ttl
I need to read 4 devices with one Rx pin on atmega2560. Devices are DS457 Motorola bar code readers. They communicate in inverted TTL at 9600 baud rate. I have already made signal inverter with 2 transistors and resistors.
My question is:
The readers will not work at the same time. Only one at a time.
For the same logic family, 74LS in this case, tie an output directly to an input. It doesn't matter what the logic function is, output ties directly to input. For the 74LS family, one output can drive 20 inputs.
You are thinking of unused inputs, and you're in for trouble. Yes, you tie an input low by connecting it to ground. For a high, though, you must not let it float. The easiest way is to use an unused gate, such as a 74LS00 or 74LS04, whose input was tied low. Another way is to use a pullup resistor. 10K to +5 will work just fine. If you let the input float, a 74LS gate will usually work, but every once in a while it will act like an antenna, and change state at apparently random intervals.
While 74LS will usually be OK when floating, the same is not true of CMOS logic. ALL unused CMOS inputs MUST be tied either to ground or power. And I'm not speaking hypothetically here - trust me.
For the 12V->5V direction a simple 2-resistor voltage divider (e.g. 7kOhm + 5kOhm) will probably be perfectly fine.
Usually you can't just assume that the +5V source can actually sink any amount of current.
Plus, normal diodes are relatively low bandwidth so that your signals may become more or less distorted even for only a few kBit/s. Schottky diodes mitigate this issue.
Another simple solution could be like this (use Schottky if possible):
simulate this circuit – Schematic created using CircuitLab
Best Answer
If you are set on using that inverter circuit you could simply repeat the left hand transistor for each input (B C in parallel and emitters to each input).
Or just buy a 4-input NAND gate instead of trying to make one.