Rockwell Automation 1785-Vx0B, D17856.5.9 PLC-5 VME VMEbus Programmable Controllers User Manual - Manual (Page 82)

Chapter 6
PLC-5/VME Processor
Communications Commands
6-13
Data-Type Field
The data-type field specifies the ID (type of data) and size (number of
bytes per element) of the data type used in these typed-write and
typed-read commands. The default data-type field (1 byte) contains an ID
format bit and value field for defining ID and size.
ID Size
Format
Bit ID Code Format
Bit Size Code
Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
The data-type field can vary in length if more descriptor bytes are required.
Either of two format bits (bit 7 and/or 3) distinguish between a 1-byte or
multi-byte field.
If the format bit is: Then the adjacent 3-bit field:
Zero contains a binary code (0-7) that specifies the data type ID or size.
One defines the number of descriptor bytes appended to the default byte.
The appended descriptor bytes specify the ID or size. The order of
descriptor bytes is least to most significant. The most significant (MS)
bytes of zero value are permitted but overlooked.
When both the ID and size codes are appended, the ID bytes precede the
size bytes.
For example, the following data-type descriptor fields have identical value.
They describe an ID code of 4 (integer) and a size code of 2 (bytes
per element).
01000010
Bit 76543210
01001001
Bit 76543210
00000010
01001010
Bit 76543210
00000010
00000000
Example Data Types
We now present examples of several data-type IDs and corresponding data
field (fields a and b in the command or reply packets).
Important: The packet for a typed-write command is limited to one
element of a specified data type except for the array and character string.