Experimental Physics and
Industrial Control System

Florian Feldbauer via Tech-talk <tech-talk at aps.anl.gov> · Tue, 12 Jul 2022 09:44:01 +0200

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
Fax:    (+49)234 / 32-14170
https://paluma.ruhr-uni-bochum.de

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System