RS485 is a communication standard that should be part of the advanced hardware hacker’s arsenal; it’s not commonly encountered, but powerful exactly when you need it. It’s a physical layer interface for wired communications that uses a single differential pair for noise immunity, has good long-distance properties, and allows many connections to a single bus. Because of that, you will encounter it in security systems and even cameras, wired sensor networks, DMX512 lighting and all sorts of industrial electronics. For our hobbyist goals, you can absolutely use RS485 to build your home (or room) automation system, or a relatively large robot – without all those worries that wireless brings.

The RS-485 line is typically terminated with 110-120 ohms resistance on both ends of the line (on short lines you might be able to live with termination on one end or even no termination sometimes).

One question that I cant figure out. I dont have any experience with modbus and RS485.

I have order several of thouse USB to RS485 adapter (haven’t got them yet). So to my question. I have a RTU modbus slave I liked
to communicate with. Is it enouhg to connect to wires between my USB adapter and modbus slave (TX and RX)?. What about the resistance that should be in the loop of a RS485 communication? Is this inside the USB adapter?

what module converter do you recoment ?

what cost and how soon you ship out .

With RS-485 networks, there are periods of time when no driver is actively driving the bus, such as when one driver relinquishes the bus to another driver. During this time, the termination resistors collapse the differential bus voltage to 0V, which is an undefined input level for many RS-485 receivers. Faced with this undefined input, a receiver might output the wrong logic state or worse yet, it might oscillate. The oscillation may be interpreted as an endless stream of message start bits, causing the controller to waste valuable bandwidth trying to service these phantom messages. Fail-safe bus biasing is one way to alleviate this problem.

Passive fail-safe biasing is accomplished through resistor networks of pull-up, termination, and pull-down resistors whose voltage divider action provide a differential dc bus voltage, VAB, when no driver is actively driving the bus

6-in-1 Multifunction USB 2.0 to CP2102 TTL RS485 RS232 6-Pin Module Serial Converter – (3.3 / 5V)
http://dx.com/p/6-in-1-multifunction-usb-2-0-to-cp2102-ttl-rs485-rs232-6-pin-module-serial-converter-3-3-5v-304669?r=8527370