In reply to cbpun:

Regarding resistor: from what I know you need 1 x 120 Ohm resistor on Master end (DeltaV) and 1 x 120 Ohm Resistor on Salve end only if the distance is quite long(can't tell you exact "quite long" how many meters it means). In my case we had around 100 m distance and it was not needed. Maybe you don't need not even for 200 m. Have you tried with 2 resistors, one at each end? Do you have more slaves on the line or only DeltaV Serial card and only one device at the other end? IF you have more, then in case of long distances you need resistors only at the ends of the bus(DeltaV and last device on segment), not at intermediate devices.

Regarding Mynah. the most common product regarding Modbus is the VIM card which is using Modbus TCP. They have a lot more other products like firmwares for standard Serial Card for Modbus RTU. This firmwares are some files. You use DeltaV Upgrade Utility tool to upgrade the DeltaV Serial Card with this firmwares to Programmable Serial Card. One of this firmwares is IOD-1134 Enhanced Modbus Driver which allows you to access all Modbus RTU registres addresses, all which the standard Serial Card can't, for example Holding Registers over 49999, incuding registers over 400001 or even over 450001.
Mynah has around 40 of this type of firmwares. You can see them on their page: http://www.mynah.com/content/serial-drivers-user-docs

In order to make some troubleshoot I would recommend to start with simple configuration. In case you have to possibility change device setting to RS485 Half-duplex and also on DeltaV side so you use only 2 wires (+ common port settings). Start configuring the datasets by reading only 1 data or maximum 10. I assume you need holding registers. Don't start from the begining of the registers with 40000 , try to read one register somewhere in between, for example the 40010 register or which ever has some value. If it start communicating with simple configuration then you slowly build up your required configuration.

There is a software called Modbus32 which can read Modbus RTU registers from any device, just for checking if the communication is working. It works on your normal computer/laptop but you need to have a serial connection.