Experimental Physics and
Industrial Control System

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.