S5-155 PLC Link


Thread Starter

Harminder Singh

I want to link Siemens PLC S5-155 to MAX-1000 DCS on MODBUS. Can any one furnish any information ? I know how to do on MAX-1000 DCS side. I want to
know what all additional harware/software requirement on Siemens PLC s5-155 side?

I would appreciate your help.

Best Regards


[email protected]
Doing the ASCII Modbus is quite easy on the S5, but you will either need a 155 with the second communications port, or a comms unit. Both of these support 'roll you own' ASCII protocolls.

Doug McQueen ESI Electric Supply Inc.

I have not yet been involved in a MODBUS to S5. Yet I know its going to be a time issue, I mean how fast do you really need the data or whatever you doing? If the unit needs to repond fast, the S5 might need to be set up with profi-bus. I would
call Siemens tech support, maybe You have already been there. If you haven't, check with them.
Maybe some pre-defined directions are available. And program examples for the S5 communication module.

Nick Haddock

I worked in a plant that linked Honeywell TDC3000 to S5 155 via MODBUS. The Siemens used a " PLC Gateway" on the TDC3000 side coupled to a "CP 525" comms processor on the Siemens S5 side. This acted as a data collector and dat was then sent to S5 155 PLC's by H1 bus (Industrial Ethernet).
The system was quite easy to follow with the standard Siemens Function blocks. I believe there were standard drivers for TDC3000 so maybe there is one for your DCS system but there is a standard RK 512 driver supplied with the COM 525
software package. Hopefully your Siemens rep can give you more info. The application I worked on was a Iron reduction plant and it all seemed to
work OK.

cheers Nick Haddock
Hi Harminder

Sorry Roger, but as a former Siemens rep I feel the the answer is definitely not so simple. In fact it is the roughest implementation of Modbus I
have ever worked with and that covers a lot of gear. I don't believe the S5-155 ever came with two comm ports which means Harminder has to use a CP525 communications processor. And writing code for the CP525 module is a PITA.

I think the simplest solution to get a S5-155U to talk Modbus is:

1) Install a CP525 communications processor (in the central chassis)
2) Purchase a 373 EPROM module
3) Purchase the CP525 Modbus Library (The CP525 is a general purpose card & needs the Modbus Procedure)
4) Design your comm. strategy. The easiest is to let the CP525 be the slave and set up two send & receive blocks
5) Burn the 373 EPROM with the AS-511 Interpreter, the Modbus Procedure, the comm. parameters and job control blocks
6) Install the EPROM
7) Program the S5155 with SEND-A and REC-A function blocks, assuming the 155 will be the Slave. This is sort of like doing BTW/BTR on a Allen Bradley PLC, except more obscure.
7) Program the S5155 with SYNCHRON function block
8) Set up your Data Blocks. If you have the 155 as master, you need to load the first few registers with constants such as the Modbus Function Codes (e.g. Send=3D16, Receive=3D03) and destination register numbers. This will then cause all the data values to be offset.
9) Sort out the non-standard wiring on the CP525 ports
10) Turn on and pray. Unfortunately it won't work.
11) You will discover there is some small error in the EPROM. Bo back to Step #5 and try all over again.

After a while you will get the system to work (at least we did) but you need a good PLC test bench, a protocol analyser, Siemens' Modbus Doc'm and a lot of patience. Once it is working it is very stable and a good system.

In short, the hardware you will need includes:
CP525 communications processor
373 EPROM module
Siemens EPROM Burner
CP525 Modbus Library
SEND-A, REC-A and SYNCHRON function blocks (usually free!)
CP525 Cabling

It has been a few years so I may have missed something. Or maybe there is a better way now so please tell us if you know another simpler way to do this

Good Luck
Eric Byres, P. Eng.
I want to link Siemens PLC S5-155H CPU948R to Daniel flow computer 2500 series on MODBUS. Can any one furnish any information ? I want to
know what all additional harware/software requirement on Siemens PLC s5-155 side. How programming on Siemens PLC s5-155 and Daniel flow

I would appreciate your help.

Best Regards

[email protected]
Install CP851, this is Intel Pentium Inside. This CP has special Handling Block to Comunicate with S5PLC and you can program the MODBUS software inside the CP581.