TTL devices can handle CMOS input. HD6309 -> 74LS612
CMOS devices may or may not handle TTL input. 74LS612 -> peripherals

62256 SRAM, 28C256 EEPROM, 16550 UART all handle TTL input.
82C42 lists it's outputs as TTL compatible, but says nothing for inputs.
Must ensure all peripherals are rated for TTL input.