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It is a half duplex protocol, meaning that only one party at a time may transmit data. The RS232 driver and receiver use separate conductors on the serial cables, enabling full duplex communications. Characters may be lost if the transmit driver is turned off while pending characters are still being transmitted. The protocols are described in turn. The dual communications channels also provide an easy way to link systems that communicate using different serial protocols. The USB cable requires a specific programming package and does not support freeport communications. First, download the latest installation package from the Python website. We will describe the steps to run the Python example in this section. A serial mouse is used as an example device. A serial communications cable is also supplied with QScreen Starter Kits. Given the availability of ready-made communications cables, it is not necessary to study or understand the following descriptions of cable connections.

If upgrading a Wiegand system to OSDP, it is possible to use the existing wiring, however, using simple stranded cable typical of Wiegand access control readers does not typically meet the TIA485/EIA-RS485 specification for twisted pair recommendations.1 If your building has both styles of access control systems, Remee provides a specific cable design (PN 725930) that allows you to migrate slowly from Wiegand to OSDP with a single cable pull. Design engineers or buyers might want to check out various Rs485 Cable factory & manufacturers, who offer lots of related choices such as electric wire, copper wire and electrical cable. There are surface mount resistor pads on the QScreen that will allow you to bring out the secondary serial port to the Field Header on pins 5-6 or 7-8 as shown with the parentheses in Table 11-3. Pads are also available to bring out the RS485 signals to the DB9 Serial 1 Connector.

Most computers conform to IBM PC AT-compatible RS232 interfaces which use 9-pin D-Type connectors, consequently the QScreen Controller brings out its serial ports to two female 9-pin D-Type connectors. The QScreen Controller combines an embedded computer based on the 68HC11 microcontroller with a touch panel and LCD (liquid crystal display) graphic user interface (GUI) that is ideal for instrument control and automation. The QScreen Controller does not differentiate between these. 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. 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. There are many possible configurations of master/slave networks. Because of this RS485 and the others can be used in situations with a severe ground level shift of several volts, where at the same time high bit rates are possible because the transition between logical 0 and logical 1 is only a few hundred millivolts.

From the QScreen Controller’s point of view, these three signals (TxD, RxD, and ground) are the only connections required to perform serial communications. 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. Differential pairs may host up to 32 devices on the standard that uses twisted pairs and a reference. Each RS422 driver transmits a differential pair of output signals at 0 and 5 volts. RS485 uses the same differential signaling scheme as RS422, and hence has the same superior signal-to-noise characteristics and range described above. 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 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. 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.