Experimental Physics and
Industrial Control System

"Mark Rivers" <[email protected]> · Tue, 9 Oct 2007 08:39:18 -0500

Hi Dominique,

> Perhaps I discovered a  modbus driver "feature"  ? 
> I created one modbus number 4 read function which read 11 bytes. I tested three waveform input records. 
> These three records read 3 different values on different offsets. These 3 records read the offset 0 ???!!!

That is the documented behavior for asynInt32Array support.  Look at 

http://cars9.uchicago.edu/software/epics/modbusDoc.html#EPICS%20device%20support

in the section on asynInt32Array support.  You will see that the "offset" field is NA (not applicable) for all Modbus functions for asynInt32Array.  The driver always uses offset 0, in other words it is designed to read the entire block of memory for that asyn port.  I think the offset function could probalby be implemented if it is desired, I don't remember why it was not implemented initially.  But since you need to use SNL or Gensub anyway, you can get access the words you need within the array.

> About ai/ao records, I will need to write and read float values with the Modbus driver. 
> I saw integer examples. Ai/ao examples work with asyn32int type. 
> Did you test asynFloat64 values ? 
> Do I have to use waveform records (ie 32bits : sign:1bit exponent:8bits fraction:23bits) ?

Float values are not a standard Modbus data type; they are not documented in the Modbus standard.  Again, because they are more than 16 bits long, they require multiple Modbus words.  I think you need to do the same thing you do for 32-bit integers, i.e. use a calc, Gensub or SNL code to convert to two 16-bit integers and then use two longout records or a waveform record.

> I have a beginner's question : does the bit left shift work like C code?
> Can I write something like A << 16 in the calc expression?

Yes, that is documented under Bitwise Operators here:
http://www.aps.anl.gov/epics/EpicsDocumentation/AppDevManuals/RecordRef/Recordref-13.html#HEADING13-0

> I tried to convert the 32 bit integer value into 2 16 bits values with a calc record. 
> Because of float conversion troubles, I don't  get through to get the right values. 

It seems to me like it should work, since I think any 32-bit integer can be represented exactly in a 64-bit double.  Can you send an example that did not seem to work?

Mark

________________________________

From: Touchard Dominique [mailto:[email protected]]
Sent: Tue 10/9/2007 3:50 AM
To: Mark Rivers
Cc: [email protected]
Subject: Re: Use of waveform records

Hi Mark,

Thanks for your answer. I agree with you. We mustn't modify the driver.

The power supply could work with two longout records. In the future, I will need to know how to work with a waveform, an SNL or Gensub code. So I decided to try these tools. I have written an SNL code which does this conversion. I know this is not the best solution. I will try a gensub record soon.
I tried to convert the 32 bit integer value into 2 16 bits values with a calc record. Because of float conversion troubles, I don't  get through to get the right values. I have a beginner's question : does the bit left shift work like C code? Can I write something like A << 16 in the calc expression?

Perhaps I discovered a  modbus driver "feature"  ? I created one modbus number 4 read function which read 11 bytes. I tested three waveform input records. These three records read 3 different values on different offsets. These 3 records read the offset 0 ???!!!

So I decided to create only one input waveform which read the whole bytes. It works fine. At present, the SNL code converts the values.

About ai/ao records, I will need to write and read float values with the Modbus driver. I saw integer examples. Ai/ao examples work with asyn32int type. Did you test asynFloat64 values ? Do I have to use waveform records (ie 32bits : sign:1bit exponent:8bits fraction:23bits) ?

Thanks for your help.

Dominique. 

Mark Rivers a écrit : 

	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