The DS Linker Writer (also known as the N-Card, ExpressCard, DS Fire Link, etc.) is a Slot-2 GBA cartridge with an USB controller; in combination with a software USB stack running on the DS itself, it allows the console to act as an USB device.

• nrio-usb-disk - USB mass storage driver, supports any DLDi flashcard
• DSMouse - mouse emulator
• DSPad - gamepad emulator
• DSPad Tilt - gamepad emulator with motion controls (DSMotion/MK6/R6 required in Slot-1)
• uDisk - official USB mass storage driver, supports N-Card only

• TinyUSB port - MIT licensed, requires a recent version of BlocksDS, examples provided
• Keil USB stack provided by the N-Card/DS Linker team, requires an ancient version of devkitARM/libnds, examples provided

These cartridges are compatible with the N-Card/DS Linker Writer. Note that the casing color differs between batches.

(Source: ApacheThunder, Disturbo)

(Source: Disturbo)

The DS Linker Writer uses an ISP1581 (or clone, depending on batch) USB controller and a 12 MHz crystal.

TODO: Mark power pin connections.

                                  _____
                                 /     \
                           DGND / 1  64 \ VCC(3.3)
                      VCC(5.0) / 2 _  63 \ TEST
                         AGND / 3 / \  62 \ WAKEUP
                   VCCA(3.3) / 4  \_/   61 \ DGND
               USB D- <> D- / 5          60 \ XTAL1 <- 12 MHz oscillator
              USB D+ <> D+ / 6            59 \ XTAL2 -> 12 MHz oscillator
                      RPU / 7              58 \ VCC(3.3)
                    RREF / 8                57 \ DATA15 <> GBA AD15
                  MODE1 / 9                  56 \ DATA14 <> GBA AD14
                /RESET / 10                   55 \ DATA13 <> GBA AD13
                  EOT / 11                     54 \ DATA12 <> GBA AD12
                DREQ / 12                       53 \ DATA11 <> GBA AD11
               DACK / 13                         52 \ DATA10 <> GBA AD10
              DIOR / 14                           51 \ DATA9 <> GBA AD9
             DIOW / 15                             50 \ DATA8 <> GBA AD8
           INTRQ / 16                               49 \ DATA7 <> GBA AD7
                /                                       \
                \          Philips/NXP ISP1581          /
            /CS1 \ 17                               48 / DATA6 <> GBA AD6
             /CS0 \ 18                             47 / DATA5 <> GBA AD5
               DA0 \ 19                           46 / DATA4 <> GBA AD4
                DA1 \ 20                         45 / DATA3 <> GBA AD3
                 DA2 \ 21                       44 / DATA2 <> GBA AD2
                READY \ 22                     43 / VCC(3.3)
                  DGND \ 23                   42 / DGND
               VCC(3.3) \ 24                 41 / DATA1 <> GBA AD1
           GBA /CS <-/CS \ 25               40 / DATA0 <> GBA AD0
           GBA /RD <- /RD \ 26             39 / AD7 <- GBA A23
            GBA /WR <- /WR \ 27           38 / AD6 <- GBA A22
           GBA /IREQ <- INT \ 28         37 / VCC(3.3)
                          A0 \ 29       36 / DGND
               GBA A16 -> AD0 \ 30     35 / AD5 <- GBA A21
                GBA A17 -> AD1 \ 31   34 / AD4 <- GBA A20
                 GBA A18 -> AD2 \ 32 33 / AD3 <- GBA A19
                                 \     /   
                                  \   /
                                   \ /      
                                    V
                                    
                            GBA cartridge port
                            
                    |________________________________|
                    |:                              :|
                    |::::::::::::::::::::::::::::::::|
                  1 |::::::::::::::::::::::::::::::::| 32 
                     |||||||||||||||||||||||||||||||'- /GND
                     ||||||||||||||||||||||||||||||'- /IREQ           
                     |||||||||||||||||||||||||||||'- /CS2
                     |||||||||||||||||||||'+++++++- A16-23
                     |||||||||||||||||||||
                     |||||'+++++++++++++++- AD0-15
                     ||||'- /CS
                     |||'- /RD
                     ||'- /WR
                     |'- PHI
                     '- VDD                     

A much rarer, alternate design uses a PDIUSBD12 chip instead.

* ISP1581 datasheet (Rev 06)