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

CHAPTER 4: USING THE IEEE 1394-BASED PROTOCOL
IEEE 1394 node appears logically as a 48-bit address space. To communicate with a device, you
read from and write to specific memory addresses in that space.
The SNAP Ethernet-based brain’s memory map is similar to a grid of mailboxes, with each
mailbox having its own memory address. (See Appendix A for the detailed memory map of the
brain.) Every I/O module appears in two ways in this memory map:
1. As bank data for block reads and writes (for example, all counters in one bank)
2. As individual point data for reads and writes of individual points.
Communication Packets
Communication using the Opto 22 IEEE 1394-based protocol basically involves an IEEE 1394
packet placed inside a TCP/IP or UDP/IP packet. These nested packets look like this TCP example:
The SNAP Ethernet-based brain uses the following types of request packets specified by the IEEE
1394 standard:
Read Quadlet: reads four bytes starting at an address
Read Block: reads N bytes starting at an address
Write Quadlet: writes four bytes starting at an address
Write Block: writes N bytes starting at an address.
To start communication with the SNAP Ethernet-based brain, the host computer sends one of
these four packets via TCP/IP or UDP/IP. To complete each transaction, the brain returns a Read
80 SNAP Ethernet-Based I/O Units Protocols and Programming Guide