Substitute SN74LVTH245A with SN74LVTH2245?

[aaronnewcomb]

Hi! Thanks for such a wonderful project!! The SN74LVTH245A (C2652121) for U4 and U9 is currently out of stock. Would it be ok to substiture with SN74LVTH2245 (C2675862)? It looks like the only difference is a built-in 20 ohm resistor on the B ports. Not sure if the additional 20 ohms would be a problem or not. I could potentially drop the resistors that are attached to those lines to a lower value if needed.

Thanks in advance!

[androda]

Unfortunately the LVTH2245 does not look like a solution. The B-side of the chip faces the SCSI bus, and the B-side can only source or sink up to 12mA. SCSI requires a chip that can sink up to 48mA, which is 4x as much.

I don't have a great solution here, because it would be a hardware redesign to use the "2245" instead of standard "245". And also either a software change to invert the direction signal, or an inverter gate onboard to avoid the software change. The redesign would not be "huge" but it's change that would need to be tested.

[aaronnewcomb]

What about the non-"H" version? Texas Instruments SN74LVT245BPWR (C74214)?

[androda]

The "H" version is required, as stated in the schematic. Bus-hold is necessary so we don't have a bunch of pulldown resistors to manage on the unused pins of the LVTH245 chips.

Some early PCB revisions could swap between LVT and LVTH chips, but the space requirements for all the extra resistors was quite a lot. Overall, changing to a different bus transceiver chip will require PCB and software changes.

[aaronnewcomb]

Thanks! I will place an order for the original ICs and wait to order until they come in.