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N JUNE, I WROTE ABOUT BREAKOUT BOXES, Ethernet device servers and legacy systems and got some interesting feedback. So I decided to use Tier-2 products, as well as that same Tier-4 product, and compare setup, performance and ease of use with varying applications.
My hope was to prove that while we get what we pay for, sometimes we don't understand what we get. The products were ezTCP from Sollae Systems, an offshore manufacturer; Co-Box from Lantronix; Digi SP and IA from Digi Corp.; and the ENet interface from Rockwell Automation.
This is not a product comparison, more an exercise to indicate we need to make careful choices, depending what is important to us. The prices range $50-500. Some required third-party software, which can run the cost up over $500.
I started with a standard marquee text display, a Uticor PMD 3000, and first tried the ezTCP device with some limited success. This device requires a third-party serial port re-director, which takes the serial port communications from any program and re-packages it for Ethernet TCP broadcast.
There are some issues with this form of package transposition as far as control lines go, so it is important to understand what form of communications you need for your device. Since the marquee display doesn't use control lines, it wasn't a problem.
Now, if you consider that I had to "discover" that I needed a COM port re-director, then find one, download and configure it, and then see if all is well, by that point I had eaten up most of the morning.
I tried the same unit using a Sick DME3000 laser measurement device with an external RS-422 to RS-232C interface board. That worked well, and because I had already set it up, the time spent was acceptable, although the data was being viewed in a telnet session, not very useful, I suppose.
I spoke to a U.S. distributor for this product and he suggested it probably should be used only with standard ASCII due to the packet nature of its board and TCP stack. Telnet sessions would be OK, but if you wanted anything two-way, then the source program would probably need some tweaking.I already had tried this with a PLC to no avail (see my June 2004 column, A Legacy of Pain, p53). It wouldn't work on a Siemens 505 either.The Rockwell ENet adapter works with Rockwell products, and also uses tunneling software for protocol ID on Rockwell's network. It doesn't like standard ASCII devices either, but works well with A-B stuff, as you'd expect.The Lantronix Co-Box, as with the Digi SP and the IA Real Port, comes with its own COM port re-director software that works flawlessly.Digi offers control line handling as well, which wasn't tested, thus the "real port" name. It looks and feels like a serial port, a novel idea.Both products use tunneling software to discover themselves on the network, and setup is painless.I used both of these products connected to the marquee and the laser device with complete success. For both solutions it was out of the box, and up and running within minutes. Wow, imagine, products that work as advertised.Next up: the Allen-Bradley Micrologix. This was the source of all the aggravation I wrote about before. Not that A-B is the only one to use a tight protocol, but it seems to be a bit less tolerant than Modbus.So with the ENET device, I set up the communication software that communicates with the PLC to use an Ethernet driver, and "saw" the PLC on the network. The benefit here: I can have multiple devices on the network and see them all. I was communicating with the Micrologix within 10 minutes out of the box.The Digi and Lantronix devices use COM ports for communication. If I have more than one device on the network, I can only see one at a time. They worked well with the Micrologix, and were really easy. These are also the least-expensive solutions.Last, I tried Digi and Lantronix devices with the Siemens 505 PLC. Since the DOS-based software only recognized Com 1 or 2, I had fiddle a bit to disable a port, not the interface's problem, and once I did that, I was home free.What I found was that the Tier-4 device that costs the most (with third-party software) did the least, and the least-expensive devices did the most and did it easily.Seems it is true, the right tool for the job and you can get more than you paid for. I think I proved it.Jeremy Pollard, CET, has been writing about technology and software issues for many years. Publisher of The Software User ONLINE, he has been involved in control system programming and training for more than 20 years. Browse to www.tsuonline.comor e-mail him at[email protected].
