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For this to happen without interference, the outputs from the U7 transmit ICs on all the other SMINI cards must be held in a high impedance state, effectively an open circuit to prevent loading down the bus. Two wires are used to send data from the PC to the SMINI cards and two wires are used to send data from the SMINI cards to the PC. Because none of the other SMINI cards were addressed, their corresponding U7’s are held in their high impedance tri-state, effectively open circuited, to prevent conflict with the operation of the addressed SMINI. Grounds between buildings may vary by a small voltage, but with very low impedance and hence the possibility of catastrophic currents - enough to melt signal cables, PCB traces, and transceiver devices. Care must be taken that an SC connection, especially over long cable runs, does not result in an attempt to connect disparate grounds together - it is wise to add some current limiting to the SC connection. The vast majority of C/MRI outputs make use of standard current-sinking, where the transistor switch, built within every C/MRI output line, completes a connection to ground to activate the connected railroad device. Clearly a physical connection to the signal is necessary.

In summary, the EIA/TIA standards (which are commonly miscalled RS standards) are the physical layer which include electrical signal levels and RS-232 includes the connector. 5 V relative to the signal ground. The driver must not exceed 10V differential, or 6V common mode (outputs relative to circuit common). A common and incorrect assumption is that the RS-232 standard defines the bit protocol of the data being transferred asynchronously through the UART. The termination also includes pull up and pull down resistors to establish fail-safe bias for each data wire for the case when the lines are not being driven by any device. The arrows indicate the direction of data transmission for each two wire set. Thus, two ICs are used where U6 is set up to receive data from the PC and U7 is set up to transmit data to the PC. 200mV as a binary value, but different manufacturers can set the threshold where ever they want. Many other IC manufacturers make a drop-in replacement for this IC, and many of them (such as the MAX483) use the same labeling.

In the end, the cable you use needs to work for the application where you use it. Then the software will work unchanged when connected to a full-duplex RS-485 network, a RS-232 network, and a variety of other communication media. Converters between RS-485 and RS-232 are available to allow a personal computer to communicate with remote devices. If an RS-232 to RS-485 converter is used, things start to get even more murky. RS-485 does not define a communication protocol; merely an electrical interface. U8, a dual RS232 transmitter and receiver IC, handles communication in both directions when using RS232. This means that only the U7 transmitter on one particular SMINI can be permitted to be active at a time, otherwise there would be more than one U7 trying to pull the transmission line high or low at the same time resulting in a confused state. RC5 used as output to drive the transmitter enable line for enabling the RS485 transmissions from SMINI back to the PC. As soon as U1 pulls the U9 enable line low, U9 latches the output data so that it stays constant until the next output data update. When the input is high the inverting output goes low, and when the input is low the inverting output goes high.

However, by contrast, there are many users that think input line filtering is one of the C/MRI’s greater assets. When 5V drivers are used, the driver typically pulls one wire to circuit common, RS485 standard and the other wire to 5V (and vice-verse for the opposite data). See the section on grounding for a discussion of this "extra wire". By substituting a 2N4403 PNP transistor and changing card jumper locations it is also possible to obtain a current-sourcing output configuration which I will cover in the next section. A termination resistor can increase the signal's quality (reduce ringing), but it will also reduce the signal's amplitude. The exact voltages will depend on the driver, the loading, biasing, termination, and any shift in ground potential between the driver and receiver. These are signals from the PC, signals to the PC and ground. Only one SMINI, as determined by the PC, may be permitted to communicate back to the PC at any point in time. When the C/MRI application software, resident in the PC, requests that a given SMINI node send the railroad data to be received as inputs on its 3 input ports, the U1 gathers the data, converts it to serial format, then activates U7’s DE enable line, which then seizes the serial bus for sending the input data to the PC.