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Data Block Length
This is set to the number of data bytes which follow in this packet. The minimum value
is 1, so if there are no data bytes, this should be set to 1 and the data block should contain
a single null byte (value 0).
Serial Number
In Command Packets this field contains the serial number of the Controller to which the
packet is intended, encoded in ASCII. In Reply Packets, this field contains the serial
number of the Controller transmitting the Reply Packet.
NB:
this refers to the serial number of the CONTROLLER, not the chamber.
Data Block
This field is used mainly in Reply Packets from the Controller to pass back the
information requested in Command Packets from the master, although some Command
Packets also have data associated with them. In general, the data bytes contained in the
Data Block are printable characters encoded in 7 bit ASCII (values 32 to 127). The bytes
may contain any 7 bit value (0 to 127).
XOR Checksum
This is an "exclusive OR" checksum calculated from all bytes from the Header to the
last byte in the Data Block (inclusive). The checksum is calculated as follows. A byte
variable C is initialised to 128. As each byte in the packet is transmitted, each bit in C
is XORed with the corresponding bit in the byte being transmitted. After all bytes have
been transmitted (up to and including the last data byte), C contains the result of the
checksum calculation and is transmitted.
Additive Checksum
This is a sum of all bytes transmitted in the packet up to and including the last data byte,
modulo 128 (i.e. the remainder, after dividing by 128 the sum of all bytes transmitted).
Compatibility with RS485 Multi-Controller Configuration
The RTS signal on the RS232 port controls data direction on the RS485 bus. Normally,
the RTS line should be left unasserted by the master. Just before the master transmits
a packet, it should assert the RTS line (to drive the master's RS485 port to Output mode).
After the master has completed transmission of apacket, the RTS line should be dropped
within 0.05s (but not before the stop bit of the last byte has been transmitted).
The Controllers normally have their RTS lines unasserted, so that their RS485 ports are
in Input mode waiting for transmissions from the master.
When a Controller is about to transmit a reply packet, its RTS line is asserted (driving
its RS485 port to Output mode). After the Controller has completed transmission of a
packet, the RTS line is dropped within 0.05s. This means that the master must wait at
least 0.05s before raising RTS to transmit the next Command Packet.
Packet Timing
In most instances, the Controller will issue a Reply Packet in response to a Command
Packet from the master within 1 second.
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