1.2 Run your first IOC

Create one process variable (PV), run an Input/Output Controller (IOC), and read and write the same value through Channel Access (CA) and pvAccess (PVA).

Open two Bash terminals on the same host with these roles and settings:

1. In two Bash terminals, activate the matching installation from lesson 1.1. Set TREE in each terminal to that installation’s absolute path, then direct clients to this host:

   source "$TREE/setEpicsEnv.bash"
   export EPICS_CA_AUTO_ADDR_LIST=NO EPICS_CA_ADDR_LIST=127.0.0.1
   export EPICS_PVA_AUTO_ADDR_LIST=NO EPICS_PVA_ADDR_LIST=127.0.0.1
   

2. In terminal 1, create a workspace outside the distribution:

   mkdir -p ~/ioc-training/first
   cd ~/ioc-training/first
   

   Save this database as water.db:

   record(ao, "temperature:water") {
       field(DESC, "Water temperature setpoint")
       field(EGU, "degC")
       field(VAL, "0")
   }
   

   This analog output (ao) record has no hardware connection. A write changes its stored value; EGU identifies its engineering units.

3. Start the installed PVXS executable in terminal 1:

   softIocPVX -d water.db
   

   Wait for iocRun: All initialization complete. At the IOC prompt, list the records:

   dbl
   

   The IOC shell prints the loaded record name:

   temperature:water
   

   Leave the IOC running in terminal 1.

4. In terminal 2, read the value and description:

   caget -t temperature:water
   caget -t temperature:water.DESC
   pvxget temperature:water
   

   CA returns the scalar value and description, while PVA displays a typed NTScalar structure:

   0
   Water temperature setpoint
   temperature:water
       value double = 0
       alarm.severity int32_t = 0
       alarm.status int32_t = 2
       alarm.message string = "UDF"
       timeStamp.secondsPastEpoch int64_t = 631152000
       timeStamp.nanoseconds int32_t = 0
       timeStamp.userTag int32_t = 0
       display.limitLow double = 0
       display.limitHigh double = 0
       display.description string = "Water temperature setpoint"
       display.units string = "degC"
       display.precision int32_t = 0
       display.form.index int32_t = 0
       display.form.choices string[] = {7}["Default", "String", "Binary", "Decimal", "Hex", "Exponential", "Engineering"]
       control.limitLow double = 0
       control.limitHigh double = 0
       valueAlarm.lowAlarmLimit double = nan
       valueAlarm.lowWarningLimit double = nan
       valueAlarm.highWarningLimit double = nan
       valueAlarm.highAlarmLimit double = nan
   

5. Write through CA, then read through both protocols:

   caput temperature:water 24
   caget -t temperature:water
   pvxget temperature:water
   

   Both reads report 24, and the first write clears the initial UDF alarm status and updates timeStamp:

   Old : temperature:water              0
   New : temperature:water              24
   24
   temperature:water
       value double = 24
       alarm.severity int32_t = 0
       alarm.status int32_t = 0
       alarm.message string = ""
       timeStamp.secondsPastEpoch int64_t = 1794424745
       timeStamp.nanoseconds int32_t = 930919750
       timeStamp.userTag int32_t = 0
       display.limitLow double = 0
       display.limitHigh double = 0
       display.description string = "Water temperature setpoint"
       display.units string = "degC"
       display.precision int32_t = 0
       display.form.index int32_t = 0
       display.form.choices string[] = {7}["Default", "String", "Binary", "Decimal", "Hex", "Exponential", "Engineering"]
       control.limitLow double = 0
       control.limitHigh double = 0
       valueAlarm.lowAlarmLimit double = nan
       valueAlarm.lowWarningLimit double = nan
       valueAlarm.highWarningLimit double = nan
       valueAlarm.highAlarmLimit double = nan
   

6. Write through PVA and read through CA:

   pvxput temperature:water 44
   caget -t temperature:water
   

   pvxput exits quietly on success, and CA returns 44 because both protocols reach the same record:

   44
   

7. In terminal 1, stop the IOC:

   exit
   

Exercise: Add an air temperature

Add a second ao record to water.db, named temperature:air, with DESC set to Air temperature setpoint, EGU to degC, and VAL to 20. Restart the IOC and read both records:

caget temperature:water temperature:air

The restarted records begin at 0 and 20. Client writes in this lesson are not saved to the database file.

To observe a failed search without changing the host firewall, direct the CA client to an unused local UDP port in terminal 2:

export EPICS_CA_ADDR_LIST=127.0.0.1:59999
caget -w 1 temperature:water

With no IOC search listener on that port, the read times out:

Channel connect timed out: 'temperature:water' not found.

Restore the address and verify that the read succeeds:

export EPICS_CA_ADDR_LIST=127.0.0.1
caget -t temperature:water

An empty address list does not reliably cause failure when automatic broadcast discovery is enabled. Keep it disabled for these local exercises.