User's Manual

COBALT UHF-SERIES CHAPTER 5: COMMAND PROTOCOLS
PAGE 57 OF 140
5.4.5 ABx Fast - Multi-Tag Command Packet Elements
Tag Limit
The Tag Limit parameter holds a one-byte value that indicates the maximum number
of tags expected simultaneously in RF range for the given command operation.
This parameter allows users to limit the number of attempted read/write operations
the controller will make per execution. Users do not have to wait for the timeout to
expire.
The Tag Limit value should be set in relation to the maximum number of tags that
could possibly be present in the reading field at any one time. Setting the value
higher increases the number of tags expected to be read in the antenna’s RF field.
Lowering the value, however, can speed up tag read operations for a small group of
tags. Setting the proper value is therefore a tradeoff between the number of
expected tags in the reading field, and the time required to read/write to them. The
permitted values range from zero to 100 (0x00 – 0x64). The Tag Limit parameter
resides directly after the “Anti-collision Mode” parameter in the command string
(when applicable).
Timeout Value
Multi-tag commands also contain a two-byte Timeout Value parameter that is used to
limit the length of time for which the Cobalt will attempt to complete a given operation.
It is important to set a realistic Timeout Value that permits enough time for the Cobalt
to read/write to all tags specified in the command. Processing multiple-tag operations
requires a longer time period than does the execution of single-tag commands.
The value is expressed in one-millisecond increments, with a maximum value of
0xFFFE (65,534 milliseconds) or approximately 60 seconds. For most single tag
read/write commands, a Timeout Value of at least 1000ms is recommended.
However, it is recommended that you allow an additional 100ms per tag for multi-tag
read operations and 150ms per tag for multi-tag writes.
Timeout Value Example
When writing to 16 different tags in RF range, for example, set the two-byte Timeout
Value to at least 0x0D48 (16 x 150ms + 1000ms = 3400ms or 3.4 seconds). A
Timeout Value of zero (0x0000) will cause the Cobalt to return a syntax error
message.
Using a Timeout Value that is too short may result in diminished read/write range.
Setting a long Timeout Value does not necessarily mean that the command will take
any longer to complete. The value only represents the period of time in which the
Cobalt will attempt to complete the particular operation. If all required tags are in RF
range when the command is sent, the time necessary to complete the command will
be approximately the same whether the Timeout Value is 1000ms or 10,000ms.
For time critical applications, the optimal Timeout Value should be obtained through
rigorous performance testing.