Because with all the bureaucracy at this place ordering anything is two full days of work, and then you still have to wait for the delivery. If i can do it with with readily available components I could easily spend three full days on it and it would still be worth it. Also, lets not forget: This is not to be used for networking, only communicating with a device 1 meter away with nothing causing unusual amounts of interference. It needs not be robust in an industrial sense, only in that it under the right circumstances will relay the data.
As far as i can figure out from the manual, the device does only use +5V and GND for signaling. It can also use rather low baud rates should speed be an issue. So hardware should not be a problem.
Interestingly, I can find nothing that sounds like a reference pin. The pins in the connector are:
1 n.c.
2 GND_B
3 +5V_b
4 RS485-
5 Rs485+
6 24V_IN
7 +5V_B*
8 GND
9 +5V
For the simply connecting to a nearby computer they suggest bridging pins 3-9 and 2-8.
So how do i determine whether to send or receive data?