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Operations Manual: HPAC-100/125/150/200/250-RM SSPA
3 Operation of Stand-Alone Unit
Packet Length: This field indicates the number of bytes contained in the
packet. The leading frame sync bytes are not included in the count.
Destination Address: The destination address field specifies the system for
which the packet is intended. This field consists of one byte containing an
integer address (0 to 31). Each unit or node in an SSPA system must deter-
mine if the packet currently on the bus is meant for it. This is done by compar-
ing the destination address on the bus to the SSPA’s System Address.
Source Address: The source address field specifies the address of the
node that sent the packet. This field consists of one byte containing an
integer address (0 to 31).
The data portion of a packet consists of two fields: Command and Data.
Command Field: The command field is a one byte field that either instructs
the destination node how to use the data that the source node is sending or
requests that specific information from the destination node be returned.
Data Field: The data field is variable in length depending upon the associ-
ated command. The first byte of the data field contains the unit ID of the
amplifier with which the controller is communicating. This byte acts as a
modifier for the destination address which is especially important when com-
municating with a redundant SSPA system. The unit ID designates which unit
should receive the command packet and also designates which unit a packet
came from. The range of valid values are 0 to 3. The value 0 indicates the
System Controller of a redundant system and the values 1, 2, and 3 correspond
to a particular SSPA within the system. The unit ID of a specific amplifier is
displayed on line 3 of the OPS menu and is determined from a factory set DIP
switch.
The unit ID of all stand-alone SSPAs is 1. The remaining bytes in
this field are for the data associated with a command or response. Some
commands require no additional data while others require several bytes. If the
logical data size of the information is 16 bits, then each data word will be placed
in the frame with its most significant byte first. This is demonstrated in Figure 3-
7.
Figure 3-7: Example of data field information in the packet.
Word 0
MSB
Word 0
LSB
Word 1
MSB
Word 1
LSB
Command
Data
FCS
The trailer component only contains one field: Frame Check Sequence.
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