Experimental Physics and
Industrial Control System

"Mark Rivers" <[email protected]> · Fri, 5 Oct 2007 11:32:58 -0500

One thought on this.

Because you must do 2 Modbus operations to write a 32-bit value with
Modbus, I suspect that the power supply only updates the value when you
write one of the 16-bit words, i.e. either the most-significant or
least-significant 16 bits.  You should read the documentation to find
out which one actually updates the power supply and make your database
write that value last.  Otherwise you will get a momentary garbage
output if you do it in the wrong order.

Mark

> -----Original Message-----
> From: [email protected] 
> [mailto:[email protected]] On Behalf Of Mark Rivers
> Sent: Friday, October 05, 2007 11:23 AM
> To: Touchard Dominique; [email protected]
> Subject: RE: Use of waveform records
> 
> Hi Dominique,
> 
> Sorry for the delay in replying to this, I was on vacation.
> 
> I think an easier solution to this problem is to use 2 calc records to
> convert the 32-bit integer to 2 16-bit integers, and write 
> those to the
> Modbus driver using function 6.  Then you don't have to write any C
> code, just a simple database.
> 
> PowerSupply (longout) -> calc1 (16 LSB) -> longout1 -> Modbus
>                       -> calc2 (16 MSB) -> longout2 -> Modbus
> 
> I don't recommend modifying the driver, because a 32-bit 
> integer is not
> a standard Modbus data type, and we could end up with all sorts of
> modifications for non-standard data types.
> 
> Mark
> 
> 
> > -----Original Message-----
> > From: [email protected] 
> > [mailto:[email protected]] On Behalf Of 
> Touchard Dominique
> > Sent: Wednesday, September 19, 2007 8:44 AM
> > To: [email protected]
> > Subject: Use of waveform records
> > 
> > Hi,
> > 
> > we are working on/with the Epics modbus tcp driver written by Mark
> > Rivers. We need to write a 32 bits integer value in a power supply
> > command control interface.  First we thought to write an asyn32Int
> > record with the number 6 modbus function.  Immediatly, we 
> saw that the
> > driver uses the number 6 modbus function which works only on 
> > a 16 bits word.
> > 
> > After reading the Modbus Driver Support documentation, we 
> saw that we
> > have to use a two value length waveform record.
> > 
> > First question : is this correct ?
> > 
> > Second question : do we have to use the gensub record to manipulate
> > values inside a waveform record ? In this case, is there any example
> > that shows how to do this ? Could somebody send us an example 
> > on this ?
> > 
> > Third question : Is a bad idea to modify the modbus driver 
> in the goal
> > to write a asyn32Int 32 bits integer record with the  
> number 16 modbus
> > function ?
> > 
> > Thanks for your help .
> > 
> > Dominique Touchard.
> > 
> > 
> 
> 

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System