Connect multiple Serial Interfaces to PLC

[jawolthuis]

I did one system with 10 RS232 scales. I used an automationdirect.com H2-DM1E with (4) H2-SERIO modules. It was easy to parse the data, and has caused no problems in the past 2 years of continuous operation. The CPU has on-board Ethernet to talk to whatever device you want to send the data to.

 

[JaxGTO]

I did a job many years ago where we used a serial multiplexer box. This had one port you connected to and based on the first sequence you send it, it would connect you to up to 32 separate serial ports. It would also receive unconsolidated message and put a wrapper around it with the port it came from. That's what really worked well for us. We told the devices to send on data change, and then just listened.

don't know if anybody still makes something like that. Maybe Black Box?

 

[briancr]

One thing Hein123 hasn't told us is what the distance from the PLC to the scales are.
Remember RS232 has a very limited cable distance. This could be a major influencing factor in determining the route to go.
30 - 40 devices indicates a far distance.

 

[robertkjonesjr]

I had to work out a solution to a similar problem a number of years ago. The plant had two rs232 devices already, but wanted to add 20 or so more due to growth (good problem to have). I wanted a different comms option on the device besides RS232, but these were 'cheaper' and they already had some, so they could buy less when they ramped up. Project Manager was all about 'minimum'.

I proposed several of these:

http://www.sealevel.com/store/seria...ethernet-to-16-port-rs-232-serial-server.html

as a starting point, but it's $1K for 16 ports. So none of this will be cheap.

Another option was these:

http://www.kksystems.com/din-rail-converters/interface-converters/kd485-ade.html

That convert addressable RS485, which would be multidrop, to RS232. But this is not cheap either, if you need one per rs232 port.

I ended up getting the more expensive devices with RS485 comms capability instead of the standard RS232, so did not deploy either of these solutions.

I kind of like the idea of a small controller at each device. Maybe something really cheap like this gear:

http://www.triplc.com/superplc.htm

But the installed cost is much more than the purchase price. Need some type of enclosure, cable sets, power supplies, whatever. Even if you do the addressable rs232 adapters, as some one pointed out, you may need local enclosures due to distance.

If I had to do it again I would probably try to get a device that handled a couple of serial ports and converted it to Ethernet with a fieldbus - not just Ethernet, but to be sure it had a fieldbus, like ModbusTCP, EtherNet/IP, whatever you need it to be. This will minimize complexity at the logic level and ease the integration into the system.

 

[Pete.S.]

I ended up getting the more expensive devices with RS485 comms capability instead of the standard RS232, so did not deploy either of these solutions.

That is the cheapest solution all things considered. Least amount of gear, least amount of problems. You need some programming skills though if it's not a standard protocol like modbus.

RS232 in a plant is ridiculous and asking for intermittent problems. Inside a cabinet yes, outside well maybe 10 feet tops.

But since we are looking for cheap here and not the best solution, I suggest a raspberry pi for each scale and then communicate with all of them over ethernet. That will be about $50 per scale, two cheap 24 port switches and then running some cat 6.

I don't know what the purpose of the PLC is but if it's just for the scales you don't even need one and could replace that with a raspberry pi too. It depends on what you are going to do with the weights you collect.

 

[Helliana]

The other consideration would be to put a small Ethernet compatible PLC at each scale using it as the converter. That would give you a more “mainstream” system and you could have each PLC react to different situations (if the scale stops sending data you could send a notification to the host PLC for example).

I think this may actually be a feasible idea. If you go the AB route, a Micrologix1100 would work great and easily be accessible over an ethernet network. You could mount the PLC on the scale so distance would not be an issue. Any issues would be easy to troubleshoot and would be isolated to 1 scale. If you needed to move the scale at a later date, all you would have to do is run a new network cable. Also, cost on a micro1100 may be cheaper than some of the serial to ethernet devices.

The only real downside i see is the need to program every device individually, but that shouldn't be a huge issue.

 

[DwSoFt]

You could also go with serial 232 to 485 converters as well they are cheaper than ethernet, and then 485 to the plc... this sounds like it will be quitge the little job...

 

[Dravik]

So, if the scales are reasonably close together(~ 50 ft), or groups of them were together, I'd use a some form of serial to ethernet server(s) that support telnet and throw a prosoft generic ascii ethernet module in either a CompactLogix or ControlLogix chassis.

 

[Firejo]

You could also go with serial 232 to 485 converters as well they are cheaper than ethernet, and then 485 to the plc... this sounds like it will be quitge the little job...

That’s assuming that each scale can be separately addressed. If they can then I agree that an RS232 to RS485 converter at each scale will do the job however if the scales are peer to peer they are most likely not addressable and then converting RS232 to RS485 won’t work.

 

[OkiePC]

I would also consider the Digi One IAP as a serial to ethernet bridge device. Matter of fact, there was one scale I used to work with that offered the Digi One as an option from the OEM for just such a purpose. I can't recall which model of scale it was.