Experimental Physics and
Industrial Control System

"Zimoch Dirk \(PSI\) via Tech-talk" <tech-talk at aps.anl.gov> · Tue, 12 Jul 2022 09:24:33 +0000

You could write an asynInterpose driver that chops the output into 16 bytes chunks. (And maybe delays in between if
necessary.)

On Tue, 2022-07-12 at 07:56 +0000, Zimoch Dirk (PSI) via Tech-talk 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
> > 
> > 

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System