network rs232

A surprising number of modules still have an interface based on RS232. No wonder, as RS232 is easy implemented on a microcontroller with two I/O pins and a line driver such as the MAX232. In the case where the master is a PC, the serial port is relatively easy to access on both Windows and Linux. Usually modules implement a text terminal interface that decodes single line commands with arguments and generate
network rs232 - schematic

a reply like this: A complication occurs when there are a number of RS232 modules in a project, as each requires a serial interface at the master. A hardware solution in the form of an RS232 multiplexer would be a solution but wouldn`t it be nice to get this functionality for free! By deviating from the original aim of RS232 as a point-to-point link, we can have an RS232 network in which all the modules share both transmit and receive lines to one master inter-face. All modules operate at the same speed, start and stop bits with no flow control. When idle, all the modules are listening for commands from the master and have their transmitters disabled. Each module is configured with an identifier consisting of a number that the master sends as a single line (e. g. 2/n` selects module 2). If a module receives an identifier that matches its own, it is selected and can decode commands and drive its transmitter for the duration of the reply. Conversely, if the identifier does not match it must not decode commands and ensure its transmitter remains disabled. In addition to some firmware sup-port, the RS232 driver electronics must be able to tri-state the transmitter while keeping the receiver operational. Sadly, the classic MAX232 driver is unsuitable but the ICL3321 and MAX242 are possible candidates for our purpose. These have low-power shutdown modes that power-down the charge pump and transmitters but keep the receivers...

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