If so, how? I have 4 wire and 2 wire devices I would like to connect on the same data line. I know you can convert 4 wire to 2 wire easily at the master, but is it possible farther down the line, say after a string of 4 wire devices?
Electrical – connect 4 wire and 2 wire RS485 devices on the same network
rs485
Related Solutions
All the ways you suggested will work to some extent, but some sort of computer based device is liable to be best . RS485 uses 2 wire differential voltage signalling and will usually have enough drive to allow an extra receiver to be attached. "Monitor" is probably a better term than "hijacker" :-).
A computer will provide the most flexible result as you can analyse the data and also display in an oscilloscope type display on the computer screen if desired.
An oscilloscope will allow you to see that signals are being sent and the levels involved but will provide little other information. Some oscilloscopes may provide means to analyses data via an RS485 interface but that is not usually an oscilloscope based function.
Special purpose hardware can easily be built and you can buy analysers with RS485 interfaces - but in many cases it is simpler to use a general purpose computer with an RS485 interface. The actual interface to a computer could be an off the shelf commercial RS485 to USB converter or RS485 to RS232 converter or you could build something using a data acquistion system.
Many examples of RS485 to USB interfaces here:
DIY RS485 - USB interface from here - with PCB layout. The circuit is copied from the FTDI data sheet but he has added much comment and construction detail.
Commercially available RS485 analyser
and here
and here , same as this
Note: I don't claim to have the most scientific answer but i will try to explain things the way i understand them.
I don't know if The Voltmeter argument is 100% valid , since it is a low speed device , a more appropriate example would be the Battery operated oscilloscope , or a differential oscilloscope probe.
Remember the definition of Voltage which is a potential difference between 2 points. The Signals A and B are differentiated by the electrical components inside the Amplifier (mainly transistors) which all have absolute maximum ratings between their base and collector/emitter .. This is referred to as differential amplifier maximum Common Mode Input Voltage with respect to its own ground. Hence the A and B voltages are meaningless without specifying what they are referenced to. For example if the difference between A and B is 2.5v but this voltage is 20v shifted above the receivers amplifier supply , would would the amplifier see (2.5 or 22.5) ?
4: The long distance outdoor cables are more prone to noise or ESD or whatever source of charge or current that may enter the bus ( that has specific impedance and DC resistance) so if amount of charge/current is high enough multiplied by the higher resistance (of long cable ) would cause a larger voltage spike at the receiver that may cause damage. Earthing in this situation can be used to provide path for the spikes hitting the shield , and may be used as a stable ground reference.
Whether the RS485 receiver circuit can be changed to be floating and fully capable of acting like the voltmeter/Oscilloscope might be entirely possible by adding extra components, isolation circuits, etc.. but with added cost, complexity and size which is all beyond the capability of a small IC such as the MAX485.
Best Answer
4 wire device provide full duplex communication and 2 wire half duplex communication. It depends what kind of transceivers you are using and what communication you would like to have (half or full duplex).
It is possible to use 4 wire transceiver on a two wire bus. With more information in your question you would get much better answer.