Experimental Physics and
Industrial Control System

Dirk Zimoch <[email protected]> · Tue, 08 Dec 2009 11:33:11 +0100

Hi Geoff,

In the current version, the gateway has the option -no_cache which allows a

client to poll through the gateway. The previous version (or when started

without -no_cache) has a very strange behavior: Only if no client monitors the

channel a poll is passed through to the IOC. If any client monitors a poll does

not pass the gateway but instead delivers the last value received by monitor.

The problem is that you can hardly know when there is any other client with a

monitor.

That is very annoying if you have values that don't cause monitors. For example

if HOPR changes dynamically and you do a caget with DBR_CTRL_DOUBLE on the VAL

field. Then you would always get the HOPR value at the time when the first

client started a monitor on the VAL field. This is because the gateway uses

DBR_STS_* for monitors (as most CA clients do). So the HOPR information in the

gateway is never updated.

We user the -no_cache option everywhere. The additional overhead is moderate. It

is only TCP traffic between gateway and IOC. The gateway still reduces the UDP

traffic, so that even a shell script doing caget in a loop does not flood the

IOC network with broadcasts.

Of course, polling over the network is generally a bad idea.

Dirk

Geoff Savage wrote:

Hi,

Is it possible to ask the gateway to poll PVs? Currently the gateway

uses monitors when it connects to the PVs on the client side. I would

like the gateway to have the equivalent of a SCAN field for a PV name it

is reading.

Thanks,  Geoff

--
Dr. Dirk Zimoch
Paul Scherrer Institut, WBGB/006
5232 Villigen PSI, Switzerland
Phone +41 56 310 5182