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Smart-Custom Cable for SmartUPSes~

You do not have this cable unless you built it yourself. The Smart-Custom cable is not an APC product.

SMART-CUSTOM CABLE~

Signal Computer (DB9F) UPS (DB9M) Function
RxD 2 2 TxD (Send)
TxD 3 1 RxD (Receive)
GND 5 9 Ground 
graph LR
    subgraph Computer [Computer DB9F]
        PC_RxD[Pin 2: RxD]
        PC_TxD[Pin 3: TxD]
        PC_GND[Pin 5: Ground]
    end

    subgraph UPS [UPS DB9M]
        UPS_TxD[Pin 2: TxD]
        UPS_RxD[Pin 1: RxD]
        UPS_GND[Pin 9: Ground]
    end

    PC_RxD --- UPS_TxD
    PC_TxD --- UPS_RxD
    PC_GND --- UPS_GND

If you have an OS that requires DCD or RTS to be set before you can receive input, you might try building the standard APC Smart 940-0024C cable (see "940-0024C Cable Wiring").