Jaguar:Behind Jaggy Lines (BJL) Mod

BJL debug bios

Behind Jaggy Lines [BJL] is a method to load a debug BIOS, capable of receiving a single ROM sent over the second controller port via a special cable. It is generally used for homebrew development, but can be used as a method to load homebrew games. Original guide by Bastian Schick, pictures by Matthias Domin.

Required Materials

You need:

• An EPROM type 27c010 or 27c1001 (this is 128K x 8) loaded with the BJL BIOS (DEBJAG.IMG)
• A 32-pin socket
• A SPDT switch (on-on)
• Two 4k7 resistors (1/4W)
• Soldering iron, solder, and wire
• A VGA port (Jaguar port) and parallel port

Console Modification

1. Open your Jaguar console and remove the shield. Locate the BIOS chip near the power socket (M27C1001, labelled U35 on the motherboard).
2. Cut /CE (pin 22) as close as possible to the PCB and bend it upwards.
   
3. Solder a resistor to the lifted pin to pin 32 (Vcc).
   
4. On the 32-pin socket, bend pin 22 upwards.
   
5. Solder the socket onto the BIOS chip, with the exception of pin 22.
   
6. Solder the second resistor to pin 22 of the socket and to pin 32 of the BIOS chip.
   
7. Solder 3 wires to the switch:
   • The middle to the pin 22 hole on the PCB
   • To pin 22 of the BIOS chip
   • To pin 22 of the socket
8. Switch your Jaguar ON to test. Then, turn it off, change the switch position, and turn it ON a second time. In one position nothing should happen, in the other, it should boot normally.
   • If nothing happens in either position, check all steps and check for short circuits.
9. Insert your EPROM into the socket and switch your Jaguar on. It should now show a red text screen with a 68k register-dump and a blue or red border.
10. Mount the switch. Common places are:
    • Near the power plug (requires cutting metal shield):
      
    • Near the joypad port 2:
      

BJL Cable

Create a cable with this wiring. The Jaguar end is the same connector as a VGA cable, and the other end is a parallel cable to connect to a PC or Parallel-to-USB adapter.