Experimental Physics and
Industrial Control System

"Mark Rivers" <[email protected]> · Fri, 24 Jul 2009 09:08:22 -0500

Hi Patrick,

To answer your second question first:  Are you using asyn for socket
communication?  If so all communication should be done either with
pasynOctetSyncIO->writeRead() which is an atomic operation, and/or in
the callback from pasynManager->queueRequest, where you can do
pasynOctet->write() followed by pasynOctet->read() and be guaranteed
that nothing else can talk to the socket at the same time.

For your first question, if you set PACT=1 then another position cannot
make it to your device support until PACT=0 from the original record
processing.

Are you aware of the EPICS "motor" record?  It handles all of these
things for you.  It does not set PACT=1 when a move it in progress, so
additional positions or stop requests can make it to device support.

The current version of the motor record uses a device-independent asyn
device support layer, a device-independent asyn driver layer, and then a
simple set of about a dozen functions that you must write for a
particular controller.  It is really quite easy to write a new driver.
It does exactly what you want: polling updates of position and status,
ability to set a new target position while the motor is moving, etc.
Plus lots of features like hard and soft limits, etc.

http://www.aps.anl.gov/bcda/synApps/motor/

Mark

-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Patrick Thomas
Sent: Friday, July 24, 2009 7:22 AM
To: [email protected]
Subject: checking for change in value in device support function

Hi,
I would like to try to implement the following pseudo code as a device 
support function for a motor. Basically, when called, it turns the motor

on, checks to see if the motor was on, tells the motor to move to a 
position, waits for it to finish or time out, turns the motor off, 
checks to see if the motor is off, then checks to see if the current 
position is the one that was requested. The functions that it calls will

be provided by driver support. The question I have is that I want it to 
immediately start moving to a new position if a new value is entered in 
the associated ao record field, even if the motor is already in motion. 
Is it possible to check for changes in the value field of the record 
from this function? Do I need to start this function as a new thread, 
set PACT to true, and then set it to false at the end of the thread? How

would the thread pick up on the fact that the record value field has 
changed?

Another question I have is, that in driver support, in the init 
function, I will open up a socket connection and spawn a thread which 
constantly polls the motor for its status, ie current position, 
trajectory mode, motion done, etc. This data will be written to a 
structure which is available to the device support. However, when device

support calls other driver support functions which communicate with the 
motor over the same socket connection, how do I ensure that these don't 
collide with the functions that are constantly polling?

Thank you,
Patrick

request_move_to_absolute_position( record val field )
{
    motor_on();

    if (read_motor_on != true)
    {
        print error;
        return;
    }

    move_to_absolute_position( record val field );

    while ( (motion_done_status != true) and (timeout != true) ) {
        if ( record val field changes ) {
            move_to_absolute_position( record val field );
            reset timeout;
        }
    }

    motor_off();

    if (read_motor_off != true)
    {
        print error;
    }

    if (read_actual_position() != record val field) {
        print error;
    }

    return;
}

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System