Re: Issue with Asyn+StreamDevice on a Raspi CM4

[Florian Feldbauer via Tech-talk <tech-talk at aps.anl.gov>]

Hi Dirk,

In my test I tried to write a parameter of type "u_short" (in the 
Pfeiffer nomenclature) which has just 3 bytes,
But many parameters are actually of type "old_bool" or "int" having 6 bytes.

> epics> dbpf SCATTERCHAMBER:VACUUM:TP:IntSelLckd 0
> DBF_STRING:         "off"
> 2022/07/12 09:59:28.743 /dev/ttyAMA0 write 20
> 0011006106000000015\r
> 2022/07/12 09:59:29.393959 rs485 SCATTERCHAMBER:VACUUM:TP:IntSelLckd: 
> No reply within 500 ms to "0011006106000000015<0d>"
> 2022/07/12 09:59:29.395 /dev/ttyAMA0 write 16
> 0010006102=?102\r
> 2022/07/12 09:59:30.033945 rs485 SCATTERCHAMBER:VACUUM:TP:IntSelLckd: 
> No reply within 500 ms to "0010006102=?102<0d>"


> Got 16 bytes:
> 0011006106000000
> Got 16 bytes:
> 15\r0010006102=?1

I also tried to use a simple python script to send messages with the 
same result. It seems the Kernel accepts more then 16 bytes but only the 
first 16 get send.
The 17th byte seems to get lost while the remaining bytes are send with 
the next write-call.

Also for those parameters having 6 byte payload data, my workaround I 
mentioned in the previous mail won't work...or I have to calculate the 
checksum outside of streamdevice.

Guess I will check if this issue is known and if there is some 
workaround independent of EPICS.

Cheers,
Florian

On 7/12/22 09:56, Zimoch Dirk (PSI) wrote:
> Hi Florian,
>
> If the output buffer is not able to accept all bytes at once, then the write() function should have returned 16 instead
> of 17 and drvAsynSerialPort writeIt() would try again with the remaining bytes, waiting for writeTimeout and failing
> with asynTimeout if it cannot send.
>
> But if the kernel driver implements write() in a way that discards bytes, then all hope is lost.
>
> Try with some other program, for example from the command line:
> echo "this is a string longer than 16 chars" > /dev/ttyAMA0
>
> Dirk
>
> On Tue, 2022-07-12 at 09:44 +0200, Florian Feldbauer via Tech-talk wrote:
> > Hey all,
> >
> > we have a small vacuum chamber to test our sensor modules after production.
> > The 2 Pumps and 2 valves are controlled via a Raspberry Pi CM4 running
> > linux-aarch64.
> >
> > The Turbo pump and digital gauge are from Pfeiffer Vacuum and controlled
> > via RS485.
> >
> > I noticed some strange issue. I was able to read all parameters from
> > those devices, but changing them fails.
> >
> > To further investigate I attached a 2nd PC to the RS485 bus just
> > listening and found the following:
> >
> > > epics> asynSetTraceIOMask rs485, 0, ASYN_TRACEIO_ESCAPE
> > > epics> asynSetTraceMask rs485, 0, ASYN_TRACEIO_DRIVER
> > > epics> dbpf SCATTERCHAMBER:VACUUM:TP:CtrlViaInt RS485
> > > DBF_STRING:         "RS485"
> > > 2022/07/12 09:25:52.084 /dev/ttyAMA0 write 17
> > > 0011006003002125\r
> > > 2022/07/12 09:25:52.721930 rs485 SCATTERCHAMBER:VACUUM:TP:CtrlViaInt:
> > > No reply within 500 ms to "0011006003002125<0d>"
> > > 2022/07/12 09:25:52.722 /dev/ttyAMA0 write 16
> > > 0010006002=?101\r
> > > 2022/07/12 09:25:52.762 /dev/ttyAMA0 read 16
> > > 0011006003002125
> > > 2022/07/12 09:25:52.766 /dev/ttyAMA0 read 1
> > > \r
> > >
> > > epics>
> > The 2nd PC received this:
> >
> > > Got 16 bytes:
> > > 0011006003002125      [30 30 31 31 30 30 36 30 30 33 30 30 32 31 32 35]
> > > Got 16 bytes:
> > > 0010006002=?101\r     [30 30 31 30 30 30 36 30 30 32 3d 3f 31 30 31 0d]
> > > Got 17 bytes:
> > > 0011006003002125\r    [30 30 31 31 30 30 36 30 30 33 30 30 32 31 32 35 0d]
> > As you can see, on the first message which was send from the IOC, the CR
> > is missing.
> >
> > I guess the issue is, that asyn tries to write all 17 bytes at once, but
> > the tx buffer on the Raspberry Pi can only hold 16 byte, so the CR gets
> > lost.
> >
> > Is there a way to limit the number of bytes written in a single write
> > call with asyn?
> >
> > Luckily in this case the length of the telegrams is well known and I
> > could simply use 2 OUT statements in my protocol file to work around
> > this problem, but what if I want to control a device where the length
> > depends on the payload data?
> >
> > Cheers,
> > Florian
--
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Dr. Florian Feldbauer
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