USB to Serial RS-232 Adapter

To connect your computer via USB to the RMC75S, RMC75P, or RMC100, Delta highly recommends:

ICUSB232FTN USB to Null Modem RS232 DB9 Serial DCE Adapter Cable

This is available from StarTech or many other resellers, such as amazon.com.

Another product is IOGear GUC232A, although it also requires a separate null-modem serial cable (http://www.iogear.com/product/GUC232A/). For RS-232 DB9 cables, see this topic.

Some converters do not work with the RMCs. For example, a customer reported trouble using a Belkin F5U409v1. RMCTools took about 20 seconds to connect and the status lights show very little activity

What about RMC75E controller. Can we not use the Modbus via RS232/Serial to communicate with USB connectivity?

Not via USB, but you can get an RS-232 to Ethernet converter for Modbus. See the Lantronix converter here: RMC150E and Serial RS-232 Communications