Danfoss FC through DP

Hi all,

Quick question.

It is our practice to continuously send either a start (47C) or stop (43C) to our frequency converter over profibus. So during operation we always have a start signal, and during stop we always have a stop signal.

We have a client who is saying this is bad practice and we should only send commands for a couple of seconds when they change.

Anyone has thoughts on this?

Thanks!

customer always right ???

That is another discussion. The customer is always right as long as he is paying for it and it does not interfere with safety and standards/guidelines.

I want this to work correctly, and I am wondering what common practice is. Regardless of who is right.

I always like to keep my start/stop signals high as it makes looking at code a whole lot easier. If the start bit is on the motor is being told to start and the same with the stop.

If you only turn it on for a few seconds then unless you were looking at the logic for those few seconds, how would you know whether the drive is being told to start or stop.

Cheers

Mark

Well, you still would know that there had been a start command. Likely, if you give a start command and don't receive a running status within a certain time period, a alarm should be initiated. The start command is still there, but it is just not being send continuously over the bus.

But I do agree with you. I think the problem here is that there are multiple control sites.

explain to you customer

the defination of START Instant or STOP Instant

and

the defination START Delay or STOP Delay

which mode he accepted.

but come in SAFETY explain to he which is better.

I am not referring to a delayed or instant start/stop.

I am asking, what is common practice. Send a continuous start or stop command, or send a single start or stop command until either running or not running status returns.

Apologize for miss-understand

If the PLC send a command to Drive to START or STOP with single or continues command

How the Drive react the command or handshake with PLC.

I think in this case that would be achieved by the reference set point.

I do exactly the same as you Boneless and I've programmed hundreds of them without a complaint from anyone.

so far i know the PLC CPU,

cyclic run or scan program or processing program every microsecond,milisecond

the command will send cyclic.

we can read from the status word

Use an internal maintained start / stop bit, and drive the command on run / not run feedback.. Have seen this done at plenty of sites.. May be something to do with the VSD command being level / transition commanded from your customers perspective..

Thanks guys

I know this is an old thread but I thought I could add value to people ever searching this.

It seems that you are writing 47C or 43C as a hex value using a move instruction. If this is the case, I believe irrespective of whether you are momentarily or constantly writing this, this is bad practice.
The control word is a combination of bits (bools) that are being turned on and off.
Moving a value to this word is only shortcutting which bits are turned on and off.

Best practice i believe is to write each bool individually with code that is relevant, which in turn will create your 47C/43C. The difference at a binary level between 47C and 43C is bit 6 turning on and off. This is the start/stop bit.

Problems with writing a value (especially constantly) instead of writing each bit is that if you want to use another command bit later (like the reset bit, or quick stop, etc) then the move instruction can potentially write over the command,
for example if you set bit 7 (reset command) high then your word would read 4BC when stopped and 4FC when running. Writing 43C forces the reset command off.

Hope this helps

FC on Prifibus

Hi Guys

Boxy is right,

Refering to wht Boxy have said, it's better to only send(one shot) the START/STOP codes(assuming that the drive is retaining the control word bits pattern), this will let you safely use other bits when you want to.

Hagos,