rmorella

Thanks again for the response. The current problem, as described by our field service tech is as follows. On a unit with 3 slaves, only two out of the three will communicate with the master. It works in one motor mode, two motor mode, but not three motor mode. When a 3rd slave is included, it doesn't work. I'm sure you are asking yourself "why the very specific focus on grounding?" The original engineer left the RS485 signal ground off the design. It is a very short network with all three slaves mounted in the same machine. From what I have been told, it has worked fine this way for all installations (numbering in the many dozens). However, recently, we have a few installations where one or more of the slave module's communication ports are repeatedly failing. Since the engineer is no longer there, I was called in to consult. I immediately noticed the lack of a signal ground wire and suggested that it be added to the design, if for no other reason than a precautionary measure. Now that the changes have been made, the field tech is experiencing the issue described above. I do not have any experience with the Unitronics RS485 communications and it's grounding, so I am wondering if my suggested design changes have not introduced other potential problems. Also, I am curious as to why Unitronics does not want us to use the same signal ground pin for RS485 as the one in the RS232 pinout. However, I will save that discussion for another day, and assume that internally they are the same as any other PLC with a non-isolated RS485 port.

Aus, Thanks for the reply. I assume that ZA is likely trying to prevent people from using the shield for signal ground, since typically the shield is grounded to chassis at one or two points (I agree with you, in that I always connect the shield only at the chassis of the PLC). I assume their biggest concern is ground loops, and differences in ground potential, should someone get it wrong. This is why, I question where connecting the RS485 signal ground to the chassis, which would be the case id if follow the Unitronics documentation. My biggest issue is that there is no accommodation for the signal ground on the Unitronics modular connector used for RS485. These seems like an oversight on the part of Unitronics, since all of the RS485 enabled devices I have encountered provide a connection for RS485 signal ground. So, my question is... How have others dealt with RS485 signal and chassis grounding of the Unitronics PLC with respect to reliable RS485 operation, considering that they do not offer an obvious signal ground terminal for RS485 communications in their documentation? I have attached a diagram of how our contractor has advised us to connect this. The drawing shows 1 slave (there can be up to 4 slave units). There is also a connection between 0V and chassis ground which is made at the 24VDC power supply. Modbus-485 Connection Diagram.pdf

We have an RS485 based communications issue We are using a Unitronics V430-J-RH6-N to communicate to a network of up to 4 slaves, consisting of Ziehl-Abegg Modbus modules. The ZA module wiring requires that, in addition to the data lines, the ground connections also be daisy chained together. This is what I am used to. The Unitronics communications connector does not specify a ground connection on the 6 pin RS12 connector provided. They only mention using pins 1 and 6 for Data A and B. 1. Where at the PLC do I terminate the ground for the RS485 circuit? Currently we have it tied to chassis. 2. Is there a possibility that tying the V0 ground at the PLC to Chassis, as suggested in the Unitronics documentation, could be causing us issues? The ZA socuentation specifically mentions NOT to connect the shield on the cable.