Reading data from bo record?

[Ryan Pierce <rdpierce@pobox.com>]

I've just started building my first EPICS project (the computer 
controlled beer brewing system I mentioned, which recently made 
Hackaday's twitter feed: 
https://twitter.com/HackadayPrize/status/570791651620347904 ) and I am 
not sure about the behavior I'm observing.

The plan is to run a soft IOC on a Raspberry Pi using asyn, 
communicating via USB serial with an Arduino running custom code derived 
from cmd_response, and (for now at least) controlling this from CSS BOY.

I've succeeded in creating a PV of type bo, as well as defining a custom 
command and custom Arduino code for it. I can do a caput to the PV of 1 
or 0, or I can use one of many binary switch objects in CSS BOY, and 
when I switch it, the Arduino takes the appropriate action. That's 
great. But I've got a few questions:

1. I noticed that for a bo record, I can define an OUT but attempting to 
define an INP produces an error. Ideally, I'd really like something 
bidirectional, e.g. I can write a 1 or 0 to it, and that will trigger 
asyn to call the command to write to it, but other actions with the 
running Arduino code can also change the physical thing represented, it 
can be queried, and the result may be the same as, or different from, 
the last thing written to it.

Physical example: I've got three propane burners. I'd like PV's for each 
of them, so I can turn them on or off. But I may want to write to a 
different set of PVs that enables a thermostat mode, where code on the 
Arduino turns the burner on or off automatically based on fluid 
temperature. I'd like to be able to query that and display it on the 
control screen.

I get the feeling EPICS is a Swiss Army knife with a lot of tools, but 
being a beginner I really don't know what the best tool for the job 
would be here. I'm thinking either:

A. I create a pair of bi and bo PV's for each burner. The bo would be 
used to send commands to turn the burner off or on. But the bi would 
indicate whether the burner actually is on of off.

B. Do something I'm missing to make the bo truly bidirectional. I'm 
looking through the docs and am wondering if DOL and OMSL would do what 
I wanted.

2. I noticed that in CSS BOY, I can define an LED with the PV name of 
the bo in addition to a switch. When I flip the switch, there's a little 
lag (due to my SCAN time, I suppose), but the LED eventually reports 
correctly. In addition, the switch itself changes its appearance. I've 
wondered whether CSS BOY is latching the last thing it wrote, or if the 
IOC is latching the last write command it received. Seeing as I can 
restart CSS BOY and these are always correct, I'm assuming the latter, Yes?

3. Doing a caget on the bo PV returns nothing. This confuses me. I'd 
assume I'd get whatever the IOC is latching. Presumably, there's got to 
be a way to get this, otherwise CSS BOY wouldn't be able to display it. 
What's happening?

FYI: There has been a release of the Raspberry Pi 2, which seems to have 
vastly higher performance than the earlier Pi's. I don't have one yet, 
but that is eventually the plan. Right now, even the Arduino dev 
environment is sluggish on the older Pi, but I'm hearing the Pi 2 feels 
like a desktop. I'd really like to be able to run Apache, Tomcat, and 
Web OPI on the Pi so I wouldn't have to rely on any external server.

Thanks,
Ryan