Yokogawa 414 Batch Controller - Manual (Page 28)

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4.1.2 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 convert 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
Host
Computer
400 Series
Instrument
400 Series
Instrument
Load
120 ohms
Load
120 ohms
Twisted Pair