Looking for a small PLC


Thread Starter


We are searching a small PLC to integrate in a 3U high (132 mm)/19 inches wide (482 mm) rack space. This space must include the space for thermal dissipation and the cabling.

We reviewed many options of many suppliers (Rockwell, Schneider, Siemens, RTP, Hima, Omron, Mitsubishi, Pilz, Wago, Phoenix contact, Gefran, Gilogik, IDEC, ABB, Honeywell, Saia-Burgess Controls, GE Fanuc, Rexroth Bosch Group, Hitachi, IMO, Omniflex, Festo, Jokab, EADS/Racal, VME solutions, Macq Electronique, B&R Automation, National Instruments) and we didn't find anything.

Thanks for you help and propositions of solutions.

The needs in inputs/outputs are the following: 3 RS485 independent channels, 2 Ethernet channels (or 1 with a switch), 7 binary inputs (24VDC), 6 binary outputs (free contacts, 1 NO + 1 NC each), 3 analog outputs (4-20mA, 12 bits minimum).

It's for an industrial use and we would like to have access to qualification information: norms, levels and criteria. Main expected norms are RCCE-2005, CEI 60439, CEI 60068, CEI 61000 and NF EN 55022.

And then, we would like the solution has a correct life time cycle: about 8 to 10 years of stability for hardware and software embedded, about 15 to 20 years of stability for the function.

Thank you.

bob peterson

I don't know of any PLC that has 3 independant RS485 channels built in. In any case, you would have to specify what protocol you wanted to use.

AB can add Modbus RS485 ports two at a time via add in modules on several of the lines of PLCs.
The 8 - 10 years of hardware and software stability might be a problem, as most of the major brands that I am aware of seem to be well into the life cycle of their current product lines for their smaller PLCs. Some of the models in fact are starting to look a bit long in the tooth. I wouldn't be surprised to see most of the existing models replaced with new (and incompatible) ones in that time period.

If this is for a machine that is in series production, you might consider using a single board computer (SBC). They tend to not change as much.

If you already have a PC in the rack, you might also consider simply doing the control in software and using Ethernet connected I/O (including for the serial ports).

Steve Jackson

I didn't see a mention of Panasonic (Aromat) on your list. They are about the smallest PLC I've found, and pack a lot of features for the size. The price is right, as well.



You may consider Schneider Electric M340 PLC:

372 mm length
100 mm highth
160 mm width

2 port ethernet (Modbus TCP)
1 port serial 232/485 (modbus RTU / ASCII)

For serial port you may use external gateway 485/ethernet
You should check Wago 750-841 (ethernet controller). The number of RS285 (422) is configurable. With a library, you can use them with Modbus (master/slave) or user-defined protocol. There is 8 DI single-ended in one board and several combinations for AI and DO. You can use Modbus/TCP for data retrieving or control.
We know the Vipa solutions. If you are looking for small, modular and inexpensive the 200 series is a nice little controller. It can have serial, Ethernet, switch,etc. everything you need.
Have a good look at the Omron CJ2 PLC series. The RS485 issue is simple - just use 2 serial cards type CJ1W-SCU31 (4 x individual RS485). Ethernet/Ethernet I/P is built in and another Ethernet card can be added. I/O is simple also just add what you require with 16/32/64 bit cards.

Omron have an excellent record for supporting older platforms for this type of PLC over a long period of time. This is a new one and I would expect the 8-10 years to be no problem. I have just replaced some C200H PLCs from the early 90's with new CS1 racks, processors and power supplies and used all the old C200H cards in the new racks. The software is converted automatically for the new processor with some minor manual adjustments.
You could mount a Schneider/Modicon Momentum or a Control Microsystems SCADAPack flat on a shelf. The I/O terminations on these controllers are oriented the right way for this.

The Momentum can be equipped with 1 RS485 and 1 RS232 Modbus serial port. The SCADAPack can be equipped with 2 RS485 and 1 RS232 Modbus ports. In either case you could add Modbus RS485 ports using Modbus/TCP to Modbus serial bridges. MOXA's bridge is very small and suitable for mounting on a horizontal shelf.

In either of these cases you would have to add at least one I/O module. You have room for that.

I do not know what the thermal implications of having the heating vents oriented that way would be. With the SCADAPack you could just remove the metal cover. But in either case, you could add a small fan to push some air though the vents.

I have used the VIPA 100, 200, and 300 series hardware for the past 7 years. Very good product.

It is a modular system allowing any combination of serial coms as well as dig, analog I/O. It is a powerful control system.

CPU can be ordered with different types of coms built in.
Check it out on http://www.vipa.de/en/home/