Experimental Physics and
Industrial Control System

Carl Lionberger <[email protected]> · Mon, 12 Apr 2004 10:56:43 -0400

I think the desire for bidirectional records reflects some personal 
preference; some people think there is some kind of elegance about it.  I 
agree with Andrew that it is not realistic.  I think it can be easily avoided 
in all cases, as I outline below.

>===== Original Message From "Thompson, David H." <[email protected]> =====
>If you are trying to do I/O on the same point you are right.  You can't
>really turn most hardware around like that anyway.  There are two or
>three cases that I think we (at SNS) need to be able to do
>bi-directional I/O; when talking to a smart device and you want to do
>bumpless transfer of control between local and remote,

This sounds equivalent to having a remote/local switch on the device; the 
switch is implemented in labview.  I thought it was SNS policy to not have 
EPICS control remote/local switches.  These should be controlled at the device 
(locally).  EPICS reads them.  Some poor guy locally testing the hardware 
should not have control taken away from the control room.  Its not safe.

[>]when you have a
>device that can set one state and depends on Epics to set the other
>state,

For this you have an output record that sets it and a readback record that 
reads it.  I find the least confusing way to do this is to have the output 
record be momentary, ie, a bo with a HIGH of about 1 so that when you push the 
button it sets the state (which is read out) and returns to standby.

[>]and when you have more than one PV pointing at the same hardware
>address in the OUT field.

In this case, last gets it. This just says you monitor the val field.  The 
difference from the normal case is that all device supports which write to the 
same hardware must post monitors when any of them controls it.  Its not really 
bidirectional control.

Carl

Carl Lionberger
SNS Controls Group
(865) 574-7636

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System