Motorola V186 User Manual - Page 103

Communications IP Ports). The RS232 ports may operate at data speeds up to 115.2 kbps (depending on the total wire length). RS485 Ports On ACE CPU modules, Serial Port 1 (SI1) can be configured as RS485 port. Additionally up to two RS485 Plug-in ports can be installed on the CPU module (on PI1 and PI2 plugin ports). The RS485 ports permits up to 32 2-wire RS485 devices to be parallel-connected (multidrop) onto one pair of wires for the exchange of data. A typical ACE3600 use for RS485 is the interconnection among multiple RTUs in the same site. RS485 is also used to connect various devices in the site to the RTU using the ModBus protocol or a user defined protocol. The RS485 Connection Box is available to make this interconnection; or the installer may make the cables by using the small handset-size connectors commonly found on modular telephones. The RS485 port may operate at data speeds up to 460 kbps (depending on the total wire length). The RS485 specification calls for the circuitry to be capable of communicating at 10 Mbit/s for 40 feet (12 meters). At 4000 feet (1200 meters), maximum cable length, the data rate is reduced to 100 Kbit/s. There are other factors involved including the network configuration; wire characteristics, the device used, biasing resistors and termination resistors (see later) that can influence the data rate. One of the most frequently asked questions and one of the most difficult to answer is the speed/distance/number of drops tradeoff. Different studies in the industry have given some of the following (often conflicting) results, however the table below provides a conservative estimate based on the assumption of a daisy chain topology with no stubs. Data Rate (Kbps)