• Symbolic Mode
• Physical Non-Blocking Mode
• Physical Blocking Mode

Make sure to check out our ControlLogix Performance Information to maximize the results from your TOP Server investment.

General Guidelines for Selecting Protocol Mode:

• Symbolic Mode - Most efficient when only a few tags are being read
• Physical Blocking Mode - This can either help or hurt, depending on the number of tags you are accessing within a structure (structure within RSLogix 5000). What this will do is pull the entire structure into the driver, and allow the driver to parse the tag addresses internally. If you are referencing only a few tags within a structure this would be inefficient. If the arrays you are accessing are all within a structure and make up most of the tags within that structure this option would improve your performance.
• Physical Non-Blocking - Generally speaking, this is the most efficient method of communications. If you’re accessing less than 30 percent of the tags within a structure, this is the recommended method.

IMPORTANT NOTE:  When using either Physical Protocol Mode, it imperative that each device be configured under a separate channel (see Multiple Connections).  Failure to do so will result in slow performance due to the TOP Server process of polling devices sequentially when defined under a single channel, combined with the pre-mapping of addresses that occurs with the Physical Protocol Modes.

Symbolic Mode Details:

Each Client/Server Tag address is represented in the packet by its ASCII character name.  Symbolic mode is convenient in that all of the information needed to make a data request lies in the Client/Server Tag's address.

To take advantage of the Multi-Request Packet optimization, you want as many tags represented in a single packet as possible. Since tag addresses are represented by their ASCII character name in the packet, this implies tag addresses should be made as short as possible. For example, MyTag is preferred over MyVeryLongTagNameThatContains36Chars.

Pros:

• Low initialization overhead. All information needed lies in Client/Server Tag's address
• Only the data being accessed in Client/Server tags is requested from the PLC
• Backward compatibility
• Best for wireless or cellular communications

Cons:

• High device turnaround time to process symbolic address
• Less requests per Multi Request Packet since each request is of variable size

Physical Blocking Mode Details:

In Physical Blocking, all data for a Logix tag is retrieved in a single request.  Blocking is possible because each Logix tag has a base physical address from which to work with.  Physical Blocking mode is ideal when most or all members/elements for a given Logix tag are being referenced by a client.

Pros:

• If 1/3 or greater of Logix tags are referenced, faster than Physical Non-Blocking
• Low device turnaround time to process physical address
• Maximum request per Multi-Request Packet since each request is a fixed size

Cons:

• Initialization overhead uploading project to determine physical addresses
• If 1/3 or less of Logix tags are referenced, slower than Physical Non-Blocking; more data being accessed from PLC than referenced in Client/Server tags

Physical Non-Blocking Details (This is the default setting):

Non-blocking physical is identical to symbolic in that all Client/Server tags are requested individually, utilizing the Multi-Request Packet. They differ in that the Client/Server tags are specified in the packet with their physical address, not their symbolic address. This is considerably faster than symbolic mode, as the device turnaround time is diminished.

Pros:

• Low device turnaround time to process physical address
• Maximum request per Multi-Request Packet since each request is a fixed size
• Only the data being accessed in Client/Server tags is requested from the PLC

Cons:

• Initialization overhead uploading project to determine physical addresses