I know the four standard addresses 0X, 1X, 3X, & 4X, and their type differences. This brings me to my question. You see, the controller used by my company gets its protocol converted to Modbus via a Modbus Gateway. On some of our site locations, the Gateway is run into a Johnson's Metasys Integrator. The Gateway designates all the digital I/O has a 1X. However, the integrator from Johnson Controls uses 0X and 1X addresses. Everything works fine, but how is it that 0X and 1X addresses are interchangeable? Wouldn't this defeat the purpose of having 2 digital address types? If the 0X and 1X addresses are mirrors of each other, why the redundancy? Second, what other documents would you recommend for help with understanding Modbus? Thanks for any and all help.