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Serial Coms connections to MJ1/MJ2 sockets


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----  Warning  ----  Warning  ----

On the XL series OCS, the serial connections are via an MJ45 socket. This socket includes both an RS232 port and an RS485 port.
On some models, MJ1 and MJ2 (separate serial ports) are included in the same socket.

Because this is the same type of socket that is used for Ethernet connections, there is an obviously easy way to make a connection, use a standard Ethernet patch lead with one plug cut off.

--- Don't Do It  ---

The Ethernet wiring in the patch lead comprises four twisted pairs and in the Ethernet structure, a different push pull signal is dedicated to each pair. Twisted pair cabling is designed to minimize cross talk and interference with other circuits in close proximity because the twisted pair carries an equal and opposite signal on the two cores. The combined electrical and electromagnetic fields from a twisted pair, will cancel to zero and as such will not induce a significant signal into another circuit in close proximity.

If you use a standard Ethernet cable you do not have a balanced differential signal applied to each pair, rather there are signals that are running on multiple pairs and the cross talk is seriously enhanced.

The RS485 circuit is a differential circuit and if you applied connections such that the RS485 circuit was on a single twisted pair, you would a) reduce cross talk for signals getting out and causing interference, b) reduce cross talk for signals getting in and causing interference.

The RS232 circuit is single ended in that there is only a single wire circuit carrying the TX information relative to a ground circuit, and a single wire carrying the RX information relative to the same ground connection. This is the wrong format for a twisted pair which must be a differential signal.

Running the MJ1/MJ2 connections together down an Ethernet patch cable causes cross talk between the RS485 circuit and the RS232 circuit and will often lock up all communications to and from the PLC on all ports.

The circuits must be separated as close to the socket as possible and if they need to run a distance down a similar path, they should be run in separate cables. The RS485 signals would ideally be run in a screened twisted pair cable and the RS232 in a screened un-twisted pair cable. 
NB: RS232 is potentially very noise sensitive and so the length / distance of the RS232 cable should be as short as possible and well clear of other electrical cables and conductors.  RS485 is inherently far more noise immune and can run considerable distances without problems, but with long distances, the RS485 cable should have an impedance of 120 ohms and be terminated in 120 ohms at each end. All RS485 connections should be "daisy chained and the intermediate connection should not be terminated.

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