Talking to the SDS1204X-E over USB from a Mac

Siglent doesn’t ship a Mac app for the SDS1204X-E and the Windows one isn’t much anyway. The scope has a USB device port that enumerates as USBTMC, so I wanted to see how far libusb alone would get me before committing to a real app.

What I tried

Plugged it in. macOS has no USBTMC driver, which turns out to be the good case: nothing grabs the interface, libusb claims it from user space, no kext. VID 0xF4EC, PID 0xEE38.

First attempt with python-usbtmc died on the first control transfer. pyvisa-py got *IDN? back. Then I wrote my own 80-line USBTMC framing in pyusb because pyvisa-py kept hanging on waveform reads, and that harness became the reference for the C++ transport later.

The command set is the old LeCroy-style short form, not the :TIMebase:SCALe tree. C1:VDIV 1.3V, TDIV 50NS, TRSE EDGE,SR,C1,HT,OFF. CHDR OFF first or every reply comes back with a header and units glued on.

What I measured

• 2.5 ms per short query, median. 70 queries in 0.3 s.
• 7 000 point waveform fetch: 81 ms. That’s 12 fps if you do nothing else.
• Full 7 M point record: 15.4 s. USB on this scope is full speed, 12 Mbit, about 500 KB/s real.
• WFSU SP,5000,NP,1400 gives 1 400 points spread across the whole 7 M record in 0.3 to 0.5 s. Sparsing only works if NP is also set, otherwise it’s ignored and you get the whole thing.
• PNSU? returns the entire panel setup as 88 KB of XML. Takes 12.8 s every time, so it’s a connect-time thing only.
• SCDP screenshot: 768 067 byte BMP in 1.3 s.
• 60 s stress of 20 queries plus 2 waveforms per round: 377 rounds, zero failures.

The volts formula checked out against the scope’s own measurement to the last digit: code × VDIV / 25 − OFST, 25 codes per division. Rising edge landed on sample 3500 of 7000, so the trigger point is dead center.

What broke or surprised me

• Bare SANU? hangs the link. SANU? C1 is fine. It’s on a denylist now.
• The scope rejects USBTMC INITIATE_CLEAR and INITIATE_ABORT, which is what most libraries send when a read times out. That’s why they hang. Recovery is a USB port reset plus draining whatever is still in the pipe, about 1.5 s.
• It pads bulk-in transfers and sometimes ends them early. You read until the header’s byte count is satisfied and ignore the packet boundaries.
• MSIZ silently ignores 14K/140K/1.4M/14M when three or four channels are on. Only the 7K series is valid then.
• If you kill the app mid-transfer, the scope keeps streaming the rest of the record at the next program that opens it. My first C++ reader treated an all-zero header as “more coming” and looped forever. The Python harness never validated tags so it never noticed. Fixed with tag checking, a deadline on every read, and a drain on open.

Next

The app shell works: live 4-channel plot, run/stop/single/force, edge trigger panel, V/div and time/div, screenshot to the Desktop with Ctrl+S, and it follows the scope’s physical knobs within about a second by polling. It looks like a Qt example though. Next is making it look and work like the actual front panel.