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<== Date ==> <== Thread ==>

Subject: Readback from AB1771-IXE used as a millivolt module
From: [email protected] (Marty Kraimer)
Date: Fri, 14 Oct 1994 08:01:14 -0600
> From [email protected] Thu Oct 13 22:11 CDT 1994
> From: Ying Wu <[email protected]>
> Date: Thu, 13 Oct 1994 23:11:12 -0400
> To: [email protected]
> Subject: Readback from AB1771-IXE used as a millivolt module
> Content-Type> : > text> 
> Content-Length: 1307
> 
>  
> 
> Hi,
> 
> Recently we fixed a problem with the readback from an Allen Bradley 
> IXE used as a millivolt module. What we have seen is that the linear 
> conversion for the device type "AB-1771IXE-Millivolt In" is
> bypassed in devAllenBradley.c. RVAL is returned as VAL even if linear
> conversion is selected in the database. 
> 
> Without further information about how to use the RVAL value, 
> I used a volt-meter to measured the voltage applied to the AB1771-IXE, 
> and then compared the measurement with RVAL. It turns out that 
> the voltage reading in millivolt is RVAL divided by 100. 
> 
> Note: one cannot distinguish a negative voltage reading from a positive 
>       one only using RVAL.
> 
> Here is the measurement,
> 
>  Raw voltage measured           VAL(or RVAL) from an
>  by a volt-meter (mV)           AB1771-IXE used 
> 				a millivolt module
> -----------------------------------------------------------------------------
>   8.5                            858
>  47.2                           4722
>  82.2                           8207
>  -8.5                            858
> -47.2                           4713
> -82.2                           8193
> -----------------------------------------------------------------------------
> 
> Could someone confirm this observation, or correct me if it is wrong ?
> 
> Thanks in advance !
> 
> Ying
> 
> 
> 

The ixe module returns ascii integer values representing values. For Tc readings
it is accurate to a degree.For millivolt valies it represents 100s of
millivolts. 

The value returned by the ixe module has a "unique" way of returning the sign.
At one time the epics allen bradley driver did not handle signs properly.
As far as I know it does now. I have only tested it with Tcs not with
millivolt inputs so there could still be a problem but I just looked at the code
and the driver handles sign bits for millivolts just like it does for Tcs.
Thus your problem with signs may be because you have an old version of the
driver.

The device support does the following:

1) For millivolt inputs it adds 10000 to the value returned by the driver and
   puts this value in RVAL. This RVAL has a value between 0 and 20000 with
   0 to 9999 representing negative values and 10000 to 20000 representing
   positive values
   If linr=1 the device support also sets
    eslo=(eguf-egul)/20000. This is a normal way of handling linear conversions

3) For Tc inputs the driver just sets val itself because the value returned by
    the driver is already in degress.  

Again I suspect that your problem with signs is an old version of the
allen bradley driver.

Marty Kraimer

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