I have a query on modbus. I haven't used this communication protocol before so my knowledge of it isn't great.

I need to connect 4 no. flowmeters to it. These instruments are in an EX area Zone 1.
The type of flowmeter will be Ex d.
The Power to the Ft is 110v and the communication protocol is modbus. My queries are

(A) What type of cable do you recomend to use as the modbus cable

(B)What type of tap boxes should be used for the modbus. Will ordinary EX e junction boxes be adequate.

(C) How do you terminate the bus

(D) If I need to replace any of the flowmeters at some stage. I will need to power down the meter but how do I handle disconnecting the bus cable. I do not want to take down the whole bus to replace one FT and I do not want to expose the modbus connections to the Zone 1 area. There is obvoiusly a voltage on the modbus cable that isn't intrinsically safe.

If anyone can provide me with some information on this I would appreciate it. Thanks

jog33 @ oceanfree.net

 

A) Shielded, twisted pair. Belden 9841 should be great, but I've used instrument cable specs also. It worked fine.

B) & C) Exe box with terminals should be adequate, separate from the FT enclosure (Exd). No need to open the FT enclosure for Modbus wire work. However, the transit conduit between the Exe and the FT should be sealed.

D) Have you considered wiring your Modbus in a star configuration instead of a multi-drop? This way, you can locate your hub in a non-hazardous location where you can disconnect any node quite easily, using ordinary terminals.

 

William, thanks for your response dated 17 January regarding Modbus in ex areas. Sorry for not getting back sooner.

I have another few questions on Modbus area.

In a 2 wire system, how many cores are needed?
I would have thought 2 but in a document I see on modbus.org it calls upon 3. The third core needs to be used when the a device needs line polarization.

I haven't implemented Modbus before so my knowledge of it is small. My questions are:
(A) What is line polarization
(B) When is it acceptable to used only 2 wires
(C) Do you need to put an external 5 volts + on to the line D1 through a resistor 650 ohms and a 5V- onto the line DO
(D) In profibus the the profibus cable receives a voltage from each device and you do not need to power the line is this system different.
(E) If one of the devices on the line can implement such a line polarization can it be mixed with ones that needs line polarization.
(F) Is there a good book available that outlines the electrical installation guide for Modbus giving you information on terminations details/fault finding tips etc.

I would appreciate anyones knowledge on this.
Thanks.