S7 Selection Advice needed

Hi

Am looking at doing a new job using my first S7 300 (my guess) which is why i thought id ask here first.

I have to take 16 readings every 40mS (although 10-20 would be good) and calculate the average over a second, this then gets converted to a reading/minute as time goes by.

Not being at all familiar with the S7 as i have only used a few s7-226's and i would like to try the 300 range i wanted to know what would be recommended.

The 16 readings will be from an Analogue card on the processor, preferably being able to take a 0-200mV signal with good resolution

0-200mv would be read as 0-10000 dec.

The S7 would be linked to a Redlion HMI so i would like 2 ports one for the HMI connection and one for my laptop.

Any Thoughts appreciated.

edit: Forgot to say

Othat than this the S7 will only be running 140 digital I/O for trips and contactors etc

I just finished my first major S7 job after many years in the AB world. I used to be a big flag waver for the S5-95u and S5-115u product line.

If you are familiar with the S7-200, then toss all that out the window. S7-300 is not a big brother to the S7-200.

For our S7 job, we chose the S7-317-2PN/DP CPU, and I am glad we did. One port was configured for profibus , to connect to remote racks and we used the ethernet for MP370s and MP270s to connect via ethernet.

Avoid the use of the CP-343-1 connected to a non-ethernet CPU, unless you are the only connection. I had an issue with slow response times as more HMIs came online with our project that pointed to the IP address of the CP-343-1. Evenyually I pointed all the HMIs to the ethernet port on the CPU, and the response issues dissappeared.I think the issue is the backplane of the CP-300 series is actually a serial bus.

I also used the ethernet port for programming as I hate having to rely on a special dedicated cable, as this cable becomes the weak link in commissioning if problems develop with it or it goes missing. Wireless ethernet connections to a PLC makes testing I/O so much easier.

In the S7-300, you can schedule an OBXX block to run every XXXmsec. This can be used to accumulate and average samples.

Ian

I use the 317-2DP almost all the time and this CPU would eat this task (and maybe your budget). I've used the 315-2DP a couple of times and this would probably suffice. I've not used the lower spec CPU's.

I take it you've got to invest in Simatic Manager as well - if you are going to use the 300 range regularly then I'd recommend getting PLCSIM as well.

L D[AR2 said:

I use the 317-2DP almost all the time and this CPU would eat this task (and maybe your budget). I've used the 315-2DP a couple of times and this would probably suffice. I've not used the lower spec CPU's.

I take it you've got to invest in Simatic Manager as well - if you are going to use the 300 range regularly then I'd recommend getting PLCSIM as well.

Click to expand...

Yes Simatic Manager, but thats just tools for the job.

Whats PLCSIM ?

What is the spec (roughtly) of the 317-DP and what ports does it have ?

typical scan times ?

http://www.automation.siemens.com/simatic/controller/html_76/produkte/produkte_s7-300_techn_daten.htm

Look here to compare the specs on CPUs

Ian

PLCSIM is a software package that runs on your pc and it simulates a plc. You download your program to the simulator and you can run (almost all) the code as if its in a real plc.

From what you say, the 315 should be perfectly adequate for your requirements. The only thing that might be worth considering if the budget allows it is going up to a 317 PN to get the Ethernet communications off the backplane. As far as I'm aware, there is no PN version of the 315 (though I could be wrong there!) From a performance point of view, the 315 and 317 are pretty much the same, except for the amount of memory.

Roy, unless the current Siemens information has been updated, the 315 is not in the same league as the 317 unless you are going to perform word operations only...

The 317 is much faster but...

Can the job be done easily with a 315? The bit operations for both the 315 and 317 are fast. If one had to do 10 bit operations for every I/O point then that would take up 140 microseconds on a 315. Let mulitply that time by 10 so the time is 1.4 milliseconds for the I/O. Adding 16 numbers and averaging them takes about 20 insructions but lets mulitply by 10 too. That is 600 microseonds. What Siemens probably isn't counting in the instruction times is all the register shuffling and openning of DBs. That could add another millisecond. Lets add 5 more milliseconds for over head. Are we at 10 milliseconds yet? I think a 315 would blaze through this too. I would bet he can do the averaging every scan and the scans would be less than 10 milliseconds.

Thanks for the Input guys.

The 315 or 317 PN/DP lists Ethernet, is this on the cpu or do you need a module for this ?

Also what about a 16 channel analogue card that will take 0-200mV dc directly ??

504bloke said:

The 315 or 317 PN/DP lists Ethernet, is this on the cpu or do you need a module for this ?

Click to expand...

I would get the Ethernet built into the CPU. The bus can't handle the Ethernet bandwidth. The other guys would know better about what the Ethernet options are. I have a S7-315DP which is fast. At least the Profibus DP part is. I have an old ethernet module and it is slow. I also have an external Profibus module and it is slow. This leads me to believe that you should get all your comms on the CPU module if you can.

Also what about a 16 channel analogue card that will take 0-200mV dc directly ??

Click to expand...

It isn't a very good idea to run low voltage any distance because of noise. I would look at amplifiers that convert 0-200mv to 4-20ma.

The 315-2PN/DP is now my "standard" CPU.
It is fast enough for my typical applications, and has a fast onboard Profibus port and a fast on-board Ethernet port.
Like others have said, the CP343-1 Ethernet modules that has to pass data via the backplane are MUCH slower than the onboard Ethernet port.

the 315 is not in the same league as the 317

Click to expand...

I was going from memory when I wrote that and really had in mind the jump from 317 to 319, which is over 10 times faster. In that context, the 315 and 317 are (for me) in much the same league, it's the 319 which is really in a different league.

RMA said:

I was going from memory when I wrote that and really had in mind the jump from 317 to 319, which is over 10 times faster. In that context, the 315 and 317 are (for me) in much the same league, it's the 319 which is really in a different league.

Click to expand...

The step from 315 to 317 is almost the same as 317 to 319.
Memory: 315PN/DP (256KB), 317PN/DP(1024KB), 319PN/DP(1400KB)
Speed: 317PN/DP is up to 10 times faster than 315PN/DP
319PN/DP is up to 8 times faster than 317PN/DP and
up to 20 times faster than 317PN/DP in Arithmetic


The execution-times in the documentation are best-case. If someone uses complex addressing, the differences of these three CPUs are much bigger!