CTI-CRYOGENICS On-Board Network Terminal, Installation, Operation And Maintenance Instructions, Page: 38 / 51

Network Terminal Installation, Operation, and Maintenance Instructions
P/N 8040375G001 B-1
C
HELIX TECHNOLOGY CORPORATION
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TI CRYOGENICS
Appendix B - Network Terminal RS-232
Interface Protocol Format
Introduction
The format of RS-232 messages between a computer device (the HOST)
and the Network Terminal (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, 9600, 14200, and 38400 baud.
The message packet is composed of a starting flag character (the $
character, hex 24), followed by a P and the pump address (example
P00=PUMP #0, P01=PUMP #1), when talking to a pump (N when talking
to a Network Terminal), followed by a message dependent data field,
followed by a message checksum character, terminated by an ASCII
carriage return code (hex 0D). The pump address is set by means of a
rotary switch on the On-Board Pump Module. 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