In this case, the device is being powered from **-5V** (well, it's clamped at -5.1) with VDD being ground and VSS at V- (provided by the RTS pin),. DTR is providing positive power for the pullup resistor (the
1.5k). If RB7 is high, then the transistor is off, and CTS is high (inactive). When RB7 goes low, it turns on the transistor, pulling CTS low, so yes, you need the transistor.
It's just a standard old NPN ( you could probably use any one of a laundry list of such devices - See this page for BC547 details:
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As to the VIL1 spec, use the hardware data sheet which shows VIL for all except OSC1 at 0.2VDD (MAX). OCS1 is 0.3VDD (MAX) for certain operational modes.
Datasheet available at
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Cheers
PeteS