Experimental Physics and
Industrial Control System

[email protected] (Jeff Hill) · Mon, 15 Mar 1999 15:42:37 -0700

Brian,

On Tuesday, March 09, 1999 1:10 PM, Brian McAllister [SMTP:[email protected]] wrote:
> 
> Is there a document that describes the CA beacon "system" ?
> 

There isnt a document dedicated to this subject, but these sections from the CA
reference manual do provide a high level description of what is occurring. Feel
free to contact me if you have more detailed questions.

Jeff

1.3 CA and Wide Area Networks
Normally in a local area network (LAN) environment CA discovers the address of the host for 
an EPICS channel by broadcasting frames containing a list of channel names (CA search messages) 
and waiting for responses from the servers that host the channels identified. Likewise CA clients 
efficiently discover that CA servers have recently joined the LAN or disconnected from the LAN by 
monitoring periodically broadcasted "beacons" sent out by the servers. Since hardware broadcasting 
requires special hardware capabilities, we must provide additional configuration information 
when EPICS is extended to operate over a wide area network (WAN).
If CA has not seen a beacon from a server that it is connected to for EPICS_CA_CONN_TMO seconds 
then an echo message is sent to the server over TCP/IP. If this echo message isn't replied to by the server 
within 5 additional seconds then the client will assume that the server is no longer present on the network 
and disconnect. Disconnecting implies notification of any application programs that may be using Channel 
Access to communicate with the server in question. Disconnecting also implies releasing any resources 
reserved while communicating with the missing server. The parameter EPICS_CA_CONN_TMO is specified 
in floating point seconds. Note that for short time outs, the delay until disconnect will also depend on the 
rate at that applications poll into the CA client library (and allow CA background activity to be attended to). 
When a CA server initializes it initially sends "beacon" messages to each address specified in 
EPICS_CA_ADDR_LIST, and also any addresses auto configured from network interfaces found, with a very 
short period between "beacons". However, this short period is doubled each time a "beacon" is sent so that it 
will rapidly reaches a plateau specified by EPICS_CA_BEACON_PERIOD. The parameter 
EPICS_CA_BEACON_PERIOD is specified in floating point seconds. For efficient operation it is 
recommended that EPICS_CA_BEACON_PERIOD be set to at least one half of the value specified for 
EPICS_CA_CONN_TMO.

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System