Connecting Mitsubishi GOT HMI to Eurotherm Controller - Modbus RTU

[cornfieldeng]

I am trying to get a Mitsubishi GT2715 HMI to read and write from a Eurotherm 3508 controller using 4 Wire RS485 Mobus RTU communications.

The Eurotherm has been ordered with a 4 Wire RS485 card and is set to Modbus communications, 9600bps, address 1.

The HMI is configured with the internal RS422/485 port as Modbus master with 9600bps, 8 data bit, 1 stop bit, no parity.

I have manufactured a short cable that is connected as follows;

HMI Eurotherm
Pin 1 - SDA HB - Rx+
Pin 2 - RDA HE - Tx+
Pin 5 - SG HD - Common
Pin 6 - SDB HC - Rx-
Pin 7 - RDB HF - Tx-

Pin 3 - RSA Linked to
Pin 4 - CSA

Pin 8 - RSB Linked to
Pin 9 - CSB


The above pinout is from the Mitsubishi Connection Manual (GOT2000-CON4-SW1-E) page 4-6, (3) RS-422/485 cable 2 (2 Pair wiring) and the Eurotherm Manual (HA027988) page 27, Figure 1-8: EIA232 and EIA485 Connections, EIA485 5-Wire Connections.

I have created a basic HMI application with a single output field looking at the actual values from the Eurotherm, which is address 400002.

When I power it up the HMI reports a communication error and the value is not displayed on the screen.

Any assistance is much appreciated.

 

[cornfieldeng]

Issue Resolved

In case it might help somebody in the future I have resolved the communications issue.

I swapped the A(+) and B(-) connections on the instrument and it now works correctly. Presumably the documentation is incorrect somewhere.

 

[danw]

The EIA-485 standard does not 'define' the labeling of the RS-485 driver lines. Therefore, some manufacturer/vendors label the driver lines backwards of the other set of vendors.


One of the rules of RS-485 implementation is to swap the driver lines at one end when there is no comm but the configuration looks reasonable.

 

[parky]

Some manuals do have errors, one of our engineers designed a MCC he used the diagrams from Telemechanique for the star/delta starters, on site while commissioning, every motor tripped with a big bang, turned out that the drawing he copied from the tele book was incorrect, looked at a previous version that was also incorrect, so in essence even though there were updated literature it contained at least some of the original errors.