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 1and 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 areprintable 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, theRTS 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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