As shown in the figures "A" and "B" of the illustrated RS-232 Synchronous Serial DB-25 connector, what pin is used as the common signal return path pin during serial communications? See illustration EL-0257.
• Look at the pinout legend beside figure A/B and find which label refers to the common signal return used by all RS‑232 data lines. • Distinguish between chassis ground and signal ground on a DB‑25 connector. • Recall that in RS‑232, the data lines need a reference voltage line so that both devices agree on what is + and –.
• On the right-hand list in figure B, which pin name would naturally serve as the reference for all signal voltages rather than just the equipment frame? • Compare the labels for pin 1 and pin 7: which one sounds like it is tied to the physical metal case, and which one sounds like it is tied to the electronics of the transmit/receive circuits? • If a serial cable had only TX, RX, and one ground wire, which ground label from the list would you expect that single ground conductor to connect to?
• Verify which pin number in the illustration is explicitly labeled Signal Ground (or equivalent wording). • Confirm that Chassis Ground is a different pin number and usually associated with the equipment frame, not the signal return path. • Before choosing, double‑check that your selected pin is the one commonly used as the reference for all RS‑232 signal voltages, not just for shielding.
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