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Python Channel Access Interface (CaChannel)

Python Channel Access Interface (CaChannel). Geoff Savage EPICS Collaboration Meeting 16 November 2000. Why Python?. “Python is an interpreted, interactive, object-oriented programming language” Fermilab experience Reduced development time Portable (Linux, NT, OSF1) Graphics (Tcl/Tk)

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Python Channel Access Interface (CaChannel)

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  1. Python ChannelAccess Interface(CaChannel) Geoff Savage EPICS Collaboration Meeting 16 November 2000

  2. Why Python? • “Python is an interpreted, interactive, object-oriented programming language” • Fermilab experience • Reduced development time • Portable (Linux, NT, OSF1) • Graphics (Tcl/Tk) • Powerful extension library • Easy to extend (SWIG) • Short learning curve • www.python.org Python/CA

  3. Introduction • Implement channel access in a Python class for use in control system applications • First the channel access library must be wrapped • Each Python function wraps a CA “C” function • Additional C functions are needed to complete the wrap • Internal functions • callbacks • file descriptor manager • Helper functions • ca macros • db macros Python/CA

  4. Software Layers • CaChannel • Python class • Implemented using caPythonfunctions and SWIG pointer library • caPython • Python wrapper around the C library • Collection of python functions • Software wrapper and interfacegenerator (SWIG) • EPICS channel access C library • C functions • Macros • Data structures • Constants Python/CA

  5. Examples Interactive >>> from CaChannel import * >>> ch = CaChannel() >>> ch.searchw('catest') >>> ch.putw(123.456) >>> ch.getw() 123.456 Script from CaChannel import * def main(): try: catest = CaChannel() catest.searchw('catest') catest.putw(123.456) print catest.getw() except CaChannelException, status: print ca.message(status) main() Python/CA

  6. CaChannel Methods • Connection to a process variable (PV) • search_and_connect • clear_channel • Write to a PV • array_put • array_put_callback • Read from a PV • array_get • array_get_callback • Monitoring a PV • add_masked_array_event • clear_event Python/CA

  7. CaChannel Methods • Send the requests (search, get, put) to an IOC (server) • pend_io, pend_event, flush_io • PV information • field_type, element_count, name, state, host_name, read_access, write_access • Convenient PV access • Added by us to make PV access easier • searchw = search + pend_io • putw = array_put + pend_io • getw = array_get + pend_io Python/CA

  8. Callbacks • Users write callback functions in Python • The standard set of EPICS callbacks is supported • Connection • Put • Get • Value, Status, Time, Graphic, Control • Monitors • Value, Log, Alarm • Internal functions move data from C to Python Python/CA

  9. Monitor Callback def eventCb(epics_args, user_args): print ca.message(epics_args['status']) print "new value = ",epics_args['pv_value'] print ca.alarmSeverityString( epics_args['pv_severity']) print ca.alarmStatusString( epics_args['pv_status']) try: ch = CaChannel() ch.searchw('catest') ch.add_masked_array_event( ca.dbf_type_to_DBR_STS(ch.field_type()), None, ca.DBE_VALUE | ca.DBE_ALARM, eventCb) ch.pend_io() except CaChannelException, status: print ca.message(status) Python/CA

  10. CA FunctionsNot Wrapped • ca_change_connection_event • use ca_search_and_connect • ca_add_exception_event • ca_replace_printf_handler • ca_replace_access_rights_event • ca_puser macro • not used • ca_test_event Python/CA

  11. Issues and Plans • Issues • Scan groups included but not tested • fd manager implementation appears to have a memory leak (Janousch Markus) • Plans • Complete the wrap • Suitability of next CA version for use in Python Python/CA

  12. Summary • Extensive experience using CaChannel • CaChannel used in all control and monitoring GUIs at DZero • Easy for novices to use Python and CaChannel Python/CA

  13. Downloading CaChannel www.aps.anl.gov/epics/->host software->CaPython Python/CA

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