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Switch or Router?
I am building a panel with a PLC, an HMI, and a cellular modem ... Do I need a switch or router?


After 30 years of panel building, this is a new one for me.

A client I have had since the 90's asked me to build a panel with remote access via the internet.

He has no land line to the site.

I did my homework and quoted it last year. I got the job a few weeks ago and have begun ordering parts.

I am using a BXR PLC (BX-BM1E-M-D) from Automation Direct. I have ordered it with an Ethernet port.

I am using a Cmore 15" touch panel (EA9-T15CL-R) from Automation Direct, again with an Ethernet port.

I have coming in today a Advantech cellular modem (BB-SL30200110-X). It is set up for Verizon (my clients preference). It has one Ethernet port.

So, I need to tie there together.

The touch panel has a communications package in it. You can access it via the web and the software onboard will make it look like you are looking at the screen. This is why I choose this unit.

The BXR I was told works well with the Cmore panel. I choose it for this reason and also because it has e-mail capabilities (the HMI panel also does but this gives me two options).

So, each device has a single Ethernet port. I need the panel to talk with the PLC (and vise versa), I need the HMI to talk to the modem and I need the PLC to talk to the modem (to send e-mails).

I have read several articles and am still confused!

I think all I need is a dumb switch but I am just not sure ... do I need a router instead?

Just a side note .... I know the are a lot "better" PLC's and HMI's out there but I am only turning on and off 12 water pumps, monitoring them if they shut down and sending a daily total of water flow (two flow meters with 4-20 mA output are installed onsite) via e-mail. The PLC and Cmore include free software. For a one time only build and limited budget (I am a one man shop), this was in budget for me and my client.

I have built panels for other clients (about 10 years ago) but I worked as a contractor using their software and both already had a standard PLC (in one case Alan Bradley and in the other GE Fanuc). I have never tried using and HMI.

Thanks ...... Mike

Buy a switch to tie your hardware together. You can get a cheapie consumer one to test it out but I would go with the industrial Ethernet switches for deployment (I've had good luck with Automation Direct switches so far). The modem you mention is a router so it should work.


I would chose an un-managed switch. Automation direct has plenty of options for those.

I would say a simple unmanaged switch would work fine here but I've never used a cell modem before so I could be wrong.

It's been over 10 years since I worked with C-More HMIs, but when I did they were very solid. As I remember, the software wasn't free back then, but it was very reasonable (again I could be wrong). They were easy to program to communicate with an AB PLC via RS232 and stood up very well to a very dusty environment (non-climate-controlled brake pad factory). If we didn't already have a large installed base of other brands (AB, Siemens, Red Lion), I wouldn't object to trying them where I work now.


Thanks guys for the advice.

This is an environmental clean up site. There are 8 pumps running 24/7. To meet government regulations, they need to record the total gallons pumped every 24 hours.

They had a company install everything (wells, pumps, communications, ... ) but it never worked right.

A client I have worked with since the 90's, got the job of fixing the site.

He has everything working in manual right now but the customer needs to drive out to the site (2 hours from the office) and record the flows every day.

So they asked me to build a control panel that will turn the pumps on and off via the internet and e-mail the flow for a 24 hour period once a day.

They still plan on going to the site once a month to check on it.

Should be an interesting project.

My typical panel is just a number of contractors to sequence pumps, compressor and monitor tank levels.

Thanks again .... Mike

Question ... along the same lines but ....

I was just talking with a friend who said I need to have a static IP for both the PLC and the HMI panel. Is this true?

My understanding was that I needed a Static IP assigned to the modem SIM card by Verizon. It was originally explained to me this was like the a phone number for a cell phone.

Once you "call" the modem (put the static IP into the browser), I though you were in (maybe I would need to assign a name myself to the unit so I can call it).

Am I wrong?

Thanks .... Mike

Your HMI and PLC will need to communicate with each other and will need to always have the same IP addresses. Most of the time, it's done with static IP addresses, but you could also leave them configured for DHCP and use a managed switch to always assign them the same address. I have always used static addresses for automation devices.

By Julian Opificius on 24 February, 2019 - 11:47 am

No, no two devices on a network may have the same IP address. They should exist on the same subnet (for the vast majority of people that's everything before the last dot in "abc.def.ghi.jkl"). The rest of the advice is valid.

>Your HMI and PLC will need to communicate with each other
>and will need to always have the same IP addresses. Most of
>the time, it's done with static IP addresses, but you could
>also leave them configured for DHCP and use a managed switch
>to always assign them the same address. I have always used
>static addresses for automation devices.

A gateway device (typically a modem of some sort) may be assigned a fixed IP address: doing that is only necessary if there is a need to access it or a device on the internal network from the Internet; if there is no explicit need then I strongly discourage doing it for security reasons. Assigning a fixed IP address is a function performed by your Internet Service Provider: he will assign you one from his pool of addresses, and also give you a gateway IP address and a subnet, all of which will need to be programmed into your gateway router.

Backing up to the original question, every user network will have one (occasionally more for large institutions) device which functions as a router. Routing means deciding what information (its source AND its data type - mail, HTTP, etc.) travels through which path to its destination, so the router is almost always the "Gateway", which is the big door into and out of your network. Your Internet connection via the modem/router is obviously your gateway to the outside world. A switch is slightly less "clever": it only switches data from one path to another based on IP address, not on content, and is typically the primary interconnecting device in your local network.

Bottom line: your modem - which moves data between two electrical protocols, such as broadband and Ethernet - usually also functions as a gateway, a router, and a switch.

1 out of 1 members thought this post was helpful...

Sorry, I wasn't clear. When I said "same addresses", I meant "unchanging addresses".

Looks to me that your cell modem has router capabilities and can act as a DHCP server.

Therefore you can map MAC addresses of the devices on your network to specific IP addresses so that they will always have the same IP address.

good luck

1 out of 1 members thought this post was helpful...

Re: DHCP server

Since ALL components of a control system are subject to change, using a DHCP server is a very bad idea, albeit one that has its proponents, especially among those who hard-sell the expensive managed switches. Using a managed switch in this application would be swatting a fly with a 15 pound sledge hammer, and a managed switch could easily cost more than the rest of the components put together.

Even though at least one person has said that the cellular modem has DHCP capability, I would strongly advise against using it. The most common control system repairs and upgrades would each require reprogramming the DHCP server, adding another level of complexity to any repair or upgrade. Since routers and modems go obsolete very rapidly, the knowledge to do this is very volatile, also. That kind of documentation never sticks around, either.

The best solution I ever saw here was a complete list of all of the static IP addresses on the network written on the inside of every control panel on that network - in Sharpie permanent marker. Replacing a component was easy - the information was right there, and the "how-to" was available at the time of purchase. We both replaced and upgraded HMIs, PLCs and remote I/O, and never glitched the network doing it.

1 out of 1 members thought this post was helpful...

>I think all I need is a dumb switch but I am just not sure
>... do I need a router instead?

Personally I'd connect the cell modem to a router thus allowing everything on your internal network to get remote access to devices such as the email servers and keeping your internal network setup with static IP addresses. This also allows you keep a firewall between your internal devices and the internet.

You may also want to have a small computer on that network so that you can have "easier" access to the system via TeamViewer or the equivalent.

It all depends on your total big picture and what functions you need as well.


You should be ok with an unmanaged switch. My concern would be the cellular modem and the security risk it poses. Something like an EWON does not offer very good encryption, and if EWON (3rd party) becomes compromised ,that could impact this site. Typically, you are right a router would be used, you can add a 4G Verizon, AT&T, Sprint EHWIC card, then that router would connect to a firewall which would protect the local network from the outside world. You would then have a VPN or Tunnel from your office network to the site. We typically use that to manage remote solar sites. There are some other off the shelf products that are more secure. I would avoid products that host the connection.

-Mike D