9 pin D-SUB male connector to the computer
This scheme is some kind of RS-232 Converter.
Almost all digital devices we use require TTL or CMOS logic levels. That"s why the first step when trying to connect the device to RS-232 port is transformation of RS-232 levels ack into 0 and 5 Volts. This is done by RS-232 Level Converters, like a Max 232.