Experimental Physics and
Industrial Control System

haquin <[email protected]> · Mon, 01 Dec 2008 10:25:22 +0100

Hi Marco,
I work with Domique Touchard who initiated this thread,

I implemented a GenSub+Waveform db in which I do :

- Read from plc via Modbus/TCP with a Waveform, recombine two 16 bits
words => one 32 bits word with a genSub

- Split one 32 bits word => two 16 bits words with a genSub, Write into
plc via Modbus/TCP with a 2-element Waveform.
Doing the Write operation with one Waveform i.s.o 2 Longout to write
into the plc to avoid the update problem.

I also tried to do this with only standard Epics Records (no sub/genSub)
and I also failed in the recombinaison operation:

- two 16 bits words => one 32 bits word, because there are no equivalent
of the subarray record but to write into a waveform.
Currently my 32 bit words are integers, but I am about to do the same
thing on another plc which uses 32 bits floats.
Don't hesitate to contact me if you have some questions about my
implementation,

I don't expect too much difficulty using float i.s.o integers, I feel
some judicious cast ...
Christophe.
marco_hair a ¨¦crit :

hi,Mark:
I searched the tech-talk and found this topic before. My situation is
on the contrary: the device need to send a 32 bit float to IOC. since
modbus tcp only support 16 bit data, this 32 bit float will be
seperated to two 16 bit data and sent. I can use gensub to implement
it. If I want to just use record to implement it, how to do this?
would you give me some suggestions? Surely I am using your latest
modbus tcp device.

Thanks a lot in advance.
ÔÚ2007-10-16£¬"Mark Rivers" <[email protected]> Ð´µÀ£º
Hi Dominique,
I just put together a test database to convert a 32-bit ao record to 2
16-bit ao records. Here is the database:
record(ao, "test:ao") {
field(FLNK, "test:calcLSW.PROC")
}
record(calcout, "test:calcLSW") {
field(INPA, "test:ao.VAL NPP NMS")
field(CALC, "A&65535")
field(OUT, "test:aoLSW")
field(FLNK, "test:calcMSW.PROC")
}
record(calcout, "test:calcMSW") {
field(INPA, "test:ao.VAL NPP NMS")
field(CALC, "(A>>16)&65535")
field(OUT, "test:aoMSW")
}
record(ao, "test:aoLSW") {
}
record(ao, "test:aoMSW") {
}
I then tested this database with various 32-bit input values to
test:ao, and looked at the 16-bit output values in test:aoLSW and
test:aoMSW.
Here are the results:
corvette> caput -s test:ao "0x1"
Old : test:ao 2.41592e+09
New : test:ao 1
corvette> caget test:ao test:aoLSW test:aoMSW
test:ao 1
test:aoLSW 1
test:aoMSW 0
corvette> caput -s test:ao "0xffff"
Old : test:ao 1
New : test:ao 65535
corvette> caget test:ao test:aoLSW test:aoMSW
test:ao 65535
test:aoLSW 65535
test:aoMSW 0
corvette> caput -s test:ao "0x7fffffff"
Old : test:ao 2.68435e+08
New : test:ao 2.14748e+09
corvette> caget test:ao test:aoLSW test:aoMSW
test:ao 2.14748e+09
test:aoLSW 65535
test:aoMSW 32767
corvette> caput -s test:ao "0x8fffffff"
Old : test:ao 2.14748e+09
New : test:ao 2.41592e+09
corvette> caget test:ao test:aoLSW test:aoMSW
test:ao 2.41592e+09
test:aoLSW 0
test:aoMSW 32768
I conclude that the database works OK until the most-significant bit
in the 32-bit test:ao record is set. The database then fails, probably
because the calc parser is converting to a 32-bit signed integer,
rather than unsigned. For numbers up to 0x7fffffff ( 2147483647
decimal) it works OK.
Mark

From: Touchard Dominique [mailto:[email protected]]
Sent: Thursday, October 11, 2007 3:46 AM
To: Mark Rivers
Cc: [email protected]
Subject: Re: Use of waveform records

Mark Rivers a ¨¦crit :
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.
You are totally right. Thanks. I'll take back my glasses. ;-)

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?
I broke the EPICS database to make a SNL code. I haven't got any
example. I tried to use the 0x0000FFFF bit mask. I thought this value
was converted in a float value and the bitwise operation didn't give
the right result when it was converted back to an integer. The
0x0001000 bit mask worked. Perhaps I made a mistake.
I will tried again with two shifts (one 16 bits left shift to lose MS
bits and and one right to give back the right bit position).
Thanks again for your help.
Dominique.

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 ¨¦crit :
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.

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Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System