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+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.