Need PLC to control via serial port ...

Hi all

I'm looking for recommendations for low cost PLC/single board controllers that I can easily access through a serial port on a PC. By this I mean that the manufacturer should provide protocols for serial communication, at least as far as the basics go - I want to be able to turn on and off digital outputs, read digital inputs and so on, and all through a standard serial port, so that I can use a PC running Linux or a Mac or whatever machine I have available. (I'm happy to have the controller running a program to provide regular polling of the inputs if that is necessary, and in general the PLC will end up running a standalone package anyway)

The background is that I dont want to be tied to proprietary software (usually windows, with the manufacturers only providing their development systems and process monitoring apps for that) when I'm doing development. When I'm working on a project, I want to be able to monitor what is going on, collect data, and override outputs and not be tied to a manufacturers proprietary software for doing so.

I program in Tcl/Tk, and its very easy to do complex graphical interfaces, number crunching, data collection and so on. I can open up the serial port and then read and write byte streams with very simple commands - all I need is the right controller and the protocols. (The program will also work on a Linux box, a mac or whatever...)

So far I have found the SplatCo controllers (the actual PC interface/development system is for windows only, but they'll give you the serial protocols), and the DataTaker, which is more of a data acquisition device but can be controlled from a dumb serial terminal. Other manufacturers seem loath to give away any useful information on communication protocols (presumably they'd rather you purchase their software and not "roll your own")

Any suggestions or feedback would be appreciated.

[Steve Bailey]

Modbus protocol is freely available and is supported by many PLC brands. It's the 'native' protocol for all Schneider products and the closest thing in PLC-land to a universal 'second language'.

You can buy a Modbus driver or roll your own. Just be aware that all PLCs that support Modbus don't necessarily support all possible function codes. Furthermore, Modbus provides access to discrete inputs, discrete outputs, analog inputs, and data registers. Some PLC brands have additional memory areas that are inaccessible via Modbus.

Also, don't plan on rewriting any ladder logic in the PLC via Modbus. PLC vendors are pretty protective of their executables. It's unlikely that anybody will provide you with any details on how to modify the program code.

[seppoalanen]

As Steve told, MODBUS is only open protocol what I know.

AB DF1 protocol is some peoples well known and it could be faund from the Web, but it is not Open as well Omron, Siemens etc.

With Windows AB, Omron, Simatic etc. are usable with their OPC-Servers and somewhere could have OPC-Servers for Linux.

So use Modbus and ex Modicon plc, http://www.schneider-electric.com/

[nodoubt]

I use WAGO. It provides this feature as well.

It does support MODBUS, and you can program it to suit your
needs the best.

Serial interface is not just one: you can use the serial communication
module you prefer: 232, 485... ecc. There are sw libraries to run
modbus and other protocols.

[seppoalanen]

Hi people, can we collect the list of PLCs who support Modbus ?

1. Modicon/Schneider http://www.schneider-electric.com/
2. Wago, nodoubt have you some link ?
3. ??

[larryhts]

Hi;

If the i/o count is not to big check out www.OPTO-22.com


Larry

[Tom Jenkins]

Look in the back of Control Engineering Magazine or other trade publications. You will find adds for all kinds of I/O devices like DGH and single board controllers like Z-World.

[jvdcande]

The S7-200 PLC from Siemens is relatively low-cost. The programming port, which normally uses a propietary protocol PPI on RS485, can be switched over to RS232 mode with so-called freeport protocol, which is standard ASCII transfer as wit a PC. Check out the Siemens website for more info. Sorry, but I don`t have the URL here since I`m abroad. I checked in to plcs.net from an internet cafe, since I`m a plcs.net addict .

Kind regards,

Jean Pierre Vandecandelaere
Momentarily in Lisbon

[Donvw]

Hi;
These products look interesting. I have not used.
http://store.actionio.com

Action Instruments busware

Don

[TimothyMoulder]

Unitronics PLCs have a function called "protocol", that allows you to freely configure the RS-232 port to practically any serial format. Using compares and ASCII commands, ou ought to be able to do what you describe.

Best of all, they have a built-in HMI, some models include analog inputs / high speed stuff, and the CANbus equipped units can communicate with each other with very little setup.

In short, you could have a whole network of Unitronics to work with, if you so desired, with a lot of expandability and special functions built right in.

Low-end units in the US start around 200-300 dollars.

www.unitronics.com

TM