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Conference helix::realtime

Title:Realtime Conference
Moderator:HELIX::LUNGER
Created:Mon Feb 24 1986
Last Modified:Mon Jun 02 1997
Last Successful Update:Fri Jun 06 1997
Number of topics:1241
Total number of notes:4452

1233.0. "DECterm with WinNT SCADA/MODBUS protocol?? anyone?" by FRIXEU::OLOBARDI (Marco - E&RT Mktg Prod Mgr - Italy) Wed Feb 05 1997 11:06

    Hi,
    	a customer needs to connect some remote PLCs (talking MODBUS 
    protocol with the RS232 variant) connected by modems to a central site. 
    
    Here we have a WindowsNT server with a SCADA package, still to  be defined
    (NOT basestar, they will chose RTAP or FIX or something else).
    
    The communication lines to the PLCs will be somehow duplicated, and
    they want to be able to switch from both to cover all kind of faults. 
    
    We would like to know if using two terminal servers would solve the
    problem and if it would work. 
    
    Any hints?
    
    
    SCADA with Modbus driver
    			Ethernet
    WinNT server
    	|
    ----+---------------+-----------------------+------------
    			|			|
    		   DECterm		   DECterm
    		    |	 |		   |     |
    		    |	 -------------|    |	 |
    		    |         --------|-----     |
    		    |	      |       |          |
    		   _____________      _____________
    		   |Line switch|      |Line switch|
    		   _____________      _____________
    		      |....|		|......|
    
    		     Modems		Modems
    
    		^-^-^-^-^-^-^		^-^-^-^-^-^-^
    	       <	     >         <             >
    		^-^-^-^-^-^-^		^-^-^-^-^-^-^
    			modem		modem
    			   |		|
    			   |		|
    			   +----RTU-----+
    				MODBUS protocol
    				data acquisition
    
    How can the term server be used? with TCP/IP protocol? setting somehow 
    a passthrough data exchange to the remote RTUs
    Would that be transparent  to the SCADA application?
    Thanks in advance
    Ciao ciao
    				Marco
T.RTitleUserPersonal
Name		DateLines
1233.1LAT???SAYER::ELMORESteve [email protected] 412-364-5893Wed Feb 05 1997 12:489
    I'm not sure I understand what you are doing or what you are asking.
    
    Anyway.  I *think* DECnet can be found for WNT.  If so, the old LAT
    protocol, along with some features in the terminal servers might be
    used.  (It's been a long time since I've used terminal servers and lat
    and I don't remember any details.  Also, I've never used DECnet on
    WNT.)
    
    --Steve
1233.2Lots of words. Some suggestions. No answer.BBPBV1::WALLACEjohn wallace @ bbp. +44 860 675093Thu Feb 06 1997 05:1639
    LAT for Windows NT is available from Meridian (who do LAT for Digital
    UNIX so it must be good :-). I don't think it's in Pathworks. It may
    also be in some of the better terminal emulation packages (KEA?).
    Make sure it's clear that you want *outgoing* LAT, where an application
    connects to a port on a terminal server. LAT was more usually used
    where a user logs in to a host i.e. the opposite of what you want.

    You could in principle also use TCP/IP (it works for VMS, therefore it
    must work on NT? I think NOT in general but...). 
    
    With either LAT or TCP the host application may need to be modified to
    set up the initial connection, something which it wouldn't need if it
    was an ordinary comm port.
    
    In fact, given the number of stages in your diagram, I am puzzled as to
    why you/they don't just use a serial card in each host and take the
    modems from there. What value are the terminal servers adding ? They
    are inevitably adding a short delay, but there may be worse problems...

    If the hardware is genuine Modbus boxes *and* they are operating in
    Modbus RTU mode (binary mode, not ASCII/printable mode) you *cannot*
    use terminal servers or any other device which "packetises" data. The
    Modbus RTU protocol has no "message size" or "end of message" flags. It
    uses the inter-character time to signify end of message. If the
    terminal server (or an error correcting modem) splits a message over
    two packets, the whole thing fails to work. So don't use them.
    
    These problems don't arise in ASCII mode but then you only get half the
    thruput.
    
    Is it time to call DIGI now and plug the modems in directly ?
    
    A radically different alternative would be to call (AEG?) Modicon and
    ask them if they still do their ISA to Modbus-plus adapter, which will
    drive reasonably long lines but may need an adapter at the other end to
    get back to vanilla Modbus.
    
    regards
    john
1233.3TALSAYER::ELMORESteve [email protected] 412-364-5893Thu Feb 06 1997 19:2519
    I cannot speak for or against this company, but...I got a flyer in the
    mail advertising the following on WNT, Windows, DOS, Win95:
    
    "RS232 Direct Into Any PC Program"
    
    Capture Data From:
    Laboratory & Industrial Instruments,
    Gages, CMMs, Bar Code Readers,
    Balances, Meters, Analyzers,
    Telephone Systems, PLCs. etc.
    ANY SERIAL DEVICE!
    
    SoftwareWedge
    
    Contact:  TAL Technologies, INC,
    2027 Wallice St., Philadelphia, PA 19130
    Telephone 215-763-7900
    email:  [email protected]
    http://www.taltech.com