Hello GloCap;

We have used both options in our old projects. Both setups ended up working, but sometimes they needed tweeking.

When working with Profibus connectors, I strongly suggest you also use Profibus cable. Standard shield twisted-pair cable tends to be too thin, compared to Profibus cable, and the earthing of the cable shield in the Profibus connector ends up too loose, and if there are vibrations in the system, the connected nodes will loose their link temporarily with their Modbus master. Working with standard STP cables and  Profibus connectorsr can become fustrating...

On the ither hand  building your own cable and connector with briged terminator will sometimes require you to modify the suggested resistor values, depending of the impedance of the nodes un the Modbus network. That has also been known to be a frustrating exercise...

Choose whatever way you want, but I suggest a bench test so you can test your solution before field installation.

Hope this helps,

Daniel Chartier

Hello GioCap;

I hope this answer helps, it often causes problems when selecting the cables for your project.

Look at the image in tyev screenshit below   it shows one pair of STP cable connecting A and B terminals, plus another wire connecting the ground terminals.

https://electrical-engineering-portal.com/correct-cabling-modbus-rs485#cable-type

So  in reality  a single STP cable is not enough to establish a functional RS 485 connection, you need a cable consusting of 2 STP,  one for the Data (A/B)  one for the ground. Make sure in this case that you use a simgle wire of the geound STP, to msintain continuity. Or simply run an extra wire between the ground connections of  the terminal blocks...

Hope this helps, 

Daniel Chartier

Hello GloCap;

How certain are you of the uality of your multimeter measurement?

I can assue you (both from work experience and an abundance of practical documents on Profibus) that the Profibus connector uses 3 connections: 

• Data line A on pin 8
• Data line B on pin 3
• Data ground (the Profibus cable shield) on pin 5.

All 3 connections must be correct and in good status for the network to function properly.

Among other many documents, Ch. 6 of the Profibus Networks System manual discusses the pinout of a standard Profibs connector is table 6.6, look at the attached screenshot below.

https://support.industry.siemens.com/cs/ca/en/view/35222591

Note that pin 6 is for a secondary 5V input, I have never had t boter with it. I belive it was to power un special units that did not have access to a 5V pwer supply. These days, it is not used.

But if you decide to use the Profibus cable and connectors, the ground connection to the shield will be active through pin 5 of the 9-pin Sub-D male connector.

Hope this helps,

Daniel Chartier

It doesn't surprise me that pin 5 is not connected to the shield.

Page 170 of the Profibus Network manual claims that contacting pin 5 with the shield is optional, which I would expect for an engineered RS-485 network; where the chassis ground provides grounding for the shield and pin 5 is a signal (ground) reference  (the cited 'differential of an RS-485 signal is driver line A referenced to signal ground minus driver line B referenced to signal ground [or vice versa, I can't remember which], so the signal reference is important.)

Although the EIA-485 spec calls for a signal ground (pin 5 of the DB9 connector), far too many implementations do not provide a signal ground connection and rely on chassis ground through the DB-9 frame.  But Profibus is an engineered product so Siemens understands the need for a signal reference.

The problem with chassis ground connections is common mode voltages that can overwhelm the 485 differential signal.

Page 170 text  (  ):
https://support.industry.siemens.com/cs/ca/en/view/35222591

Optional – Contacting pin 5 with the shield

Note

Contacting the shield with pin 5 is not recommended, see: PROFIBUS Interconnection Technology Guideline V1.4.

If shield contact using pin 5 is specified by the manufacturer of your device, follow the steps below:

1. Perform steps 1 to 4 as described above.

2. Strip the insulation from the ends of a 2.5 cm long piece of stranded wire 0.75 mm² (k) as shown in the diagram.

3. Fit a wire end ferrule 0.75 mm² (m) to the prepared 5 mm long end of the wire and fit this in pin 5 (shield contact) of the socket or plug insert (h/i) and screw tight.

4. Fit the 0.5 mm² wire end ferrules to the PROFIBUS stranded wires. Fit the green and red PROFIBUS wires into the screw terminals of the socket or plug insert (h/i) and screw tight.

5. Fit the other end of the stranded wire for the shield contact into one of the four grooves on the side of the insulator body.

6. Push the coupling sleeve (f) onto the socket or plug insert (h/i) so that the metal bar F1 on the inner side presses the shield contact wire into the groove to contact the shield.

7. Screw the coupling sleeve (f) to the socket or plug insert (h/i).