Experimental Physics and
Industrial Control System

Florian Feldbauer via Tech-talk <tech-talk at aps.anl.gov> · Tue, 12 Jul 2022 10:12:50 +0200

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

--
Ruhr-Universität Bochum
AG der Experimentalphysik I
Dr. Florian Feldbauer
NB 2/131 / Fach 125
Universitätsstr. 150
D-44801 Bochum

Office: NB 2/134
Phone:  (+49)234 / 32-23563
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Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System