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J-Link JTAG Isolator

The J-Link JTAG Isolator can be connected between J-Link and any ARM-board that uses the standard 20-pin JTAG-ARM connector to provide electrical isolation. This is essential when the development tools are not connected to the same ground as the application. It is also useful to protect the development tools from electrical spikes that often occur in some applications, such as motor control applications. Another typical field of application is development of products with sensors or other analog circuitry, in which case the target hardware is protected from electrical noise originating from the development PC.
The J-Link JTAG Isolator is compatible with J-Link BASE, J-Link PRO, J-Link ULTRA+, J-Link PLUS and Flasher ARM.

Documentation download 


J-Link JTAG Isolator can be purchased directly from SEGGER. For more information regarding pricing and part number of the J-Link JTAG Isolator, please refer to the price list.

Power supply

Both sides, target and emulator, are totally isolated from each other and separately powered. The target side draws power from pins 1 or 2, the emulator side draws power from pin 19. To avoid current drawn from the target an external power supply can be used to provide power to pins 1 and 2.


  • 1kV DC isolation
  • 3.3V and 5V target operation supported
  • Powered from emulator and target
  • JTAG standard 20-pin connection supporting TRST, TDI, TMS, TCK, RTCK, TDO and RESET signals
  • Power consumption on target side: 35mA typ. / 50mA max.
  • JTAG frequency: Up to 4MHz
  • 3 LEDs to indicate emulator power, target power and target RESET

Connectors and indicators

The JTAG Isolator uses high speed optocouplers that allow a very low propagation time between input and output. It comes with the following connectors and indicators:

  • 20-pin female EMULATOR connector which can be plugged directly into J-Link
  • 20-pin male TARGET connector for connection of the target cable
  • Green LED indicating power on the emulator side
  • Green LED indicating power on the target side
  • Red LED indicating RESET

Block diagram

The following functional block diagram illustrates the functional connections between the emulator and target.

Target connector

The following picture shows the target side pinout of the J-Link JTAG Isolator:

The Emulator side of the Isolator is plugged directly into the Emulator. The Target side is connected to the target via a 20-pin flat cable.


Pin Signal Type Description
1 VCC Output The target side of the isolator draws power over this pin.
2 VCC Output The target side of the isolator draws power over this pin.
3 nTRST Output JTAG Reset. Output from J-Link to the Reset signal on the target JTAG port.
Typically connected to nTRST on the target CPU. This pin is normally pulled HIGH on the target to avoid unintentional resets when there is no connection.
5 TDI Output JTAG data input of target CPU.
It is recommended that this pin is pulled to a defined state on the target board.
Typically connected to TDI on target CPU.
7 TMS Output JTAG mode set input of target CPU.
This pin should be pulled up on the target.
Typically connected to TMS on target CPU.
9 TCK Output JTAG clock signal to target CPU.
It is recommended that this pin is pulled to a defined state on the target board.
Typically connected to TCK on target CPU.
11 RTCK Input Return test clock signal from the target.
Some targets must synchronize the JTAG inputs to internal clocks. To assist in meeting this requirement, you can use a returned and retimed TCK to dynamically control the TCK rate. J-Link supports adaptive clocking, which waits for TCK changes to be echoed correctly before making further changes. Connect to RTCK if available, otherwise to GND.
13 TDO Input JTAG data output from target CPU.
Typically connected to TDO on target CPU.
15 RESET I/O Target CPU reset signal. Typically connected to the RESET pin of the target CPU, which is typically called "nRST", "nRESET" or "RESET".
17 N/C N/C This pin is not connected on the target side of the isolator.
19 N/C N/C This pin is not connected on the target side of the isolator.

Pins 4, 6, 8, 10, 12, 14, 16, 18, 20 are GND pins connected to GND.

Using the Isolator with J-Link

In order to use the Isolator, follow these steps:

  • Plug the Isolator directly into J-Link
  • Power J-Link
  • Make sure the green LED on the emulator side is lit. If it is not, please follow the instructions below
  • Connect the target to the target side of the Isolator
  • If the target is powered, the green LED on the target side should be lit

The red LED on the target side is lit when a Target RESET is active (low).

Preparing J-Link to supply power

J-Link needs to supply 5V power to the emulator side of the adapter on pin 19. In order to do this, you may have to configure J-Link once as follows:

  • Make sure that SEGGER J-Link software is installed on your machine. It can be downloaded from here
  • Start J-Link Commander, which can be found under “Start -> Programs -> SEGGER -> J-Link ARM”
  • Enter the following command: power on perm
  • Plug in the adapter: The LED on the emulator side should now be lit

Using the Isolator with another ARM emulator

The Isolator has been designed for J-Link, but can also be used with other ARM emulators with the same pin-out. In this case, you should make sure that 5V are supplied to pin 19 of the emulator connector and that your emulator is not damaged when applying 5V to this pin. Do this at your own risk!