Hints – Westermo MD-42 User Manual
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6152-2002
Hints
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 correctly install, an RS-422/485 network should be terminated at the correct points.
The recommendation is to terminate the receiver 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).
On 4-wire systems when the MD-42 is on a slave system, it’s transmitter is linked to
the same bus as all the other slave transmitters. A status signal RTS or DTR is used to
control the MD-42’s transmitter, to ensure only one slave is active on the bus at one
time. The status signal is also used to control direction for RS-485 (2 wire) transmission.
If any problems do occur on set up of the MD-42, the LED’s will be helpful.
• PWR:
The unit has power.
• RD:
Data received on the RS-422/485 interface.
• DCD:
Simulated carrier due to the setting of S2.
• CTS:
Follows RTS.
• RTS:
Indicates that the RS-422/485 transmitter is activated.
• TD:
Data received on the RS-232/V.24 interface.