Crc; Sequence Numbering; Response To Link And Physical Errors; Reserved Addresses - Honeywell UMC800 Reference Manual

Protocol Description

2.4 CRC

The 16 bit cyclic redundancy is computed using a CRC-16 algorithm (as opposed to the CCITT CRC
algorithm). It will be placed in the packet LSB first. The check will include everything between the packet
header and footer. It will not include any inserted DLE characters. This is because the DLE insertion must
protect the CRC in the event that CRC == DLE|STX or CRC == DLE|ETX.
For the CRC-16 Calculation, refer to Appendix A.

2.5 Sequence Numbering

The sequence number is a serial number that uniquely identifies a message. The host generates an arbitrary
sequence number when it sends a request to an instrument. The instrument echoes this number in its reply.
This is so that the host can verify that the instrument is replying to the correct message.
Imagine the following scenario: the host sends a message with sequence number 5 to the instrument.
However, due to a firmware update, it now takes the instrument 10 seconds to reply to this particular
message. After 3 seconds, the host times out. A few seconds later, the host sends a different message with
sequence number 6 to the instrument. By this time, the instrument has just finished processing message
number 5 and sends the reply. The host is able to detect the out-of-synch error because the request had
sequence number 6 but the reply had sequence number 5.
Note that it is not necessary for the sequence numbers to be in any sort of order. Therefore, for the sake of
efficiency, the host may optionally skip sequence number 16 to unburden the instrument from having to
DLE-strip the sequence number. This is not a feature of the protocol. The instrument should not assume
that it would never get a sequence number 16.

2.6 Response to Link and Physical Errors

The instrument will not respond to bad incoming messages in any way. It will be the host's responsibility to
detect the timeout and retransmit the message. The host will respond to bad replies from the instrument by
retransmitting the message without incrementing the sequence number, as mentioned before.

2.7 Reserved Addresses

Unit address 1 is reserved for broadcast messages, which all of the instruments on the link will obey, but
none of the instruments will reply to. Unit address 255 is reserved for the host.

2.8 Static and Dynamic Parameters

There are two types of parameters that can be accessed:
STATIC – a static parameter is a configuration parameter
DYNAMIC – A dynamic parameter is an Input/Output parameter.
Each of these parameter types has a specific index number set.
Configuration parameters (Static) and I/O parameters (Dynamic) cannot be accessed in the same message
since both the Static parameters and the Dynamic parameters each start at Index Number 0.
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