2020-04-05 Best-guess reverse engineering of BEEBOP board Semiconductors IC1 74LS245 Buffer senses STEbus TRFERR* and SYSRST*, drives ADRSTB, DATSTB, DATACK* IC2 74LS245 Buffer drives STEbus A7-0 IC3 74LS245 Buffer drives STEbus D7-0 IC4 74LS245 Buffer drives STEbus CM2-0, A19-16 IC5 74LS374 Latch AD7-0 to hold A7-0 for IC3 IC6 74LS374 Latch wired as a one-byte memory IC7 74LS07 Buffer, open-collector, drives STEbus TRFERR* and SYSRST* IC8 74LS245 8-bit buffer drives STEbus A15-11 IC9 74LS75 4-bit complementary latch IC10 74LS04 Buffer, inverting IC11 6522 Port A drives BA15-8, port B drives AD7-0 IC12 PAL16R8 PAL, STEbus arbiter IC13 74S04 Part of 16 MHz oscillator circuit IC14 74LS74 Dual D-type latch IC15 74LS74 Dual D-type latch IC16 74LS02 Quad NOR gate IC17 74LS00 Quad NAND gate IC18 74LS138 1-of-8 decoder IC19 74LS02 Quad NOR gate IC20 6522 Drives STEbus control signals (Port B drives BCM2-0 and BA19-16) IC21 74LS367 Control-signal buffer IC22 74LS245 Bidirectional data buffer IC23 74LS Address buffer TR1 NPN Drives /IRQ of BBC 1 MHz bus XL1 16 MHz Quartz crystal Resistors R6,7,8 Pull-up bus-arbiter PAL inputs from LK5 R9,10 Part of 16 MHz oscillator circuit R11 Pull-up /IRQ of 6522 chips R1 Pull-up SYSRST* driver R12 Pull-up TRFERR* driver R13 Pull-up PA7 of IC20 R14 Limits base current to TR1 Capacitors C1,2 22u Decouplers, tantalum C3 Part of 16 MHz oscillator circuit C4-8 100 Decouplers, ceramic Link functions: LK1 Routes BUSRQ0* and BUSAK1* LK2 Routes BUSRQ1* and BUSAK1* LK3 Enables 16 MHz to STEbus LK4 Routes ATNRQ3-0* drive/sense signals LK5A Select Permanent Master LK5B Select Release-On-Request LK5C Select RL1 LK6 Enable SYSRST if fitted PAL pins 1 CLK Clock 2 RA1 Request? 1 3 RA0 Request? 0 4 /BR Bus Request 5 Not shown in schematic 6 /LOCK From IC20 port A bit 7 7 RL1 8 ROR Release-On-Request 9 PM Permanent Master 10 GND 11 /OE Output Enable 12 /R0 Request 0 13 Not shown in schematic 14 Not shown in schematic 15 Not shown in schematic 16 /BG0 Bus Grant 0 17 /BG Bus Grant to IC20 port A bit 5 18 /BG1 Bus Grant 1 19 /R1 Request 1 20 VCC