Appendix B - Network Terminal RS-232 Interface Protocol Format
B-2
P/N 8040375G001
C
HELIX TECHNOLOGY CORPORATION
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TI
CRYOGENICS
it used to produce the data field, and appends it after the field just prior to
the CR code terminator. The receiving unit performs the same checksum
algorithm on all characters which it receives between the ’$’ character and
the character just prior to the CR terminator (non-inclusive). 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.
Checksum Algorithm
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
checksum 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 Network Terminal
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 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
