04. RS-232 Serial Communication

What you'll learn

• 1Explain RS-232 physical traits (1:1 point-to-point, TX↔RX cross + common GND, matched baud)
• 2Know COPEN(SER)'s first argument means serial, vs TCP/IP's ETH
• 3Encode/decode {}·, packets and compare with non-format (ASCII decimal)
• 4Run pyserial echo over a com0com virtual COM pair (COM5↔COM6)

Introduction

Unlike Ethernet where many nodes share a wire, only two devices connect directly with one cable. The command flow and packet format are identical to lesson 3's TCP/IP — only COPEN's first argument changes ETH→SER.

Key concepts

1) 1:1 point-to-point + TX↔RX cross

   Robot controller        Host (PC/PLC)
     TX  ─────────────────►  RX
     RX  ◄─────────────────  TX     ← cross
     GND ─────────────────  GND     ← common ground

2) TCP/IP vs RS-232

Key: the commands and packet format are the same; only the physical layer and COPEN's first argument differ. Non-format mode drops the start/end·separators and reads each byte as ASCII CODE (decimal): "AB"→[65,66].

Core example

Implement the RS-232 packet encode/decode in pure Python and round-trip-verify it (no port needed).

def encode_packet(values):
    return "{" + ",".join(str(v) for v in values) + "}"   # [123,456] -> "{123,456}"
def decode_packet(packet):
    return [int(t) for t in packet.strip().strip("{}").split(",")]
encode_nonformat = lambda text: [ord(ch) for ch in text]    # "AB" -> [65, 66]

# After making a com0com virtual COM pair (COM5↔COM6) (optional)
python ../../_shared/serial_echo.py  --port COM6 --baud 9600   # Terminal A echo
python ../../_shared/serial_client.py --port COM5 --baud 9600 --value 123  # Terminal B
# Both --baud must match. One at 19200 → corrupt or b'' reply

Common mistakes

Q. The reply comes back as b'' (empty) / characters corrupt.

A. The echo server isn't up, or baud mismatch. Start the echo server first and set both --baud the same (9600), keeping the default 8N1. RS-232 has no auto-negotiation.

Q. SerialException: could not open port 'COM6'.

A. The com0com pair isn't made, a port typo, or another program holds the port. Check COM5/COM6 in Device Manager and close other serial programs.

Q. Nothing shows on the HRSS comm screen.

A. "Show message" is unchecked in the RS-232 settings (off by default). Check it before testing.

Summary

• RS-232 is 1:1 point-to-point; wiring key is TX↔RX cross + common GND, and matched baud on both sides
• The command flow (COPEN→CWRITE→CREAD) and packet format ({ }·,·non-format) are identical to TCP/IP
• Only the physical layer and COPEN's first argument (ETH→SER) change
• com0com reproduces real serial transfer on a PC

Exercises

1. Round-trip-verify [123,456] and [90000,-45100,30]
2. Change one side's baud to 19200 to reproduce corruption, and explain why
3. Convert "AB" to [65,66] and back with non-format