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 - Manufacturer ID

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 - Screen Refresh Rate

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.

CPC6128:LK5,LK6,LK8 - Disable 128K RAM Banking

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 signals directly to unbanked RAM). The CPC+ has a similar feature (see R128 below).

DDI-1:LK1 - Autoboot CP/M

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).

DDI-1:LK2,LK3 - Disc Write Precomp

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.

CPC6128:LK7 - Autoboot CP/M

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

CPC Plus:LK105 - Autoboot CP/M

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

CPC Plus:LK106 - Unknown

Purpose unknown...? (related to Upper ROM Bank signals /IORQ, A13, /WAIT, /RESET)

CPC Plus:R128 aka R28 (10kOhm) - 128K RAM Enable

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

CPC Plus:R129 aka R29 (10kOhm) - Disc Vers Enable

Purpose Unknown !!!

• The ASIC (where R129 connects to) should hava 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.
• Possibly, removing R129 might disable that feature (so cartridge bank 3 could be only accessed as ROM bank 83h, but not as ROM bank 7, nor 0.

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

CPC Plus:R130 aka R30 (10kOhm) - 8bit Printer Enable

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.

CPC Plus Cartridges:LK1-LK6 - EPROM Address Lines

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.

GX4000:LKs

• 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 (use same supply for computer and modulator)