Westermo MD-44 - Manual (Page 6)

12 6155-2002
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RS-422/485 was designed for multidrop applications. When a system is installed it should
form a bus structure (see diagrams). Star shaped networks should never be created,
there are other Westermo products designed to work in star net applications. To get a
correct installation according to the RS-422/485 specification it´s very important that the
line is terminated at the correct points. The recommendation is to terminate the recei-
ver on the master unit and the final bus slave unit. See diagrams for details of how this is
done with RS-485 (2-wire) and RS-422 (4-wire).
The line transmitter used in the MD-44 is activated by data received on the RS-232
interface, unlike conventional converters that rely on a control signal (e.g. RTS).
If any problems do occur on set up of the MD-44, the LED’s will be helpful.
• PWR: The unit has power.
• RD: Data received on the RS-422/485 interface.
• DCD: Follows RD in two wire operation. Always active for four wire.
• CTS: Follows RTS.
• RTS: Status of RTS from the RS-232 interface.
• TD: Data received on RS-232 interface.
=Termination Max 32 connections
Max 1 200 metres
Max 0.3 metre
=Termination Max 32 connections
A
B
A B A B A B
A’
B’
A
B
B’ A’ A’ B B A B’ A’ B A A B’
N.B. R+/R–, T+/T– definitations are not standard, it can help to shift A and B
if the unit does not work.