Specifications

Allen-Bradley ControlLogix Ethernet Driver Help
Choosing a Protocol Mode
Symbolic Mode
Symbolic Mode represents each client/server tag address in the packet by its ASCII character name.
Benefits Detriments
1. All the information needed to make a data request
lies in the client/server tag's address.
2. Only the data that is being accessed in the
client/server tags are requested from the PLC.
3. Backward compatible.
1. High device turnaround time when
processing the symbolic addresses.
2. Less requests per multi-request packet
because the size of each request varies.
Note 1: To take advantage of the multi-request packet optimization, as many tags should be represented in a
single packet as possible. Since tag addresses are represented by their ASCII character name in the packet, the
tag addresses should be as short as possible. For example, "MyTag" is preferred over
"MyVeryLongTagNameThatContains36Chars."
Note 2:When the default data type parameter is set to "Default," automatic tag generation creates tags with a
data type that matches the type in the controller.
Logical Modes
Logical Non-Blocking and Logical Blocking encapsulate two read protocols. The protocol used is automatically
determined by the driver and is based on the controller revision. The table below summarizes the modes and the
protocols to which they map.
Protocol Mode Read Protocol Used Write Protocol Used
FRN V21 and Higher FRN V20 and Lower All FRN
Symbolic Symbolic (Non-Blocking) Symbolic (Non-Blocking) Symbolic
Logical Non-Blocking Symbol Instance Non-Blocking Physical Non-Blocking* Symbol Instance
Logical Blocking Symbol Instance Blocking Physical Blocking* Symbol Instance
*Deprecated in V21.
The information necessary to perform Logical reads is retrieved in a controller project upload sequence
performed automatically by the driver. For the sake of brevity, the term "Logical Address" represents the Symbol
Instance ID or Physical Address, depending on the protocol used. The Logical Modes avoid the time-consuming
address parsing and lookups that are required for every symbolic request.
Note: These Logical Modes are not available to Serial Gateway models.
Logical Non-Blocking Mode
Logical Non-Blocking Mode requests all client/server tags individually and at a fixed size.
Benefits Detriments
1. Contains the maximum request per multi-request
packet because each request is a fixed size.
2. Low device turnaround time because the
client/server tags are specified in the packet with the
logical address.
3. Only the data that is being accessed in the
client/server tags are requested from the PLC.
Initialization overhead when uploading the
project to determine the logical addresses.
Note: This mode is preferred when the minority of Structure tag members are referenced by a client/server.
Logical Blocking Mode
Logical Blocking retrieves all data for a Logix tag in a single request that may be initiated by only one client/server
tag. When the data block is received, it is placed in a cache in the driver and then time stamped. Successive
client/server tags that belong to the given Logix tag then get their data from this cache. When all tags are
updated, a new request is initiated provided that the cache is not old. The cache is old when the current time >
cache timestamp + tag scan rate. If this case holds, another block request is made to the device, the cache is
refreshed, and the cycle repeats.
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