Serial Device Server and VPN

I thought I’d post this and see if anyone has any experience. I have a customer that is running a remote PC pulling data from their control system using a leased line modem connected to the serial port. They are investigating putting Internet access at both sites and getting rid of the leased line. My question is, could we set up a VPN between the sites and use a serial device server to allow the remote PC to connect, or will latency cause too many issues? And, unfortunately, switching to an Ethernet controller is not currently an option. Thanks in advance.


With this kind of setup the only way to be sure is to try it! Not very helpful I know. You might want to try a similar setup between 2 sites which already have Internet access.

I would look at your relative throughputs. Your serial line is maybe only giving you 19.2k (at very best 115k). Even a poorer quality ADSL connection should give you at least 256k upload. Modern VPN are capable of handling megabits of throughput, so I don’t see them adding much latency.

Once you move to an IP-based solution there’s no turning back. You just have far more options and flexibility. Be aware however that you probably have some kind of service level agreement for your leased line, which you won’t get with Internet access without paying a lot more than standard ADSL. Saying that, even a more expensive business-class service should be much cheaper than a dedicated leased line.

Well Al is right as far as TIAS (try it and see), but I’ll share some anecdotal experience with a similar situation.

I’ve done a leased-line to VPN conversion project, and it went well. The only headache we had was keeping the VPN up over our remote connection which was a cell-phone modem (it was an early-gen Proxicast). Once we got that solid though, throughput/latency was not an issue at all.

VPNs are usually slow on reconnections. The best I have seen is OpenVPN. Reconnects are seemless, like 10 seconds. We were using Sonicwalls, but they would take upwards of 5 minutes. You can alway install Ignition OPC and Ignition SQL Bridge, then even if you drop connections, itll buffer the data.

Thanks for the info all. I don’t really anticipate issues, as the existing system is running at 1200bps, but I haven’t messed with serial device servers too much, and this particular site is about 7 hours away, so I didn’t want to propose something that absolutely wasn’t going to work. Basically, they have a PC that I just set up at their plant running Ignition, and the remote PC with Ignition using the leased line. They’re paying $500 a month for their leased line, but have DSL at the remote location and wireless Internet is scheduled to be available at the plant this month, so they are very interested in finding a solution that will work here. The VPN end doesn’t worry me much, but I was a little hesitant about jumping into the serial conversion without doing a little research. I’m trying to steer them towards upgrading their entire control system ini the next year, but for now, they just want to use what they have over IP. Thanks again!

You provide the serial server, and Ill have our host hit it for data.

You should check out Ubiquiti’s router product called EdgeMAX lite. Its the worlds first sub $100 million packet per second router. It also supports every networking protocol and VPN protocols IPsec PPTP OpenVPN.