Interface ICs

CH340 is 5-6× cheaper and adequate for consumer and hobbyist products. CP2102 offers native CDC-ACM drivers on Windows 10/11 and macOS (no install needed), ±0.5% baud accuracy versus ±2%, ±15 kV ESD tolerance versus ±4 kV, and on-chip EEPROM for custom VID/PID. For professional, industrial or field-deployed equipment, CP2102 is worth the premium. For cost-sensitive consumer products, CH340C (with integrated oscillator) is the right choice.

No. ULN2003 is seven unidirectional open-collector channels — it sinks current to ground but cannot reverse current direction. It works perfectly with unipolar 5-wire steppers like the ubiquitous 28BYJ-48. For bipolar 4-wire steppers, use an H-bridge driver like A4988, DRV8825 or L293D.

No crystal — the oscillator is integrated. But yes, it needs four 100 nF ceramic capacitors for the internal charge pump that generates the ±5.5 V RS-232 rails from a single 3.3 V or 5 V supply. Use X7R ceramics in 0402 or 0603 package; do not use electrolytic capacitors, which have too much ESR for the 300 kHz charge pump.

The RS-232 standard targets 15 m at 20 kbps with 2500 pF load capacitance. At 115,200 bps you are limited to about 3-5 m with standard cable. For longer runs, convert to RS-422 (differential, 100 m at 10 Mbps) or RS-485 (differential, multi-drop, 1200 m at 100 kbps) instead.