I have a power-hungry 74ls digital logic project that I want to connect a USB-connected Arduino Nano to, but I'm not sure how design the interface between them safely.
The project has an external 5V power supply (a USB wall-socket power adapter,) and I'm concerned about what may happen if USB power is supplied to the Nano without the project's power supply turned on. I'm much less concerned about the Nano getting fried (which probably has all kinds of protection circuitry) than I am about my (poorly-designed) project, which is composed of ~50 chips.
The Nano needs to drive a 74ls245 (bus transceiver, including its direction and enable pins), the inputs of a 74ls173 (4-Bit D-Type Register), and an input of a 74ls04 (inverter). The Nano needs to accept input from the bus transceiver and 2 TTL-compatible CMOS EEPROM output pins.
My first thought was to connect the project's ground to the Nano's ground, but leave the Nano's 5V and VIN pins disconnected. However, the Nano will be outputting digital high to the unpowered chips above, and I'm concerned that that may damage the chips, and/or cause those chips to malfunction and damage the 50 other chips they are connected to.
If I connect the project's +5V to the Nano's VIN, I imagine that the Nano would be safe when the project is powered and the Nano's USB not plugged in.
How can I keep my project safe when the Nano's USB is plugged in but the project's power supply is off?
I have everything wired up, but am terrified of powering it up.