VCR of 3051

[Guruprasad K V]

Hi,
I am going through the 3051 manual, when i struck with two lines related with VCR.
The lines are "There are a total of 20 VCRs. Two are permanent and 18 are fully
configurable by the host system. Thirty link objects are available"

The questions are what it means permanent VCR's? and what is configurable VCR.I have some idea like if any block are connected with the blocks in the other devices then VCR is needed it might be called as configurable.

Suppose in case of 3051, it is having 18 configurable VCR, is this limits my usage of all the blocks of 3051?
and in the same line the document mentions about network parameter,
Slot Time 6
Maximum Response Delay 4
Maximum Inactivity to Claim LAS Delay 60
Minimum Inter DLPDU Delay 7
Time Sync class 4 (1ms)
Maximum Scheduling Overhead 21
Per CLPDU PhL Overhead 4
Maximum Inter-channel Signal Skew 0
Required Number of Post-transmission-gab-ext Units 0
Required Number of Preamble-extension Units 1

how this parameter effect my block execution or configuration.?

waiting for your valuable reply,

Advance thanks,
regards
Guru

[agupta]

Permanent VCR
Every device requires one such VCR. To connect to the device for any configuration requires one VCRs. Without this permanent VCR, the host cannot connect to the device to do any configuration including VCR. The 3051 has two such VCR. It is probably to allow two hosts to be able to connect to the device, even if all other VCRs have been used up for other purposes such as Functionn Block links and Alerts.
The device manufacturer specifes the device capability for the network parameters. In any installation, the Data Link has to be configured for the worst case of all devices.
For example, the Minimum Inter DLPDU Delay for a slink (segment) has to be configured to be equal or more than the largest value specifed for all devices in the link. If one device has 7 and another device has 8 in its specification, then the link has to be configured for 8 or more. Most configuration tools use the following default values specified in the FF Communication Profile specification:

"The default value for the DLL Slot time is: 8. Support for smaller values is recommended.
The default value for the DLL Max response delay is: 10. Support for smaller values is recommended.
The default value for the DLL Inter Pdu Gap is: 16. Support for smaller values is recommended."

Note that if the DLL Inter Pdu Gap for a Link is configured for larger than required by all devices, the bus utilization goes down. This is because this is the required gap between each frame and thus bus stays idle during this gap. Some of the configuration tools alllow you to set this value other than default, using 'Advanced' settings. But be careful, not to set it lower than required by all devices.

The Link configuration does not affect the block execution.
For every block link across the devices and between device and host requires one Link object and one VCR. The Alerts also require VCR. Therefore, if a device has a 'large' number of blocks then that device should also have 'large' number of VCRs. The device manufacturer knows this and I expect that there are ufficient number of VCR in the device. The limit of VCR count is usually due to non-volatile memory in the device.

[jigish_jepal]

While checking FF specs. of several makes of pressure transmitters, I noticed that for one perticular model there are no VCRs !!!

MaximumNumberOfClientVcrs = 0
MaximumNumberOfServerVcrs = 0
MaximumNumberOfSourceVcrs = 0
MaximumNumberOfSinkVcrs = 0
MaximumNumberOfPublisherVcrs = 0
MaximumNumberOfSubscriberVcrs = 0

For this product only PID block appears as registered block. AI or other blocks are not appearing as registered or other blocks.

How this is possible?

[agupta]

The list of VCRs is used by the Host configuration tools. These tools may use CF (common file) to find the number of VCR in a device. There are ususlly sufficient VCRs in most devices. If the tool does not use this count, and if all VCRs are used up, then it will not be able to program the VCR and thus not able to make a connection.
Although device spec lists max count of different types of VCRs, all devices are built such that any VCR can be used for any type. Thus one max number applies to all.
The Function Blocks are specified in the DD (device description) file of a device. The separate specification is not used. As long as the DD is correct, the device can be used.

[Jyrgen]

The cff is probably erroneous. If the entries for the VCRs are wrong, there is some chance that the entries for the blocks are also incorrect or incomplete...

[ccarter]

Jigish,
You mentioned:

While checking FF specs. of several makes of pressure transmitters, I noticed that for one perticular model there are no VCRs !!!

MaximumNumberOfClientVcrs = 0
MaximumNumberOfServerVcrs = 0
MaximumNumberOfSourceVcrs = 0
MaximumNumberOfSinkVcrs = 0
MaximumNumberOfPublisherVcrs = 0
MaximumNumberOfSubscriberVcrs = 0

For this product only PID block appears as registered block. AI or other blocks are not appearing as registered or other blocks.

How this is possible?

I wonder if the device you mention uses dynamically instantiable blocks and if that may have anything to do with the file not showing the VCR availability. Just a thought.

Chuck Carter