BRX Sales

[BobO]

1.2 ms scan rate for analog inputs, current and voltage, sounds very good.   the t1H modules are that fast now so hopefully you can do it too.

BRX onboard analogs are that fast. The new modules use the same design as the onboard.

[Ingeniero]

Hi Bob O.

How are those new fast analog cards coming along?

When I read the spec for the Brick analog inputs I see 1.2 msec conversion time.  How are conversion time and scan rate related?

[BobO]

Hi Bob O.

How are those new fast analog cards coming along?

When I read the spec for the Brick analog inputs I see 1.2 msec conversion time.  How are conversion time and scan rate related?

Expected launch is late March. As with all launch dates, things can change.

Analog conversion is independent of the PLC scan, but the image register is updated by the PLC scan.

The new modules user the same analog chip. Performance is comparable to the onboard channels.

[Ingeniero]

 

Expected launch is late March. As with all launch dates, things can change.

Analog conversion is independent of the PLC scan, but the image register is updated by the PLC scan.

The new modules user the same analog chip. Performance is comparable to the onboard channels.

So, how fast will the PLC scan time be with the new cards.

[Controls Guy]

I don't think the analog affects the scan time, I think he's saying that scan time poses a practical limit on effective update time into your logic.

[ADC Product Engineer]

I don't think the analog affects the scan time, I think he's saying that scan time poses a practical limit on effective update time into your logic.

Correct.  The two devices are asynchronous.   

The PLC has a scan time, bound by the limits of the minimum and maximum times it takes to scan the program logic.

The IO cards have their own update time, which is not tied to the PLC scan time.  When a card has new data ready it is placed into the backplane queue to be pick up when the PLC gets to the appropriate point in the scan (usually at top of scan).

PLCS do not go ask the card for data and then the card goes out and at that instant the card reads the inputs.  Only some digital input cards that have no buffer work that way.

[BobO]

Some modules do significantly affect the scan time, but our new analogs do not. We did a bit of characterization and it looks like the scan bump is ~30us per module.

That doesn't include scaling, which will add some additional time per channel scaled.

[Controls Guy]

Yeah, the ADC could be 1 microsecond, and if the PLC only looks every 3ms because of scan time, you still have effectively a 3ms update.   But, in that case, at least you can improve it by optimizing scan time, especially on a platform with as many tools to do so as Do-More.

[ATU]

If you have the analog in a remote rack, what is the added delay?

[BobO]

If you have the analog in a remote rack, what is the added delay?

It'll be scan dependent. If scan is short, a few milliseconds.

[Ingeniero]

I want to use BRX for an analog process, other PLCs controlling this process the scan time for the cards and rungs is set to 10 milliseconds. The process is high speed rotating equipment that has to be controlled within a few RPM.

The IO cards have their own update time, which is not tied to the PLC scan time.  When a card has new data ready it is placed into the backplane queue to be pick up when the PLC gets to the appropriate point in the scan (usually at top of scan).

And where is this "update time" given for the analog inputs on the BRX bricks?


I think the BRX might work, I have to break down and buy one of the bricks with analog input and outputs, wire an output to an input and see what the trends show.

[Controls Guy]

I think I'd want an encoder and dedicated motion controller.

[Ingeniero]

I think I'd want an encoder and dedicated motion controller.

They use magnetic speed pick ups and a toothed wheel. these are already installed. The pulse from the mag pick up has to be converted to 4-20 ma to interface with the PLC. There are commercially available modules to do this.

[BobO]

If you could bring it in as a pulse, it would be faster. Either way, BRX should be fast enough to do what you want.

[Garyhlucas]

I saved a ton of money at my last job using BRX High Speed Counter inputs to take pulses from flow meters rather than 4-20ma analog. Adding converters that need to be programmed adds lots of cost and makes your whole system less reliable. That left the analog channels available for thing like pressure, and temperature where analog was really needed. The less conversions the faster and more accurate it will be.