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On-Board FastRegen Control Module Programming and Operation Instructions
Appendix B - On-Board Module RS-232
P/N 8040406
Interface Protocol Format
The format of RS-232 messages between a computer device (the HOST)
and the On-Board module (slave) is the same for both directions of
message flow (HOST transmitted or slave transmitted). Each message
consists of a series of ASCII characters transmitted via a standard RS-232
asynchronous framing convention of one (1) start bit, seven (7) data bits, a
parity bit generated for even parity, and one (1) stop bit; at a transmission
rate of 2400 baud.
The message packet is composed of a starting flag character (the $
character, hex 24), followed by a message dependent data field, followed
by a message checksum character, terminated by an ASCII carriage return
code (hex 0D). The starting flag character serves the unique purpose of
synchronizing the receiver to the transmitter, by signaling the start of the
message packet. This '$' code is not contained in the set of characters used
to construct the data field or the checksum character, and therefore
establishes a fixed reference point to sync up data flow. Whenever either
receiver (HOST or slave) receives a '$' character, all history and status of
previous partial packet data (if any) is aborted and lost, and a packet
message is started anew.
The data field consists of from one (1) to a maximum of fourteen (14)
ASCII characters, the meaning of which is defined in the Pump Command
List for commands and responses. All characters with the exception of '$'
and Carriage-Return (0D hex) may be employed in the data field, if
suitable.
The message checksum character is employed to guard against garbled or
incorrect messages being received and acted upon, causing undesirable or
damaging results. Only messages which are conveyed accurately and intact
from the master to slave (or visa versa) are accepted and acted upon. The
checksum character which follows the data field is computed by a modified
binary sum technique (described later) over the characters composing the
data field. The transmitting unit generates this sum based on the characters
it used to produce the data field, and appends it after the field just prior to
the CR code terminator. The receiving unit performs the same checksum
algorithm on all characters which it receives between the '$' character and
the character just prior to the CR terminator (non-inclusive).
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C
TI
CRYOGENICS
HELIX TECHNOLOGY CORPORATION
B-1
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