Dell UPS 3750R - Manual (Page 36)

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Additional UPS Features
Table 8. RS-232 Communication Port Pin Assignment
Pin Signal Name Function Direction from the UPS
1 Unused —
2 Tx Transmit to external device Out
3 Rx Receive from external device In
4 Unused —
5 GND Signal common (tied to chassis) —
6 Unused —
7 Unused —
8 Unused —
9 Unused —
*Unused pins must be left free on all models.