Can't connect to the emulator

[diggit]

OS: Archlinux
J-Link SW package: V9.14a, same happens with V9.16
pylink: v2.0.0 via PKGBUILD
emulator: J-Link compact, 12.1, SN(redacted): 8xxxxxx84

I believe some recent change in the J-Link software package broke pylink compatibility.

Usage snippet:

try:
	# Connect to the target device
	logger.info("opening J-Link connection")
	jlink = pylink.JLink()
	jlink.disable_dialog_boxes()
	jlink.open()
	jlink.set_tif(pylink.enums.JLinkInterfaces.SWD)
	
	logger.info("Connecting to target") # this is never reached
	jlink.connect(args.target)
	# more code...
except pylink.JLinkException as e:
	logger.error(f"An error occurred: {str(e)}")

Raises error:

Traceback (most recent call last):
  File "eol.py", line 983, in <module>
    jlink.open()
    ~~~~~~~~~~^^
  File "/usr/lib/python3.14/site-packages/pylink/jlink.py", line 734, in open
    raise errors.JlinkException('Could not connect to default emulator.')
          ^^^^^^^^^^^^^^^^^^^^^
AttributeError: module 'pylink.errors' has no attribute 'JlinkException'. Did you mean: 'JLinkException'?

because there is a typo, a lower case l in

pylink/pylink/jlink.py

Line 734 in a6f0868

raise errors.JlinkException('Could not connect to default emulator.')

After fixing that, the log looks like this:

Log, click to expand

[    INFO] EOLTST : opening J-Link connection
[   DEBUG] pylink.jlink : T34C9DBC0 000:013.613
[   DEBUG] pylink.jlink : Firmware: J-Link V12 compiled Dec 18 2025 15:13:50
[   DEBUG] pylink.jlink : 
[   DEBUG] pylink.jlink : T34C9DBC0 000:014.079
[   DEBUG] pylink.jlink : Firmware: J-Link V12 compiled Dec 18 2025 15:13:50
[   DEBUG] pylink.jlink : 
[   DEBUG] pylink.jlink : T34C9DBC0 000:030.021
[   DEBUG] pylink.jlink : Hardware: V12.00
[   DEBUG] pylink.jlink : 
[   DEBUG] pylink.jlink : T34C9DBC0 000:030.216
[   DEBUG] pylink.jlink : S/N: 8xxxxxx84
[   DEBUG] pylink.jlink : 
[   DEBUG] pylink.jlink : T34C9DBC0 000:030.309
[   DEBUG] pylink.jlink : OEM: SEGGER
[   DEBUG] pylink.jlink : 
[   DEBUG] pylink.jlink : T34C9DBC0 000:030.396
[   DEBUG] pylink.jlink : Feature(s): GDB
[   DEBUG] pylink.jlink : 
[   DEBUG] pylink.jlink : T34C9DBC0 000:031.214
[   DEBUG] pylink.jlink : Bootloader: 2022 Oct 19
[   DEBUG] pylink.jlink : 
[   DEBUG] pylink.jlink : T34C9DBC0 000:032.382
[   DEBUG] pylink.jlink : EMU: Cleaned up zombie connection 0. PID: 0x5615 (22037)
[   DEBUG] pylink.jlink : 
[   DEBUG] pylink.jlink : T34C9DBC0 000:032.748
[   DEBUG] pylink.jlink : USB speed mode: High speed (480 MBit/s)
[   DEBUG] pylink.jlink : 
[   DEBUG] pylink.jlink : T34C9DBC0 000:033.080
[   DEBUG] pylink.jlink : TELNET listener socket opened on port 19021
[   DEBUG] pylink.jlink : 
[   DEBUG] pylink.jlink : T34C9DBC0 000:033.342
[   DEBUG] pylink.jlink : WEBSRV WEBSRV_Init(): Starting webserver thread(s)
[   DEBUG] pylink.jlink : 
[   DEBUG] pylink.jlink : T34C9DBC0 000:033.486
[   DEBUG] pylink.jlink : WEBSRV Webserver running on local port 19080
[   DEBUG] pylink.jlink : 
[   DEBUG] pylink.jlink : T34C9DBC0 000:033.578
[   DEBUG] pylink.jlink : Looking for J-Link GUI Server exe at: /tmp/JLinkGUIServerExe
[   DEBUG] pylink.jlink : T34C9DBC0 000:033.630
[   DEBUG] pylink.jlink : Looking for J-Link GUI Server exe at: /opt/SEGGER/JLink/JLinkGUIServerExe
[   DEBUG] pylink.jlink : T34C9DBC0 000:033.678
[   DEBUG] pylink.jlink : Forking J-Link GUI Server: /opt/SEGGER/JLink/JLinkGUIServerExe
[   DEBUG] pylink.jlink : T34C9DBC0 000:051.626
[   DEBUG] pylink.jlink : J-Link GUI Server info: "J-Link GUI server V9.14a "
[   DEBUG] pylink.jlink : T34C9DBC0 000:052.381
[   DEBUG] pylink.jlink : - 50.875ms returns 0x00
[   DEBUG] pylink.jlink : T34C9DBC0 000:052.523
[   DEBUG] pylink.jlink : JLINK_ExecCommand("SuppressControlPanel", ...).
[   DEBUG] pylink.jlink : T34C9DBC0 000:052.567
[   DEBUG] pylink.jlink : - 0.044ms returns 0x00
[   DEBUG] pylink.jlink : T34C9DBC0 000:052.603
[   DEBUG] pylink.jlink : JLINK_ExecCommand("DisableInfoWinFlashDL", ...).
[   DEBUG] pylink.jlink : T34C9DBC0 000:052.651
[   DEBUG] pylink.jlink : - 0.049ms returns 0x00
[   DEBUG] pylink.jlink : T34C9DBC0 000:052.693
[   DEBUG] pylink.jlink : JLINK_ExecCommand("DisableInfoWinFlashBPs", ...).
[   DEBUG] pylink.jlink : T34C9DBC0 000:052.723
[   DEBUG] pylink.jlink : - 0.030ms returns 0x00
[   DEBUG] pylink.jlink : T34C9DBC0 000:052.782
[   DEBUG] pylink.jlink : JLINK_SelectUSB(Port = 0)
[    INFO] pylink.jlink : JLINK_SelectUSB() may not be called after JLINK_Open(). Call ignored.
[   DEBUG] pylink.jlink : T34C9DBC0 000:052.843
[   DEBUG] pylink.jlink : 
  ***** Warning:
[   DEBUG] pylink.jlink : 
[   DEBUG] pylink.jlink : T34C9DBC0 000:052.888
[   DEBUG] pylink.jlink : JLINK_SelectUSB() may not be called after JLINK_Open(). Call ignored.
[   DEBUG] pylink.jlink : 
[   DEBUG] pylink.jlink : T34C9DBC0 000:052.940
[   DEBUG] pylink.jlink : - 0.159ms returns 1
[   ERROR] EOLTST : An error occurred: Could not connect to default emulator.
[   DEBUG] pylink.jlink : T34C9DBC0 000:061.752
[   DEBUG] pylink.jlink : JLINK_IsOpen()
[   DEBUG] pylink.jlink : T34C9DBC0 000:061.926
[   DEBUG] pylink.jlink : - 0.173ms returns 0x01
[   DEBUG] pylink.jlink : T34C9DBC0 000:061.974
[   DEBUG] pylink.jlink : JLINK_EMU_IsConnected()
[   DEBUG] pylink.jlink : T34C9DBC0 000:062.010
[   DEBUG] pylink.jlink : - 0.036ms returns TRUE
[   DEBUG] pylink.jlink : T34C9DBC0 000:062.043
[   DEBUG] pylink.jlink : JLINK_IsOpen()
[   DEBUG] pylink.jlink : T34C9DBC0 000:062.073
[   DEBUG] pylink.jlink : - 0.030ms returns 0x01

I also tried explicitly selecting my probe by serial: jlink.open(serial_no=8xxxxxx84) and jlink.open(serial_no="8xxxxxx84")
In both cases connecting to the emulator fails:

Log, click to expand

[    INFO] EOLTST : opening J-Link connection
[   DEBUG] pylink.jlink : TA8C16BC0 000:013.608
[   DEBUG] pylink.jlink : Firmware: J-Link V12 compiled Dec 18 2025 15:13:50
[   DEBUG] pylink.jlink : 
[   DEBUG] pylink.jlink : TA8C16BC0 000:014.086
[   DEBUG] pylink.jlink : Firmware: J-Link V12 compiled Dec 18 2025 15:13:50
[   DEBUG] pylink.jlink : 
[   DEBUG] pylink.jlink : TA8C16BC0 000:030.010
[   DEBUG] pylink.jlink : Hardware: V12.00
[   DEBUG] pylink.jlink : 
[   DEBUG] pylink.jlink : TA8C16BC0 000:030.244
[   DEBUG] pylink.jlink : S/N: 8xxxxxx84
[   DEBUG] pylink.jlink : 
[   DEBUG] pylink.jlink : TA8C16BC0 000:030.341
[   DEBUG] pylink.jlink : OEM: SEGGER
[   DEBUG] pylink.jlink : 
[   DEBUG] pylink.jlink : TA8C16BC0 000:030.429
[   DEBUG] pylink.jlink : Feature(s): GDB
[   DEBUG] pylink.jlink : 
[   DEBUG] pylink.jlink : TA8C16BC0 000:031.423
[   DEBUG] pylink.jlink : Bootloader: 2022 Oct 19
[   DEBUG] pylink.jlink : 
[   DEBUG] pylink.jlink : TA8C16BC0 000:034.268
[   DEBUG] pylink.jlink : EMU: Cleaned up zombie connection 1. PID: 0x580C (22540)
[   DEBUG] pylink.jlink : 
[   DEBUG] pylink.jlink : TA8C16BC0 000:034.711
[   DEBUG] pylink.jlink : USB speed mode: High speed (480 MBit/s)
[   DEBUG] pylink.jlink : 
[   DEBUG] pylink.jlink : TA8C16BC0 000:035.094
[   DEBUG] pylink.jlink : TELNET listener socket opened on port 19021
[   DEBUG] pylink.jlink : 
[   DEBUG] pylink.jlink : TA8C16BC0 000:035.323
[   DEBUG] pylink.jlink : WEBSRV WEBSRV_Init(): Starting webserver thread(s)
[   DEBUG] pylink.jlink : 
[   DEBUG] pylink.jlink : TA8C16BC0 000:036.548
[   DEBUG] pylink.jlink : WEBSRV Webserver running on local port 19080
[   DEBUG] pylink.jlink : 
[   DEBUG] pylink.jlink : TA8C16BC0 000:036.680
[   DEBUG] pylink.jlink : Looking for J-Link GUI Server exe at: /tmp/JLinkGUIServerExe
[   DEBUG] pylink.jlink : TA8C16BC0 000:036.759
[   DEBUG] pylink.jlink : Looking for J-Link GUI Server exe at: /opt/SEGGER/JLink/JLinkGUIServerExe
[   DEBUG] pylink.jlink : TA8C16BC0 000:036.826
[   DEBUG] pylink.jlink : Forking J-Link GUI Server: /opt/SEGGER/JLink/JLinkGUIServerExe
[   DEBUG] pylink.jlink : TA8C16BC0 000:055.071
[   DEBUG] pylink.jlink : J-Link GUI Server info: "J-Link GUI server V9.14a "
[   DEBUG] pylink.jlink : TA8C16BC0 000:055.811
[   DEBUG] pylink.jlink : - 54.324ms returns 0x00
[   DEBUG] pylink.jlink : TA8C16BC0 000:056.007
[   DEBUG] pylink.jlink : JLINK_ExecCommand("SuppressControlPanel", ...).
[   DEBUG] pylink.jlink : TA8C16BC0 000:056.095
[   DEBUG] pylink.jlink : - 0.087ms returns 0x00
[   DEBUG] pylink.jlink : TA8C16BC0 000:056.169
[   DEBUG] pylink.jlink : JLINK_ExecCommand("DisableInfoWinFlashDL", ...).
[   DEBUG] pylink.jlink : TA8C16BC0 000:056.236
[   DEBUG] pylink.jlink : - 0.068ms returns 0x00
[   DEBUG] pylink.jlink : TA8C16BC0 000:056.276
[   DEBUG] pylink.jlink : JLINK_ExecCommand("DisableInfoWinFlashBPs", ...).
[   DEBUG] pylink.jlink : TA8C16BC0 000:056.307
[   DEBUG] pylink.jlink : - 0.032ms returns 0x00
[   DEBUG] pylink.jlink : TA8C16BC0 000:056.372
[   DEBUG] pylink.jlink : JLINK_EMU_SelectByUSBSN(SerialNo = 8xxxxxx84)
[   DEBUG] pylink.jlink : TA8C16BC0 000:056.407
[   DEBUG] pylink.jlink : - 0.035ms returns 0xFFFFFFFF
[   ERROR] EOLTST : An error occurred: No emulator with serial number 8xxxxxx84 found.
[   DEBUG] pylink.jlink : TA8C16BC0 000:065.957
[   DEBUG] pylink.jlink : JLINK_IsOpen()
[   DEBUG] pylink.jlink : TA8C16BC0 000:066.190
[   DEBUG] pylink.jlink : - 0.234ms returns 0x01
[   DEBUG] pylink.jlink : TA8C16BC0 000:066.246
[   DEBUG] pylink.jlink : JLINK_EMU_IsConnected()
[   DEBUG] pylink.jlink : TA8C16BC0 000:066.283
[   DEBUG] pylink.jlink : - 0.037ms returns TRUE
[   DEBUG] pylink.jlink : TA8C16BC0 000:066.325
[   DEBUG] pylink.jlink : JLINK_IsOpen()
[   DEBUG] pylink.jlink : TA8C16BC0 000:066.364
[   DEBUG] pylink.jlink : - 0.039ms returns 0x01

[hkpeprah]

Is it possible you're on a much older version? Line 734 in your trace doesn't match what is currently in the repo: https://github.com/square/pylink/blob/master/pylink/jlink.py#L734

Whoops, I misread. Github is acting up for me. You're absolutely right, we have a typo!

[diggit]

If I comment out this block:

pylink/pylink/jlink.py

Lines 728 to 734 in a6f0868

	else:
	# The original method of connecting to USB0-3 via
	# JLINKARM_SelectUSB has been obsolesced, however its use is
	# preserved here to simplify workflows using one emulator:
	result = self._dll.JLINKARM_SelectUSB(0)
	if result != 0:
	raise errors.JlinkException('Could not connect to default emulator.')

connection succeeds.

It looks like emulator connection is opened by the DLL immediately. Could be a bug on Segger side. Though I don't have their SDK available, so I can't test and report that.

[HuanQian571]

I also met this issue when use Jlink V9.16a, it works well on previous Jlink V886.

`Traceback (most recent call last):
File "C:\Python313\Lib\site-packages\pyocd\probe\jlink_probe.py", line 192, in open
self._link.open(self._serial_number_int)

File "C:\Python313\Lib\site-packages\pylink\jlink.py", line 725, in open
raise errors.JLinkException('No emulator with serial number %s found.' % serial_no)
pylink.errors.JLinkException: No emulator with serial number 1064xxxxx2 found.`

[diggit]

I have interesting observation!
This sequence fails

jl = pylink.JLink()
jl.disable_dialog_boxes()
jl.open()

but this sequence passes

jl = pylink.JLink()
jl.open()
jl.disable_dialog_boxes()

Just call open() asap, before any other command, even if the command is not directly related to the probe.

[diggit]

As I don't have access to SDK docs, I try my best with pulling info from LLMs (sorry)...
(Promp was question about JLINKARM_ExecCommand pre-requisities.

Before you can successfully dispatch a command, the DLL must be in a specific state:
JLINKARM_Open() must be called first: The DLL must be initialized...

Please confirm this statement if you have access to SDK docs.

Thus addition of decorator @open_required to exec_command should be enough and errors would result in meaningful error messages.