I'm designing a 4-bit TTL computer and I want to use the CY62256N for system RAM.

As you see, the computer is a 4-bit computer which means the main data bus is only 4 bits wide. The CY62256N is an 8 bit x 32k SRAM, 8 of the data pins are used for both input and output.

The problem is that the RAM data line is going to be directly connected to the main data bus, so there are 4 I/O pins of the CY62256N that are not going to be connected.

Since those are inputs I thought about tying them to ground, but those are output too, so I'm afraid that if I connect them to ground they can damage the RAM by overcurrent on the output. The PCB of the computer is going to be pretty small and has barely any space left, so I don't think that I can add some resistors.

So should I tie those 4 I/O pins to ground or just leave them floating?


1 Answer 1


You shouldn't leave them floating, nor should you tie them hard to ground or Vcc (because they can be outputs). Connect each line through a resistor to ground (or Vcc). I would use a 10k, but it isn't critical.

  • 1
    \$\begingroup\$ If space is critical, consider a multi-resistor "array" package. Can find these in very tiny surface-mount size. \$\endgroup\$
    – rdtsc
    Aug 3, 2022 at 14:03
  • \$\begingroup\$ @rdtsc I like the suggestion, I'll use that for the board, thanks \$\endgroup\$ Aug 3, 2022 at 14:17
  • 1
    \$\begingroup\$ Why not use those 4 data lines to double the size of your RAM? \$\endgroup\$
    – D Duck
    Aug 3, 2022 at 21:56
  • \$\begingroup\$ Because their CPU only uses 4 bits. Can't use all 8 bits of 8 bit ram in a 4 bit computer. \$\endgroup\$
    – UStralian
    Aug 4, 2022 at 6:01
  • \$\begingroup\$ @DDuck If I split the I/O pin of the Ram to 2x4-bit, I need an external circuit to prevent the other 4 bits be written while not in used. Since size of the board is critical, I can't add it to the board. \$\endgroup\$ Aug 5, 2022 at 2:54

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