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

APPENDIX A: SNAP ETHERNET-BASED BRAIN MEMORY MAP
IP address data type consists of four 1-byte integers. As shown in the example below, the
lower-numbered address reads or writes the first byte:
Example:
This address
FFFF F030 0034
FFFF F030 0035
FFFF F030 0036
FFFF F030 0037
Byte Ordering and Data Ordering
All non-mask data for points in bank and point areas are arranged in low point/low address order,
as follows:
Point 0 at 0x....00
Point 1 at 0x....04
Point 2 at 0x....08
Etc.
All masks and multi-byte values (floats, integers, and so on) are in Big-Endian format, which
means that the higher-ordered byte is in the lower-ordered address.
See page 41 for more information and examples.
General Notes
Reads/writes this byte
10. 192. 55. 123
For these brains, you cannot read or write more than 2,034 bytes at a time via TCP. Via UDP, the
limit is 1,480 bytes.
Within these limits, you can read or write to an entire area within the memory map, using a block
read or write. (Each area of the map is shown under a separate heading in the following pages.)
For example, using TCP, you could read or write up to 2034 bytes of data—about a third of the
entire area—in the “Analog and Digital Point Configuration Information—Read/Write” area
shown on page 125. If you are reading, just ignore any data in the reserved addresses between
points.
If you read or write beyond the last valid address in any area, however, you may receive an error.
Reading or writing in multiples of four bytes is generally faster than accessing a number of bytes
that is not a multiple of four.
CAUTION: In certain areas, if you read or write less than a quadlet, the data will be useless. For
example, reading two bytes of a float won’t give complete data. Even more important, if you read
only two bytes of a float in an area such as the Read and Clear area of the map, not only will the
data you receive be useless but also the information in the memory map will be erased (cleared).
104 SNAP Ethernet-Based I/O Units Protocols and Programming Guide