General FAQs
- What is the difference between RS-422 and RS-485?
- Can the MA-12 be connected to the MA-21?
- I have a PC with RS-232 and a PLC with RS-485. Between them I have an MA-42 but I can not get it to work.
- To improve an existing RS-422 network suffering occasional interference I have connected an RA-48 in the middle. Now I do not have any connection with the equipment after the RA-48.
- When do I need termination?
What is the difference between RS-422 and RS-485?
RS-422 uses 2 wires for simplex transmission. RS-485 can use 2 wires for half duplex transmission. Full duplex systems are implemented using 4 wires. Both RS422 and RS485 can be used in multidrop configurations however precautions must be taken to prevent more than one transmitter being active on the same bus at once.
Can the MA-12 be connected to the MA-21?
Not on the line side because the MA-12 and MA-21 have different interfaces. The MA-12 use Westermo’s own interface, a 10 mA balanced current loop. While the MA-21 use a 20 mA current loop. The two are not compatible.
I have a PC with RS-232 and a PLC with RS-485. Between them I have an MA-42 but I can not get it to work.
The switch settings could be wrong in the MA-42 or the control signals between PC and converter could be missing. RS-485 which is a 2-wire system, uses the same cable pair for transmitting and receiving data. The converter must have a physical control signal from the RS-232 interface to know whether it should receive or transmit data. Usually the handshake signals RTS or DTR are used to switch transmitting direction. To make this work the cable between the converter and PC must contain connections for the handshake signals and the program in the PC must be able to handle this kind of handshaking. Also the switch settings in the MA-42 must be changed from always active to transmitter activated by RTS or transmitter activated by DTR.
To improve an existing RS-422 network suffering occasional interference I have connected an RA-48 in the middle. Now I do not have any connection with the equipment after the RA-48.
A common problem with some equipment is that the polarity of the line is opposite to Westermo’s interfaces. This depends on how one defines polarity-at rest or when there are some communication on the line. If the line has incorrect polarity one can usually see this on the RA-48 LED’s or the connected units, these light up when they ought to be off and vice versa. If the system has not been changed other than the connection of the RA-48, try changing the polarity off the connections, that is the wire connected on pin 3 changes place with pin 4 and the wire on pin 1 changes place with the wire connected to pin 2 on the screw terminals on both sides of the RA-48.
When do I need termination?
When using RS-485/422 it is recommended to terminate the line with a circuit of equal impedance to the characteristic impedance of the line. A 120 ohm resistance is recommended. The termination should be placed in the beginning of the line and in the last connected equipment. Our product includes termination with fail-safe. (activated with a DIP- switch).