 0x48
• The most and least significant bytes are swapped
◊ crclsb = 0x48
◊ crcmsb = 0x34
• Four-bit permutations are done:
the crclsb is exclusive or'd with the top 4 crclsb bits shifted down 4 bits
crclsb = 0x48 ^ (0x48 >> 4) = 0x48 ^ 0x04 = 0x4C
• The crcmsb is exclusive or'd with the bottom four crclsb bits shifted up 4 bits
crcmsb = 0x34 ^ (0x4C << 4) = 0x34 ^ 0xC0 = 0xF4
• The crcmsb is exclusive or'd with the top 5 crclsb bits shifted down 3 bits
crcmsb = 0xF4 ^ (0x4C >> 3) = 0xF4 ^ 0x09 = 0xFD
• The crclsb is exclusive or'd with the bottom three crclsb bits shifted up 5 bits
crclsb = 0x4C ^ (0x4C << 5) = 0x4C ^ 0x80 = 0xCC
So the CRC saved for the next byte will be 0xFD, 0xCC .
The reason for all this calculation is to insure detection of all single bit transmission errors . Obviously, the calculation must be
done the same way on both ends or a mismatch will occur and no reply will be sent .
Measurement Value
The device data display shows the static tag information and data, such as setpoint, position, and error, which is updated . :
Setpoint
This is a 4-byte real, expressed as percent . Since it is the digital setpoint, Setpoint source should be set to Digital in the
configuration screen . This parameter provides a read back of the setpoint value that the positioner is currently using .
(Default range is: 0 .0…100 .0 %) .
Position
This is a 4-byte real, expressed as percentage . The normal operating range for the valve position is 0 to 100% of the full scale .
Additional Status
Page 90 August 2014 POS-UM-00011-EN-02