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.
1. All the information needed to make a data reque lies in the Client/Server Tag's address.
1. High device turnaround time when processing the symbolic addresses.
2. Only the data that is being accessed in the Client/Server Tags will be requested from the PLC.
2. Less requests per Multi Request Packet because the size of each request varies.
3. Backward compatible.
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 will create tags with a data type that matches the type in the controller.
Read Protocol Used
Symbolic Logical Non-Blocking Logical Blocking
FRN V21 and Higher Symbolic (Non-Blocking)
FRN V20 and Lower Symbolic (Non-Blocking)
Symbol Instance Non-Blocking Symbol Instance Blocking
Physical Non-Blocking* Physical Blocking*
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.
Write Protocol Used All FRN
Symbolic Symbol Instance 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" will be used to represent 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.
1. Contains the maximum request per Multi-Reque Packet because each request is a fixed size.
Initialization overhead when uploading the project to determine the logical addresses.
2. Low device turnaround time because the Client/Server Tags are specified in the packet with their logical address.
3. Only the data that is being accessed in the Client/Server Tags will be requested from the PLC.
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