snap ethernet-based i/o units protocols and programming guide

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CHAPTER 4: USING THE IEEE 1394-BASED PROTOCOL
Parameter Full Name Description
quadlet_data Quadlet data Data being delivered to the target node. If it is not
an even four bytes of data, pad with zeros at the
end, not the beginning.
data_block Data parameter Data being transferred to the target device.
Packet Structure
The following pages show the structure for read and write request and response packets. The
SNAP Ethernet-based brain does not use the parameters destination_ID, rt, or pri (or reserved
areas). These areas must be zero filled. They are shown shaded. The source_ID parameter
(described in the previous table) is optional. If you do not use it, fill it with zeros.
IEEE 1394 packets have boundaries at four bytes (a quadlet). When you send a Write Quadlet or
Read Quadlet Request, if the data you enter in the quadlet_data parameter is not in even
multiples of four bytes, fill the remaining spaces with zeros to complete the quadlet. Zero fill at
the end, not the beginning, of the data.
You do not need to zero fill data fields in Write Block or Read Block Requests.
Write Quadlet Request
Tcode = 0
destination_ID tl rt tcode pri
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
source_ID destination_offset
Write Block Request
Tcode = 1
destination_offset
quadlet_data
destination_ID tl rt tcode pri
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0
source_ID destination_offset
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
destination_offset
data_length extended_tcode
94 SNAP Ethernet-Based I/O Units Protocols and Programming Guide
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0