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The first of the RS standards was RS232, or more correctly RS-232. We’ll use code from the GETSTART.c program which was introduced in the chapter titled Your First Program. In fact, the program works the same as it did before, but now it is using the secondary serial port instead of the primary port - and you didn’t even have to recompile the code! If your application requires RS485, you can use the secondary serial port (serial2) to program and debug your application code using the RS232 protocol, and use the primary serial port (Serial1) for RS485 communications. For Serial1 RS232 operation: Remove the jumper shunt from "1 485En" (J4). For RS232 operation: Remove the jumper shunt from J3. For Serial2 RS232 operation: Remove the jumper shunt from "2 485En" (J7). In this case, cable connections may be made to Serial 2 on either the 10-pin PDQ Board Serial Communications Header, or the Docking Panel’s 10-pin right-angle Serial Header, or the Docking Panel’s Serial2 DB-9 Connector. By default, the RS485 connections are not brought out to the Docking Panel’s DB-9 Serial1 Connector.

If your application requires communicating with a device that expects to receive a parity bit, the generation of a parity bit and selection of even or odd parity, and whether there are seven or eight data bits in each byte, is performed by setting or clearing bits in the configuration registers SCI0CR1 for Serial1 and SCI1CR1 for Serial2. In either of these cases, a source of noise that caused one bit to be received incorrectly would invalidate the received byte, since the total number of '1' bits would be odd rather than even. Bias termination - Using resistive termination decreases noise immunity, particularly if the cable is loaded with many devices. In that case, when using very long cables you can improve noise immunity and assure a valid idle level when the transceiver is not active by installing bias resistors. This allows for basic error detection, in that if noise on the transmission line causes one bit to be received incorrectly, either received as a '0' when transmitted as a '1' or vice-versa, the error would be detected due to the count of '1' bits in the byte being odd when it is expected to be even, or vice-versa depending on the parity checking settings.

RS485 avoids this issue which also allows for shielding of the cables to prevent noise. This allows either twisted pair cables to be used with terminal blocks, or Ethernet Cat5e or Cat6e cable to be used with RJ45 jacks. SPIE is a local interrupt mask that allows an interrupt to be recognized when an SPI data transfer has completed, or if a write collision or mode fault is detected. For this reason, frame-level cyclic redundancy checks are much more widely used for validating data from serial links, network connections and storage media. The SPI can transfer data much more rapidly than an asynchronous serial link - its maximum rate is 2 Megabits/second. Much like RS232, RS485 doesn’t have an actual standard connector. If this limitation is not a problem, you can reverse the roles of the Serial1 and Serial2 ports, because they have identical communications capabilities. The Serial1 and Serial2 ports are is supported by the HCS12's dual on-chip hardware UARTs, and do not require interrupts to work properly. The BAUD routine described at the start of this subsection configures the baud rate of each of the Serial1 and Serial2 channels. A jumper labeled "1 485En" (J4) enables RS485 operation on the Serial1 port if the jumper cap is installed, and configures Serial1 for RS232 operation if the jumper cap is not installed.

A jumper labeled "2 485En" (J7) enables RS485 operation on the Serial2 port if the jumper cap is installed, and configures Serial2 for RS232 operation if the jumper cap is not installed. Enabling RS485 on the Serial2 port is parallel to the process described above. Typing a carriage return at the terminal should now produce the familiar "ok" response via the Serial2 port. For Serial2 RS485 operation: Install the jumper shunt onto "2 485En" (J7). For Serial1 RS485 operation: Install the jumper shunt onto "1 485En" (J4). By default, the RS485 connections are not brought out to the Docking Panel’s DB-9 Serial1 Connector, although custom placement of zero-ohm surface-mount resistors on the Docking Panel can route the RS485 signals to the DB-9. In this case, cable connections may be made to Serial 1 on either the 10-pin PDQ Board Serial Communications Header, or the Docking Panel’s 10-pin right-angle Serial Header, or the Docking Panel’s Serial1 DB-9 Connector. Contact Mosaic if you require RS485 signals to be routed to the DB-9 Connector. Please contact Mosaic Industries if you need these custom configurations.

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