PLC H/W design questions


Thread Starter

Suttipan Limanond


I'm trying to find out what type of relays
are using in modern days PLC. For example, are
they PCB mount type or just a regular type mounted
to chassis and wired to the controller board?
The former seems to be more expensive. Also, are these relays need to be power relays?
I saw some general purpose relays that also has
high contact rating. So I'm not sure which one
should I use?

Another question that I'd like to ask is about the
`high-speed' input. I saw some interface circuits
and it seems to me that the difference between
the regular and `high-speed' input are just the
way resistor and capacitance are connected after
the optoisolator. Can anyone help me understanding
this difference?

Thank you very much,

Suttipan Limanond.

Terry Howsham

First of all the Output Modules normally only have a PCB mounted relay rated at no more than 3Amps in most cases. The idea is to have a "volt free contact". To drive anything requiring substantial current you need to turn on a contactor using one of these outputs.

As for the "High Speed , I think what happens is that they get a higher priority in being read by the PLC
With out seeing the circuit in question I would suggest that the difference is due to the response time of the RC circuit, possibly at the expense of noise immunity.

Obviously, the presence of the RC network increase the time it takes for the change in voltage to rise/fall above/below the switching thresh-hold of the input circuit.

Different values, or configurations would have different response times.
If you're looking for a compact, reasonably high current PCB mount relay have a look at the Omron G6 series, they're good for 5A @ 30V DC.

I don't know exactly how many PLC manufacturer's have implemented their digital inputs but I would suggest that there are two factors affecting the
"speed" of a raw digital input.

The first is, as you say, how "vicious" the filtering on the input is. Heavy low pass filtering with large capacitors will limit the rise time of input signals and will thus effectively block any high speed input signals.

The second is whether or not the inputs generate an interrupt on the PLC's processor. If they do not then they are being polled by the processor and hence the speed of input signal detection will be dependant upon the scan rate (the slower the scan rate, the longer a signal must be to be spotted during a scan and hence the lower the frequency).

If inputs generate interrupts then any change of state on any input should be recognised within the interrupt latency period of the processor, which is governed mostly by the clock speed of the processor and the length of the particular processor's instruction cycle (and maybe where the input's interrupt priority is and maybe how busy higher level priority interrupts are).

Generally speaking you can assume that the processor will start "doing something" to process a change of state of a digital input within a few
processor clock cycles.

Depending on how you decide to do things you could have minimal hardware filtering on your digital inputs, have any change of state generate an interrupt then do whatever filtering, debouncing or whatever in software.

This makes for a flexible design without limits imposed on the types of input you can connect to any input but it does mean that you need a little
more logic "outside the processor" to detect changes of state on your inputs. If you can afford the small added expense this is trivial in
programmable logic.

Hope this helps maybe

Geoff Moore
Straight Forward Solutions Ltd
Maynooth Road, Prosperous, Naas,
Co.Kildare, Ireland
+353 (0)45892739
[email protected]
Dear Suttipan:

We have sold thousands of relay output modules and all of them are printed circuit board (PCB) mounted units. The relay has to be on the PCB so they are removed and replaced when the output module itself is replaced or serviced. Currents are limited by connector space, the size of traces that can be practically placed on the PCB, and heat dissipation. If you need really big currents, external relays or contacters are required.

I wonder if you realize that by far most PLC outputs are NOT really mechanical relays, but instead are solid state devices such as TRIACs?
These too are mounted to the PCB. They have a leakage current (mechanical relays have none), more heat dissipation than mechanical relays, and
differences in isolation of the switched circuit.

"High Speed" inputs have several meanings. Some io modules labeled "high speed" are DC units that capture short pulses and either latch or count
them. These "high speed" inputs often even have built in logic capabilities (input to PLC is "on" if channel one is on before channel two is on, etc. These often are specialty modules.

However, the "high speed" you seem to be referring to would be an AC (presumably 120/240v 50/60 Hz sort of thing) input with faster response than the two to three AC cycles typical of most inputs. The design of AC inputs
is a tricky business. They typically bring in the AC, rectify it, and then use an RC circuit to integrate the input voltage over some period of time. This average voltage is then passed through some variation of a voltage compariter (along with some signal conditioning and the order of the steps will vary with the particular module's design). Without the RC circuit, the input will turn on and off during each AC cycle depending upon the instantaneous AC voltage. (On SOAPBOX /FLAME shields on: i always get a real kick about folks doing control using 50/60 Hz AC and claiming 10ms or even faster control times.... anything faster than the AC cycle is bogus!
Off SOAPBOX) If such a non-filtered AC input is read at any time other than during the high voltage part of the AC cycle it would appear off. So the AC input must be filtered for at least at full AC cycle to avoid cycling
(turning on and off) with the AC cycle. And in most plants, a missing (or small) AC cycle or two is very common (as motors start etc). So, unless the integration longer than these common short brownouts, the AC input will still report false off states. As a result, most AC inputs for PLCs are filtered for at least two or three AC cycles in order to avoid these false off signals. Using such a "high speed" AC input would require careful engineering to avoid the problems with AC cycles being off a large part of
each cycle and missing or low level AC cycles...

Hope this helps, steve
Steven B. Cliff title: VP, Research & Development
Control Technology, Inc. email: [email protected]
5734 Middlebrook Pike web:
PO Box 59003 voice: (423/865) 584-0440
Knoxville, TN 37950 fax: (423/865) 584-5720
Agreed your point on the speed of ac inputs!!

I just used Analog Devices AD736 rms to dc converter on a recent product.

It worked great and was very accurate BUT the settling times for low frequencies are ENORMOUS ie >100mS and you can just forget about d.c.
coupling it at such low frequencies so you have to do a.c./d.c. input type selection.

It makes a bit of a mockery of fast A to D converter requirements sometimes asked for by customers. Sure we can do 200k samples per second plus but only after we've waited 120mS for the input to settle!!

It's another of those examples of keeping requirements within perspectivce


Geoff Moore
Straight Forward Solutions Ltd
Maynooth Road, Prosperous, Naas,
Co.Kildare, Ireland
+353 (0)45 892739
[email protected]