REMOTING SIX SENSORS ON ONE TWISTED PAIR - PART 5

Glovisol · Post by **Glovisol** » Thu Feb 11, 2016 9:55 am

: N=3, 3db Chebyshev lowpass filter attenuation

5. MODEM CONNECTIONS; RS485 STANDARD AND TWISTED PAIR/COAXIAL PAIR MEDIA WITH LAB SIMULATION DATA

The RS485 Modems used in this work can be obtained here,

http://yourduino.com/sunshop2/index.php ... tail&p=323

but any modem using the MAX 485 chip will work. Be careful about shield type modems, as some have the signal pins (e.g. RO & DI) directly plugged into pins 0 & 1 of Arduino, thereby interfering with USB communications. With these shields you must first upload the sketch and then plug the shield in and start working: not very practical.

The RS485 Modems must be interconnected as shown in the enclosed schematic diagram, e.g. terminal A to A and B to B. Some extremely important issues about the transmission medium must now be discussed.

a) GROUNDING
Many advise to use just one wireline and to connect the Master and the Slave to their local grounds. This advice is wrong because it is most likely that ground loops will be formed and differences in circuit potential will come in as common mode, disturbing the signals.
Worse: one can come to the situation where common mode signal levels in the ground loop will be variable, so that the System will sometimes work and not work at other times, driving crazy all involved.
On the other hand grounding is necessary to minimize RFI (Radio Frequency Interference) generated by transmission of square waves. The solution is to ground the system in one place only, either at the Master or at the Slave. Added 4/3/2016: a ground conductor is necessary to ensure safety for the Modems. See Part 10 following.

Normally laptops with separate mains power supplies are insulated from ground, so there is no danger of spurious ground connections at the PC level. The other critical spot is the +9V supply for the Slave unit: isolation from ground is assured by using a switch mode supply. Amended March 1, 2016: see Part 10. Switch mode supplies cannot be used, because their noise degrades System operation. A small linear PSU must be used.

b) TWISTED WIRELINE CONNECTION
From what we have seen about grounding, it becomes clear that a good and reliable connection must have THREE conductors: two for the differential signal and one for ground. Using a Standard EIA 568, Class 3 wireline, the schematic diagram gives the data for loop resistance and loop capacitance vs. length. For instance a 600 m/2000ft run will have
a resistance for each conductor of 60 Ohm and a total capacitance of 40 nf. This run can be simulated in the lab with an 8 resistor – 3 capacitor ladder network having each R = 15 Ohm , each C = 15 nF and the ground resistor 4R = 60 Ohm. Data for simulating other runs is given up to 4,000 ft.

The system is working at very low frequencies (57,600 Baud) so the influence of line inductance is negligible and therefore not calculated.

c) COAXIAL LINE CONNECTION
If better protection against noise and interference is necessary, then a coaxial medium should be considered. Coaxial cable with 4 mm diameter (used in video & satellite dish to TV box connections) is both inexpensive and generally available. Two cables are used, with the shields connected together at both ends, halving the ground resistance. The internal conductors work in place of the twisted wire, while the shields carry the ground connection. With this arrangement the capacitance between conductors is reduced by a factor of 4. Simulation data for System testing in the lab is also given in the uploaded schematic.

Using the above simulation data, the system worked well with line lengths of 3,000 ft for both twisted and coaxial line, not bad for a complete RS485 Comm System costing less than £ 50- in components.

Next posts will give the full six channel software in WORD format.