Short Tutorials

Synchronous Actions

Each of the actions (search, put, get) ending with a “w” signify that the action completes before the function returns. In CA terms this means that a call to ca_pend_io() is issued to force the action to process and wait for the action to complete. When an exception occurs the offending CA status return is printed using print ca.message(status).

from CaChannel import CaChannel, CaChannelException
try:
    chan = CaChannel('catest')
    chan.searchw()
    chan.putw(12)
    chan.getw()
except CaChannelException as e:
    print(e)

Multiple Synchronous Actions

Connection

Multiple channel access connection requests.

from CaChannel import ca, CaChannel, CaChannelException
try:
    chan1 = CaChannel('catest')
    chan1.search()
    chan2 = CaChannel('cabo')
    chan2.search()
    chan2.pend_io()
except CaChannelException as e:
    print(e)

Write

Multiple channel access write requests.

from CaChannel import ca, CaChannel, CaChannelException
try:
    chan1 = CaChannel('catest')
    chan1.search()
    chan2 = CaChannel('cabo')
    chan2.search()
    chan2.pend_io()
    chan1.array_put(1.23)
    chan2.array_put(1)
    chan2.flush_io()
except CaChannelException as e:
    print(e)

Asynchronouse Actions

Asynchronous execution does not require that the user wait for completion of an action. Instead, a user specified callback function is executed when the action has completed. Each callback takes two arguments:

  • epics_args: arguments returned from epics.

  • user_args: arguments specified by the user for use in the callback function.

Since we don’t need to wait for actions to complete we use flush_io() instead of pend_io() as in the synchronous examples. Flush_io() starts execution of actions and returns immediately.

Note: The callback function is not executed in the main thread. It runs in an auxiliary thread managed by CA library.

import time
from CaChannel import ca, CaChannel, CaChannelException
def connectCB(epics_args, user_args):
    print("connectCB: Python connect callback function")
    print(type(epics_args))
    print(epics_args)
    print(user_args)
    state = epics_args[1]
    if state == ca.CA_OP_CONN_UP:
        print("connectCB: Connection is up")
    elif state == ca.CA_OP_CONN_DOWN:
        print("connectCB: Connection is down")

def putCB(epics_args, user_args):
    print("putCB: Python put callback function")
    print(type(epics_args))
    print(epics_args)
    print(ca.name(epics_args['chid']))
    print(epics_args['type'])
    print(epics_args['count'])
    print(epics_args['status'])
    print(user_args)

chan = CaChannel()
chan.search_and_connect('catest', connectCB)
chan.flush_io()
time.sleep(1)
chan.array_put_callback(3.3, None, None, putCB)
chan.flush_io()
time.sleep(1)

Asynchronous Monitoring

Watch for changes in value or alarm state of a process variable. A callback is executed when a change is seen.

import sys
import time
from CaChannel import ca, CaChannel, CaChannelException
def eventCB(epics_args, user_args):
    print("eventCb: Python callback function"
    print(type(epics_args))
    print(epics_args)
    print(epics_args['status'])
    print("new value =", epics_args['pv_value'])
    print(epics_args['pv_severity'])
    print(epics_args['pv_status'])

chan = CaChannel()
chan.searchw('catest')
chan.add_masked_array_event(
    ca.DBR_STS_DOUBLE,
    None,
    None,
    eventCB)
chan.flush_io()
time.sleep(5)