dbx quick_guide - Manual (Page 2)

note: there must be at least one byte in the data section. A length of zero will indicate
that 256 bytes are in the message frame.
Acknowledging a guaranteed message:
Even if you are not decoding received messages, if a guaranteed delivery message is sent
to a controller, the controller must acknowledge the message to keep the protocol
running.
A guaranteed message will have the format:
<0x77><0x80 | src>
<0x7b><0x80 | src>
After the ccit is received, send an ACK (0xA5) to the DriveRack.
Addresses:
400 series DriveRack units can be assigned addresses 0-99. A device address 0 will
indicate that this DriveRack can be an access point to control multiple DriveRack devices
though the multidrop 485 bus that attaches 400 series processors into a single network. A
232 connection to the master provides access to the entire DriveRack network.
The control panel may use either address 0x7b or address 0x77. If you are attached to a
network master (device address 0) use an address of 0x7b. If you are attached to a
network slave (device address 1-99) use address 0x77 as the controllers address.
200 series DriveRacks are assigned the address 1.
The ccitt checksum:
The ccitt checksum has some advantage s over a standard checksum.
Standard sum of bytes checksum.
The string: <0x00><0x00><0x00> has a checksum of <0x00>.
The string: <0x00><0x00> also has a checksum of <0x00>.
The string: <0x00><0x01><0x02><0x03> has a checksum of <0x06>.
The string <0x01><0x00><0x03><0x02> also has a checksum of <0x06>.
The ccitt checksum of the above 4 strings are: <0xd2>, <0x81>, <0x33>, <0x8d>.
Some common com faults such as dropped bytes or reversed bytes are effectively
checked using the ccitt checksum where a sum of bytes would fail to detect a fault.
The 8 bit ccitt checksum used by the DriveRack series can be caculated with a table
lookup and an exclusive OR logic function – so it is relatively easy to calculate.