ProSoft Technology 5204SE-MNET-PDPMV1 - Manual (Page 16)

5204SE-MNET-PDPMV1 ♦ ProLinx Gateway Functional Overview
Modbus TCP/IP to PROFIBUS DP-V1 Pass-Through Master User Manual
Page 16 of 235
ProSoft Technology, Inc.
December 22, 2009
o The MNET Server must pass the acyclic message and any associated
data through to the PROFIBUS DP-V1 Master.
o The PROFIBUS Master must insert this command in between normal
slave data polling messages (send an acyclic message to a particular
slave or group of slaves.)
o The PROFIBUS Master must receive a response from the addressed
PROFIBUS slave.
o The PROFIBUS Master must return any PROFIBUS slave response data
to the MNET Server.
o The MNET Server must create and return a Modbus TCP/IP response to
the processor containing the data, if any, from the PROFIBUS slave or
slaves.
o
Some acyclic messages cause data to be returned. Some acyclic
messages return no data. Acyclic messages sometimes fail.
Communication failures could happen on either or both the Modbus
TCP/IP protocol or the PROFIBUS protocol. Any data or error response to
an acyclic message returned by the PROFIBUS Master or the MNET
Server will be available in the provided Variables or DDTs after being
placed there by the triggered DFB that initiated the process cycle. User-
created logic in the processor must process data received in the
PROFIBUS DP or Modbus TCP/IP response, if any.
o If no Modbus TCP/IP response is received within the time value specified
in a Timeout variable, the triggered DFB will set a Message Error bit flag
to indicate the message sequence failed and should be retried by
triggering a new message cycle.
You can see from the amount of processing involved that it will take
somewhat more time for the gateway to respond to acyclic message
commands than it will take to respond to cyclic I/O, status and diagnostics
requests.
6 Due to the nature of the communication routines used in the processor and
the Unity Pro programming language, only one Modbus TCP/IP command
can be "active" or "in process" at any one time. All the provided DFBs have
internal checks built in to prevent more than one at a time from being active.
Therefore, any control or sequencing logic you create must respect and
accommodate this processor/language limitation. Part of the
accommodations you will have to make is to allow for the differing amounts of
time it takes to process cyclic I/O and status commands as well as the
increased time it takes to process acyclic messages.
7 All provided DFBs have binary status bits available which can be monitored
by your control and sequencing logic to be sure you are not trying to activate
more than one DFB-created Modbus TCP/IP message at a time. There is a
Message Done bit to indicate the communication cycle completed
successfully; and, there is a Message Error bit to indicate the communication
cycle did not complete successfully.