aioca is an asynchronous Channel Access client for asyncio and Python using libca via ctypes. It exposes a high level interface similar to the commandline tools:
- Returns a single snapshot of the current value of each PV.
caput(pvs, values, ...)
- Writes values to one or more PVs.
camonitor(pvs, callback, ...)
- Receive notification each time any of the listed PVs changes.
- Optionally can be used to establish PV connection before using the PV.
To use these functions a certain amount of setup work is required. The following code illustrates a simple application which reads a value from one PV, writes to another PV, and monitors a third until terminated with control-C:
from aioca import caget, caput, camonitor, run async def do_stuff(): # Using caput: write 1234 into PV1. Raises exception on failure await caput('PV1', 1234) # Print out the value reported by PV2. print(await caget('PV2')) # Monitor PV3, printing out each update as it is received. def callback(value): print('callback', value) camonitor('PV3', callback) # Now run the camonitor process until interrupted by Ctrl-C. run(do_stuff(), forever=True)
run function is just a convenience function to allow async code to be
tested, it just creates an asyncio event loop and runs the code under it. If you
already have an asyncio event loop you can just call your async function within
it and dispense with the
If running under IPython you can also do awaits from the interactive console:
In : from aioca import caput In : await caput("PV1", 5678)
How do I pronounce aioca?¶
Good question. The closest we have to a canonical pronounciation is eye-oak-ah, as the alternatives are a bit of a mouthful…
A number of environment variables affect the operation of channel access. These
can be set using the
os.environ dictionary – but note that these need to be
set before loading the
aioca module. The following are documented in the
EPICS channel access developers manual.
- Configures the maximum number of bytes that can be transferred in a single channel access message.
- A space separated list of channel access server addresses.
- If set to
NOthe automatic scanning of networks is disabled.
- Connection timeout, 30 seconds by default.
- Beacon polling period, 15 seconds by default.
- Set these to configure the ports used to connect to channel access. By default ports 5064 and 5065 are used respectively.
import os os.environ['EPICS_CA_MAX_ARRAY_BYTES'] = '1000000' # Note: the first import of catools must come after the environ is set up. from aioca import *