Appendix B - On-Board Module Rs-232 Interface Protocol Format - HELIX CTI-CRYOGENICS On-Board FastRegen Programming And Operation Instructions

Appendix B - On-Board Module RS-232 Interface Protocol Format

B-2
If this sum matches the final character preceding the CR terminator, then
the message is validated and processed by the receiver. If not, then an error
has occurred and the action taken depends on whether the receiver is the
HOST or slave unit. The checksum algorithm generates a character
between ASCII '0' and 'o' (30 hex to 6F hex) inclusive.
The checksum algorithm is defined as follows: Perform the 8 bit (modulo
256) sum of all the ASCII characters sent in the data field (with the most
significant bit cleared to 0, ignore parity). This is performed for one to
fourteen character code bytes. Fold the resulting eight bit sum into six bits
by exclusive oring the two MSBs of the sum (D7, D6) with the two LSBs
(D1, D0) of the sum such that the new D1 is the old D1 XOR D7 and the
new D0 is the old D0 XOR D6. The resulting lower six bits (D5 . . D0) are
then masked off, producing a code range of from 00 to 3F hex. This is then
added to the ASCII code for '0' (30 hex), generating the final printable
checksum character in the range of 30 hex to 6F hex ('0' . . 'o').
Message traffic is always originated by the HOST unit. This message is
referred to as a command or query. Commands cause specific actions to
occur in the On-Board Module. Queries request that the On-Board Module
reply with status or other parametric information. The On-Board Module
responds to all such correctly received messages with a response message.
The pairing of these command-response or query-response message sets
defines a transaction or exchange. If a faulty message is received by the
On-Board Module (due to improper production in the host, or transmission
media failure), the message is discarded and no response is sent back to the
host as a reply. The host must be able to detect that either no response or an
invalid response was received from the slave (through time-out and check-
sum detection), and if desired, repeat the message to the slave in an attempt
to secure a valid transaction.
All communications between a host computer and the On-Board Module
will occur within this message transaction framework. Software operating
on the host computer must generate and interpret the message-response
pairs to properly execute and control remote operation and data-logging of
the On-Board Module. Software within the On-Board Module interprets
these valid messages and returns appropriate replies, as documented in the
On-Board Module RS-232 command list. Error code messages may be
returned by the On-Board Module if a valid message packet is received but
the data field contents are not correct and cannot be interpreted. This is not
a communications error, but a software error. Invalid commands, improper
parameter ranges, or requests to perform operations which are disallowed
for some reasons all result in an error message reply.
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C TI CRYOGENICS
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