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As you can see, each device is connected to 3 wires (Data+, Data- & Ground), and there is a termination resistor at each end of the wiring (or bus).
The ground connection is their so the adapter has a referance point for the voltages in Data+ & Data-, and there is some level of circuit protection.
The termination resistors are their to prodide clear signals and stop reflection. Here is a detailed article on that

Protocol

See:

http://doityourselfchristmas.com/forums/archive/index.php/t-14596.html?s=ae1a73bb141ff476470ef72d8725fe06

http://doityourselfchristmas.com/forums/showthread.php?14596-USB-to-RS-485-used-to-reverse-engineer-something&s=4c54477f671a75a57ae844bbc5424268

Table 1 of the manual you posted lists the connections as
Red: +10V
Black: SD+
Yellow: SD-
Green: GND

Serial port runs at 9600 baud, no parity, 1 stop bit, 7-bit chars only.
DTR must be HIGH. RTS must be LOW to receive and HIGH to transmit!

All packets are formatted as follows:

DLE STX <data> <checksum> DLE ETX

Sometimes NULs are sent before and after the packet.
note: if DLE occurs in packet it is escaped as DLE NUL!!!

<data> := <dest><command><args>

<checksum> := the 7-bit sum of bytes DLE, STX and <data>

<dest> := a single byte representing destination for packet

<command> := a single byte with command for destination device

<args> := optional string of bytes for command data

References

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