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If CAT-5 (100 Ω cable) cable is used, the termination resistors should be 100 Ω. These examples are for a single 120 Ω termination. A single 8-wire I/O data bus connects the PIC16F877 to each of the 9 I/O buffering functions. Although all the I/O lines are physically on the same SMINI card it is convenient to talk about them as being separate (I/O) cards within the same (SMINI) card. For better or worse, a lot of external peripherals require 5 V power and I/O. Starfish runs on 24 VDC, interfaces with some noisy peripherals, and needs to be resilient against common power faults. Note that the local and the remote must share a common ground, so a minimum of 5 wires are required for full duplex RS422 communications: two transmit wires, two receive wires, and a common ground. Note that the local and the remote must share a common ground, so a minimum of 3 wires are required for full duplex RS232 communications: a transmit wire, a receive wire, and a common ground. In other words, each local UART on the wildcard can both send data to and receive data from a remote UART on the other end of a connecting serial cable.

The serial interface is asynchronous, meaning that there is no clock transmitted along with the data. This way, the lines will be biased to known voltages and nodes will not interpret the noise from undriven lines as actual data; without biasing resistors, the data lines float in such a way that electrical noise sensitivity is greatest when all device stations are silent or unpowered. In reality either or both the driver or receiver are going to meet the more modern RS-574 requirements and none of the RS-232 limits will apply. The driver must output between 5V and 15V in magnitude into a load of 3kΩ to 7kΩ. The driver must not be able to output more than 100mA when shorted to any other conductor in the cable, must not be able to output more than 25V and must be able to handle an open circuit, or a short to any other conductor in the cable. The RS422 driver and receiver use separate differential conductor pairs on the serial cables, enabling full duplex communications.

A UART is a Universal Asynchronous Receiver/Transmitter that converts parallel data from the host processor (any Mosaic controller) into a serial data stream. The serial data stream at the UART is conditioned by serial driver chips that transmit and receive the data. Because a single pair of conductors is used for both transmission and reception, RS485 is useful for multi-drop applications in which a master communicates with multiple slave serial devices, or nodes. In the simplest scheme, all RS485 transceivers come up in receive mode when the interface is initialized, and each transceiver node has a unique address known to it and the master. In fact, a single driver chip on the UART Wildcard is used to implement both RS422 and RS485 communications for a given serial channel. Newer protocols include the full duplex RS422 and the half duplex RS485 protocols, each of which drives differential 0 to 5 volt signals on the serial cable. The protocols are described in turn. Both the generator (driver) and the receiver of all devices on the network are connected to the two wires.

This device connects any serial device (RS232, RS485) to an Ethernet network. The RS232 driver and receiver use separate conductors on the serial cables, enabling full duplex communications. The RS-485 standard is obsolete and has been superseded by TIA-485, but many if not most engineers and applications guides continue to use the RS designation even though it has officially changed. The RS485 standard only defines the electrical specifications for differential receivers and senders in digital bus systems. Each RS422 driver transmits a differential pair of output signals at 0 and 5 volts. These are signals from the PC, signals to the PC and ground. Each RS232 driver uses inverting logic and implements a single-ended bipolar output voltage (that is, one signal that swings above and below ground). The receiver's input impedance is specified in terms of a "unit load" where a unit load is specified as input current in mA at a voltage referenced to ground. As the unit also has a task engine, multiple functions can be performed from a single contact closure. The driver has to be capable of driving 32 unit loads.