How to account VCR in a segment.

[labingtone]

Dear All,

We have a field bus demo unit in our office and Im doing a self study. Please clrify me the following:

VCR is one of the factor for designing a FF segment. How to evaluate the number of VCR in a segment.

My segment consist of the following instruments:


1. DP Transmitter : 1 Resources Block, 1 Transducer Block, 1 AI Block
1 PID Block.

2. Valve Positioner: 1 Resources Block, 1 Transducer Block, 1 AO Block

The Loop controls are program in the instrument.


Many Thanks,


Lovington Dela Cruz

[jberge]

Counting VCRs manually is too troublesome. If you had to do that, then Engineering of Fieldbus would end up more difficult than 4-20 mA which counters the reason for using it.

You should use devices and control systems with interfaces that have "more than sufficient" VCRs so that you don't have to worry about it. In the end, the configuraiton tool will make these final verifications for you.

[labingtone]

jberge,



what is the maximum number of VCR for monitoring and for control



Lovington Dela Cruz

[tibor]

Dear Labingtone,

The limit of used VCR's depends on the control system (HOST).
Please check the limit in it's user guide or help, or let me know the type of your Host to be able to give you further info.

Best regards,
Tibor Farkas
Pepperl+Fuchs Hungary

[labingtone]

Tibor,



Im using yokogawa ALF111 module as a host.



regards,



Lovington Dela Cruz

[Robin Burnikell]

Hi Labingtone,

To answer your question is not so simple because there are different types of VCRs (Client/Server, Report Distribution, Publisher/Subscriber) . There are also VCRs for H1 and HSE. Also sufficient VCRs are required in BOTH the Host and the devices for your strategy. A VCR of the relevant type exists at the start and the end of each Virtual Communication Relationship, so in very simple terms one link, i.e. between an AI block in one device and a PID in another device uses two VCRs.

Hence, as Jonas said, although for any one specific circumstance you can calculate how many VCR's you need of each type, this is very time consuming and will not cover you for future strategy changes or system expansion. Hence make sure the host and devices you use support as many as possible. I do know of several FF users in the UK that have non-smar host systems that are unable to put all the control they want in the field because they have run out of VCRs.

The number of VCRs is not something many manufacturers advertise so the best bet (as most times) is look at the capability files for hosts and devices to see what they support.

If you look at the capability files published on the FF website you will typically see the following;

Smar devices - 44 VCRs (Smar Stack)
Emerson devices - 20 VCRs (Rosemount Stack)
ABB 2600T series devices - 24 VCRs (Softing Stack)
Yokogawa EJA devices - 17 VCRs (Yokogawa/Softing Stack)

I know the Smar DFI302 host supports 100 VCRs, I do not know the other host systems but again this can all be found in the capability files on the FF website.

I hope this helps?

Regards,

Robin

[tibor]

Dear Labingtone,

The VCR limits of ALF111:
- client: 31
- think: 2
- subscriber: 48
- publisher: 48

And please consider the FF standard allows either 128 and 256 byte data package. Most of suppliers use 128 bytes size but Yokogawa uses 256 bytes thus if you do use the limit value of VCR's, memory overflow may occures. Recommended to div. the limit values by 2. Or use the Yoko site to download the correct cff files. All of these notes belong to field devices of third parties which use 128 byte data package. Yoko's uses 256 bytes.
However if you want to compare to the others the limit should be multipled by 2.

Regards,
Tibor Farkas
Pepperl+Fuchs, Hungary

[Robin Burnikell]

Hi Tibor,

Until now I had not responded to your post because I was unsure of the detail, however, I was intrigued by what you said, so I consulted our R&D guys.

I have been informed that although the specification does allow for either 128 or 256 bytes per VCR this does NOT mean a 256 byte VCR is equivalent to 2*128 byte VCRs. Hence, it is NOT possible to get a 256 byte VCR to act as 2 VCRs.

Of interest I am told most VCRs are published links with only 5 or 2 bytes of data.

Hope this helps,

Robin