Yokogawa 415 Steam and Gas - Manual (Page 46)

4.2.2 Multipoint Communication
Multipoint Communication is a system whereby a number of instruments can be
addressed over a dual twisted pair interface. Up to 32 instruments can be
connected to a common bus using the RS422 and RS485 interfaces as shown
below.
To covert the RS422 interface to an RS485 interface, the RS422 (-) Data In
Terminal must be connected to the RS422 (-) Data Out Terminal and the RS422
(+) Data In Terminal must be connected to the RS422 (+) Data Out Terminal.
These connections will convert the RS422 4 wire interface to the RS485 2 wire
interface, as shown in figure 2.
Each instrument can be programmed with a unique address which is used by the
Master Controller (ie IBM/PC) to identify each instrument. The Controller will
send the address down the line and will alert the relevant instrument.
Subsequent software protocol will control the flow of data between the Controller
and the Instrument.
Figure 1 RS422 Interface
44 Options
Host
Computer
Load
120 ohms
Twisted Pair
Load
120 ohms
400 Series
Instrument 400 Series
Instrument