LK Links

Links - Amstrad has used a number of LKs (Links) on CPC mainboards and on peripherals like DDI-1 and CPC+ Cartridges. The LKs are simply a piece of wire that is (or is not) installed between two soldering points.

Resistors - The CPC+ does additionally have some optional 10kOhm resistors with similar functions (they connect to the ASICs MUX pins, which are used as configuration input (affected by the resistor), and as ADC channel select output (not affected by the resistor; but won't work when using wires instead of resistors). Note: In schematics, these resistors are labeled R128,R129,R130 - but, on the mainboard they are labeled R28,R29,R30.

LK1,LK2,LK3 connect to PPI Port B, Bit1-3. The links select the manufacturer name (which is displayed by the BIOS in the power-up message). These LKs exist on all CPC mainboards (in CPC Plus/GX4000 schematics they are named LK101,LK102,LK103).

LK4 connects to PPI Port B, Bit4. The link tells the BIOS whether it should set-up the CRTC for 50Hz or 60Hz refresh rate.

The CPC6128 contains 128K RAM, RAM banking is controlled by a PAL (which assists its Gate Array). However, the 6128 mainboard can be also used for 64K computers - in that case, only 64K would be installed, the PAL would be missing, and LK5,LK6,LK8 would be installed (bypassing the RAM banking logic, and passing /CAS,A14,A15 signals directly to unbanked RAM). The CPC+ has a similar feature (see R128 below).

In the DDI-1, LK1 connects both to the Expansion Port /EXP pin (which goes to PPI Port B, Bit5, and to the AMSDOS ROM bank number selection logic. The latter one allow to map AMSDOS to bank 7 (default), or bank 0. If AMSDOS sees itself mapped to bank 0, then it does automatically load & start the boot sector from drive A (equivalent as when typing |CPM after power-up).

In DDI-1, LK2-LK3 connect to the P1/P0 pins of the FDC9229BT Disc Data Separator (P0=LK3, P1=LK2, and fixed: P2=GND). According to FDC9229BT datasheet: P2-P0 select the amount of precompensation applied to the write data.

Same as DDI-1:LK1 (see there).

Same as DDI-1:LK1 (see there). Bug: There's a bug in the System Cartridge, which always attempts to execute the Burnin' Rubber bootmenu at address 7:C072h, this is done even if bank 0 and 7 are exchanged, causing the CPC to crash when LK105 is changed. Workaround would be to replace opcode LD C,07h (at 05EBh in system cart) by LD C,83h (this fixes the bug), or to replace opcode LD HL,C072h (at 05E8h in system cart) by LD HL,0000h (completely disables the bootmenu, which was a rather idea annoying anyways). The same bug occurs when Connecting a Disc Interface to CPC464+.

Purpose unknown...? Usually LK106 is not installed. (related to Upper ROM Bank signals /IORQ, A13, /WAIT, /RESET)

Installed for 128K DRAM, not installed for 64K DRAM.

Purpose Unknown !!!

• The ASIC (where R129 connects to) should have nothing to do with the FDC. However, it does control the AMSDOS ROM bank selection: translating ROM bank 7 (or 0) to cartridge bank 3. However, bank 7 is used by the Burnin' Rubber boot-menu, so bank 7 must exist regardless of R129.
• The ASIC does output some FDC related signals /MOTOR and /FDC. Maybe removing R129 disables these, though that would be of not too much use (unless one wants to disable an internal FDC, in order to use an external FDC instead of it).

This is not verified, can somebody test what happens when installing/removing R129 ?

When installed (default), enables the 8th printer bit, controlled via CRTC Register 0Ch, Bit 3. When removed, outputs LOW as 8th bit (as on classic CPCs with 7bit printer port). See also: 8bit Printer Ports.

The six cartridge LKs are for A15,A17,A18 (address lines, of course, not the "A1-A18" pins).

 VCC ---LK1--- EPROM.A18 ---LK2--- CA18
 VCC ---LK3--- EPROM.A17 ---LK4--- CA17
 VCC ---LK5--- EPROM.A15 ---LK6--- CA15

Some cartridge PCBs don't have any LKs installed, instead, the etched circuit has hardwired connections between some of them. One could scratch them off, and then use the LK soldering points to reconfigure the board for eproms of other size.

• LK1-LK4 - same as CPC, though not being actually used, due to absent BIOS
• LK5 - none such...?
• LK6 - same as LK106 on CPC+
• LK7 - shortcut +5V with +5VMOD (installed in french version, which has no TV modulator)