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These signals may alternatively be redirected to the digital inputs and outputs used by the second serial port if hardware handshaking is required. Its simplest implementation requires only three wires: one to transmit serial data, a second to receive serial data, and a third to provide a common ground reference. It is important to note that when the CPHA bit is 0, the /SS line must be de-asserted and re-asserted between each successive data byte exchange (M68HC11 Reference Manual, p.8-3). The SPI control register, SPCR, contains 8 bits which must be initialized for proper control of the QVGA Controller’s SPI (M68HC11 Reference Manual, p.8-7). If the 68HC11 is initialized as a master by setting the MSTR bit, then bit 5 of the Port D data direction register (PORTD.DIRECTION) determines whether /SS is an input or an output. The /SS input in turn determines whether the slave responds to the SCK input, as described in a previous section.

Clearing the MSTR bit in the SPCR control register automatically configures the /SS (slave select) pin as an input. By setting this output LOW, the slave’s input /SS is pulled LOW. In this example, the QVGA Controller selects the serial A/D by outputting a LOW signal on /SS. If the CPHA bit is 1, the /SS line may be tied low between successive transfers. One of the effective ways to eliminate common-mode interference is to use the shielding layer of the 485 communication line as a ground wire, connect the equipment in the network such as machines, computers, etc., and connect them to the ground reliably from one point. If more than one slave tried to drive the transmit line simultaneously, their serial drivers would fight with each other for control of the bus. The Serial 2 port is dedicated to RS232 communications at up to 4800 baud. If you are running Serial2 at 4800 baud, the rest of your application must be able to function properly using the remaining portion of the CPU time. This means data can be transmitted in both directions to and fro devices one direction at a time.

It is a half duplex protocol, meaning that only one party at a time may transmit data. Rather, the UART deduces the correct time to sample the incoming signal based on the start and stop bits in the signal itself. Rather, it relies on software handshaking via transmission of XON/XOFF characters to coordinate data transfer and ensure that information is not lost when one of the communicating parties is busy. The RS232 protocol provides for four handshaking signals called ready to send (RTS), clear to send (CTS), data set ready (DSR), and data terminal ready (DTR) to coordinate the transfer of information. The secondary serial port is connected similarly except that the onboard connection of RTS to CTS, and DSR to DTR are permanent. RS232 uses inverse logic; that is, a positive bit at the 68HC11 UART is inverted by the onboard RS232 driver chip and appears as a negative signal on the serial cable. Thus in Table 9 6 , RTS1 is connected to CTS1, and DSR1 is connected to DTR1 and DCD1 onboard the QScreen Controller using zero ohm shorting resistors. For on-site debugging, you must carry a few converters that can connect to long distances and multiple loads, a commonly used computer notebook, a multimeter to test the circuit break, and a few 120 ohm terminal resistors.

RS485 cable is a type of communication cable used in industrial applications to transmit data between devices over long distances. So remember, differential, any noise subjected to the cable is canceled out. If not, check out our other blog post What is RS232. Computer is connected to the converter rs232 rs422 ATC - 105. Converter is also connected to external power, supporting RS485 port. From what I’ve read, the modbus is a protocol for communication between a computer and an RS485 device, if this is true, I shouldn’t need a modbus lib for this application. Devices utilizing RS485 can communicate with central control systems using communication protocols such as Modbus and ASCII. The use of shielded twisted-pair cables helps to reduce and eliminate the distributed capacitance between the two 485 communication lines and the common mode interference generated around the communication lines. Generally, there are three levels of 485 chip load capacity-32 units, 128 units and 256 units. 5 Volt supply by a dual RS232 driver chip that has a built-in charge pump voltage multiplier. These characteristics include: definition of a unit load, voltage ranges, open-circuit voltages, thresholds, and transient tolerance. The RS485 protocol uses differential data signals for improved noise immunity; thus RS485 can communicate over greater distances than RS232.

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