Electronic – UART pins interfacing with Barcode without handshaking signals

Clock and data are open-drain on the old AT keyboards. Put some pullup resistors (like 4K7 on each) and have another go at it.

Okay, given that you actually had the resistors, how about that 5kSa/s on your scope waveform. I'd expect to see more like 1Gsps on a proper scope.

Those angled lines are just the scope connecting the dots with really horrible horizontal resolution.

The 5v signals between the Max232 and your Arduino are inverted (due to the internal gates of the chip). Is the Arduino's 5v RS232 format set up to read data that way? If not try placing extra inverter gates between the Max232 and the Arduino input and output pins. (But if the setup works for another RS232 device perhaps it is ok, then see next paragraph.)

Be sure that the handshake input of the scanner is being satisfied, (otherwise there will be no output data from it). Alternately set the scanner's handshake parameter to "None". If there is no way to disable the handshake you might wire the remaining Max232 buffer (T1out or T2out) up to the scanner's handshake pin, (possibly pin 7 CTS - but check the scanner manual), then fix the other side high or low to satisfy the scanner. Some RS232 devices can be fooled to always output data by shorting there own handshake pins together - CTS & RTS, (possibly on pins 7 & 8 - but again check the manual). Of course verify that you are matching the scanner's other RS232 parameters such as baud, parity, bits, etc.