Unlike the standard RS232 protocol, RS485 allows many communicating parties to share the same 3-wire communications cable. Done that establishes the RS485 receive mode is coded such that it waits until all queued (pending) characters have been transmitted before the driver chip is taken out of transmit mode. If your computer does not have an RS-232 serial port, low cost USB-to-RS-232 serial cables are available; contact Mosaic Industries for details. The default serial routines used by the onboard kernel assume that full duplex communications are available, so you cannot use the RS485 protocol to program the controller. 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. A single master can broadcast commands to all the slaves, and can direct commands to an individual slave using its unique address. The primary and secondary serial communications ports are accessible through the QScreen's 10 pin, dual row Serial Header (H5) which is typically not installed, the 24 pin, dual row Field Header (H3), and through the individual DB-9 Serial 1 and Serial 2 connectors.

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. Function prototypes for this function and other versatile serial I/O routines are defined in the COMM.H header file, and are described in detail in the Control-C Glossary. The QScreen Controller’s kernel software contains a complete set of high level driver routines for the Serial2 port, and these functions are summarized in the Control-C Glossary. The Control-C Glossary contains a list of functions that temporarily disable interrupts, and the glossary entries give further information regarding how long interrupts are disabled. Electrical signaling is balanced, and multipoint systems are supported. The foreword to the standard references The Telecommunications Systems Bulletin TSB-89 which contains application guidelines, including data signaling rate vs. After the experiment is completed, the class can view statistics from the recorded data including instant, average, rs485 cable min and max values.

Let’s do a quick experiment to see how easy it is. Configuration and data accessibility were a snap: the FieldLogger's user-friendly configurator software was accessible by Ethernet, USB or RS485, and also provided means for online monitoring, downloading of all logged data, and quick exporting to spreadsheets and other formats. An optional exclusive color HMI was chosen for local indication and configuration. Note that the local and the remote must share a common ground, so a minimum of 3 wires are required for half duplex RS485 communications: a pair of transceive wires and a common ground. The QScreen Controller controls the RS485 transceiver with bit 5 of Port D of the processor. In a computer system, SCSI-2 and SCSI-3 may use this specification to implement the physical layer for data transmission between a controller and a disk drive. 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. Care must be taken when changing an RS485 node from transmit mode to receive mode, or visa versa. RS485Receive() clears bit PD5 to place the transceiver in receive mode, and RS485Transmit() sets bit PD5 to place the transceiver in transmit mode.

After the slave transmission is complete, the slave puts itself back into receive mode so that the master can transmit additional commands. If you do this now, remember to move the QScreen Controller’s serial connector back to Serial Port 1, and to change the terminal’s baud rate back to 19200 baud using the "Comm" item under the terminal’s "Settings" menu. It is supported by virtually all personal computers, and is the default protocol for both of the QScreen Controller’s serial ports. Likewise, the terminal’s transmit signal TxD is connected to the QScreen Controller’s receive signal RxD1. Although the RS232 protocol specifies functions for as many as 25 pins, each communications channel requires only three for simple serial interfaces: TxD1 (transmit data), RxD1 (receive data), and DGND (digital ground). Because the software UART is interrupt based, competing interrupts that prevent timely servicing of the Serial2 interrupts can cause communications errors on the secondary serial channel. Data translation between different machines can be performed with ease, and applications that communicate via the primary serial port can be debugged using the secondary channel. A jumper, J3, configures the primary serial port for either RS232 or RS485 operation. We can gain insight into the operation of the RS232 protocol by examining the signal connections used for the primary serial port in Table 9 6. The transmit and receive data signals carry the messages being communicated between the QScreen Controller and the PC or terminal.