Header, Packet Count, Serial Number Length

7.3.3PACKET LAYER DEFINITION Data Packet Structure

Byte	Description	Byte Value range
1	Header	128 to 255
2	Packet Count	0 to 127
3	Serial No. Length S	1 to 127
4	Data block length D	1 to 127
5to(4 + S)	Serial No.	32 to 127
(5 + S)to(4 + S+D)	Data Block	0 to 127
(5 + S + D)	XOR Checksum	0 to 127
(6 + S + D)	Additive Checksum	0 to 127

Minimum packet size is 8 bytes.

Maximum packet size is 260 bytes.

The same packet structure is used for both transmissions from the master (Command Packets) and transmissions from the Controller (Reply Packets). A dialogue is always initiated by a Command Packet from the master.

Header

This byte, which must have bit 7 set (i.e. value between 128 and 255), identifies the purpose of the packet (a full description of all the different types of packet is given later).

Packet Count

This byte may take a value between 0 and 127, and should be incremented by 1for each Command Packet transmitted by the master, except when the packet is a retransmission due to the fact that no reply was received to the last Command Packet (in which case the Packet Count should remain at the value of the original Command Packet).

When the Packet Count reaches 127, the Packet Count of the next Command Packet should be 0. When the Controller replies, the Packet Count byte in the Reply Packet is set to the same value as in the corresponding Command Packet. This allows the master to positively associate Reply Packets with Command Packets.

The Controller doesperform validation of the Packet Count field in some circumstances. For example, when transferring a file from the master to a Controller, the file is split into packets and transmitted one packet at a time. As the Controller receives each packet it acknowledges this to the master. It is possible, however, that the acknowledgement is lost (e.g. due to noise), and the master has no way of knowing whether the Controller ever received that packet or not (as far as the Controller is concerned, the transaction was completed successfully). In this case, after the time-out period has elapsed (see below; Packet Timing) when the master re-transmits the Command Packet, the Packet Count will be the same as in the preceding Command Packet. The Controller, upon receiving this duplicate packet (which it can identify by the Packet Count) simply acknowledges the packet, but does not store the data.

Serial Number Length

In Command Packets this is set to the length (in characters) of the serial number of the Controller to which the packet is intended, hi Reply Packets, this is set to the length (in characters) of the serial number of the Controller which is transmitting the reply.

550 specifications

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