PLC Vs DDC

[tripper]

Hi, I'm new to this forum, but I was wondering how many control dudes have been in both areas(ddc/plc),

I moved from 14yrs of ddc (to plcs) & thought I knew it all until I was learnt the plc way of doing things,

...just something that bugs me... interested in other comments
thanks

[mationman]

i had a plc background and went to ddc. i like plcs better than ddc, the technology is far more advanced and you can do much more with a plc with less effort. however plcs cost more, and building owners would never purchase these plc systems. such a shame....

[sysint]

i like plcs better than ddc, the technology is far more advanced

Clarify --- Thanks.

[tripper]

Originally posted by sysint
i like plcs better than ddc, the technology is far more advanced

Clarify --- Thanks.

from what I've noted so far:

DDC:
better, ie. simpler, control loops without having to understand PID algorithm,
Better front end graphics (i know i know means nothing.... but sells$$)
Kiss principle....and works..,
The ddc systems I've workedwith so far do not limit software prices to tags (points)doesn't matter how many point are on system, you have payed for software licence & end of story.

plc: Mili second updates, robust/unbreakable comms, better hardwear, better tech support, better training.(ddc training nearly non-existant.

[sysint]

Lonworks can be very "robust".

Newer controllers are dispensing with PID theory.

Graphics? There are many graphical programs every bit as comprehensive. I've done flash automation before. Personally, it's a waste of time.

On the DDC software - so you are saying it's a benefit to pay for software one time or not?

Milli-second updates -- not necessary unless possibly watching Generator RPM.

Even on the hardware I'd say depends. You can buy Sysmik controllers and they use Phoenix Contact I/O so there's one example where it's industrial grade.

I do know there is more money in industrial as opposed to the HVAC market. You get what you pay for---- and the customer typically doesn't want industrial grade.

[sparky666]

Comparing lonworks to a PLC would be like comparing a Dodge Neon to a Viper.....Ya get what ya pay for!

[tripper]

Originally posted by sysint
Lonworks can be very "robust".

true, but not same league

Newer controllers are dispensing with PID theory.
[/B]
?

Graphics? There are many graphical programs every bit as comprehensive. I've done flash automation before. Personally, it's a waste of time.
[/B]
?

On the DDC software - so you are saying it's a benefit to pay for software one time or not?
[/B]
I'm saying it is better to pay once, just pointing out the differences and how the two industries dont talk/share ides.

Milli-second updates -- not necessary unless possibly watching Generator RPM.
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not comparable to motion control etc
[/B]

Even on the hardware I'd say depends. You can buy Sysmik controllers and they use Phoenix Contact I/O so there's one example where it's industrial grade.
[/B]
?

I do know there is more money in industrial as opposed to the HVAC market. You get what you pay for---- and the customer typically doesn't want industrial grade.
[/B]
we agree

[/B]
I think sysint we have been on different pages here, just noting differences, some good/some bad.

[sysint]

Lonworks is used in some industrial settings. Theoretically, there is no "speed limit" to the medium(s) and it has excellent collision avoidance. Much better than Ethernet.

There is a standard for Trains, particularly in the braking systems so I'd say it is in fact as robust as the competitors, or more so in certain areas.

However, for cheaper speed on limited networks Echelon has come out with Pyxos, which is faster.

Again, in the HVAC market, there just isn't the need for hi-end capacity. Although, I'm sure you could do motion control with Sysmik because their I/O is industrial grade, or even a Lonworks Mitsubishi PLC (by example)

I think with Lonworks, the limitations are more set by the manufacturers that don't need the hi-end results. When you think about it - what's the difference between the 78k and the 1250k wire? (Nothing) It's just the manufacturers are all selecting the 78k transceiver because it's good enough. It's just now you are seeing IP852 nodes....

Lonworks isn't bacnet. It is actually very robust and scaleable.

Let's put it this way -- if all you saw were Trane's LON implementation, you would think Lonworks is junk. Then you discover a Circon system and then Lonworks looks significantly better. Now you have companies like Loytec out there expanding the capabilities even further and Lonworks is still handling those applications. It's a much better protocol than many understand. Case in point: Many are still using bacnet ms/tp. (talk about poor performance)I can see why the specification engineers like it- passing the same crap along whether you want it or not with a supervisory engineer putting out directives.

[gwizz]

Are we comparing protocols or controllers?

From what I've seen PLCs are more standardized, with much quicker scan times for the I/O and process loops, they're also more expensive than a DDC controller, and also tend to be more "industrial strength" with high accuracy I/O and a LOT of 4/20 ma analog.

DDC controllers come in all sizes and flavors. An application-specific controller with a pre-engineered application can be very economical both in initial cost and total costs of installation. No need for millisecond response time or 16-bit A/D convertors when doing room temperature control. Fully programmable DDC controllers can rival a PLC in function.

PLCs seems to follow ladder logic or other common programming modes, where commercial DDC programming is all over the map. If you learn how to program one manufacturer's PLC, you'll be in good shape to lean anothers... not so in the DDC world.

Commercial HVAC DDC systems with multiple communication protocols can also communicate with both HVAC DDC controllers (Lon, BACnet, etc) and PLCs (Modbus, etc).

Really apples and oranges. For commercial HVAC, I think HVAC DDC controls work fine. They're fast enough, low cost enough, and dog-goneit, people like them. :)

[ctrlguy]

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[sysint]

Some of their "options" are probably expensive.
Especially when those options are analog or communication.
Modbus can be cumbersome to deal with also.