ProSoft Technology MVI56-AFC - Manual (Page 130)

Events MVI56-AFC ♦ ControlLogix Platform
User Manual Liquid and Gas Flow Computer
Page 130 of 316 ProSoft Technology, Inc.
February 25, 2011
9.5.4 Security and Optimization
Two features are available that can improve security and throughput of a log-
download session. These are:
1 Session ID - This is a value between 0 and 255 that is chosen by the host at
the opening of the session and must be used in all transactions of the
session. While a session is open, transactions that do not supply the correct
Session ID are rejected. The AFC never displays the ID of the open session,
so only the initiating host knows its value.
2 Collapsed Acknowledgement - This allows the Acknowledgement
transaction of one F&A cycle to be embedded in the Fetch transaction of the
next, reducing by almost half the number of transactions required for the
session.
Use of either of these features, excepting only the use of Session ID 0, requires
non-standard tweaking of the Modbus command packets of Fetch transactions,
hence applications that cannot perform such tweaking are limited to the use of
Session ID 0 and the non-Collapsed acknowledgement methods.
All session transactions except Fetch are Modbus writes; in those, the Session
ID is included as an element of the written data. Fetch transactions are Modbus
reads, which contain no data in their command packets; in those, the Session ID
and (if used) the Collapsed Acknowledgement are encoded in the high-order 9
bits of the "number of registers" field, so that:
Bits 0 thru 6 contain the true "number of registers", which 7 bits are, for holding
register access, sufficient to carry any value that is valid according to the
standard Modbus protocol.
Bit 7 is used for the Collapsed Acknowledgement.
Bits 8 thru 15 contain the Session ID.
The tweaking of the Modbus read command packet is performed before
calculation of the CRC or LRC. See Sections 6 and 7 for more detail.
All Modbus transactions of a log-download session, whether read or write,
address offset 0 of the LDW, either to its Primary Slave location of 65400 or to its
Virtual Slave mapped location configured by Ph00147, hence validation of
Session ID, and recognition of tweaked Modbus command packets, are
performed only for commands that address that location. Any attempt to address
any other offset into the LDW is rejected with a Modbus exception code.