RS-485 signals are used in a wide range of computer and automation systems. RS485 extends the common mode range for both drivers and receivers in the "tri-state" mode and with power off. A true multi-point network consists of multiple drivers and receivers connected on a single bus, where any node can transmit or receive data. RS485 meets the requirements for a truly multi-point communications network, and the standard specifies up to 32 drivers and 32 receivers on a single (2-wire) bus. 100 ohms, Cat 5 cables almost meets the electrical requirements for RS-485 cables. The EIA has officially disbanded and the standard is now maintained by the TIA as TIA-485, rs485 cable but engineers and applications guides continue to use the RS-485 designation. With Modbus, BACnet and Profibus, A/B labeling refers A as the negative green wire and B as the positive red wire, in the definition of the D-sub connector and M12 circular connector, as can be seen in Profibus guides. Each serial port can be configured for the RS232 or RS485 protocol, and runs at standard baud rates up to 115,200 bits per second. GasLab® runs on Windows XP or higher, and free to use with our products.

Be sure to use a wire pair such as the blue and blue/white pair for the data lines and a third wire or pair twisted together for the common connection. In its simplest form, a pair of converters from RS232 to RS422 (and back again) can be used to form an "RS232 extension cord." Data rates of up to 100K bits / second and distances up to 4000 Ft. A 2-wire synchronous IIC (Inter-IC) bus provides multi-drop signaling at rates up to 100 Kbaud. To solve the "data collision" problem often present in multi-drop networks hardware units (converters, repeaters, micro-processor controls) can be constructed to remain in a receive mode until they are ready to transmit data. They translate the bit-by-bit data on the serial cable into bytes of data that can be interpreted by the operating system or by your application program. HP is a company that makes and market particular hardware / software for speciallist application. In general if you are not connected to a modem the handshaking lines can present a lot of problems if not disabled in software or accounted for in the hardware (loop-back or pulled-up). Without termination resistors, signal reflections off the unterminated end of the cable can cause data corruption.

Interoperability of even similar devices from different manufacturers is not assured by compliance with the signal levels alone. The RS232 signals are represented by voltage levels with respect to a system common (power / logic ground). The "idle" state (MARK) has the signal level negative with respect to common, and the "active" state (SPACE) has the signal level positive with respect to common. It also defines three generator interface points (signal lines); A, B and C. The data is transmitted on A and B. C is a ground reference. RS485 is in its design a different interface from RS232. Of the processor’s three synchronous SPI (Serial Peripheral Interface) ports, two are available for inter-processor communications on multi-processor systems, and the third is brought out to the Wildcard expansion bus. Any number of characters can be sent, and the transmitter will automatically re-trigger with each new character (or in many cases a "bit-oriented" timing scheme is used in conjunction with network biasing for fully automatic operation, including any Baud rate and/or any communications specification, eg. Ideally, the two ends of the cable will have a termination resistor connected across the two wires. RTS (Request to send) does have some utility in certain applications.

This page describes the serial ports and how to use them for instrument control and automation applications. All of the serial ports are supported by pre-coded C-language software drivers that make it easy to exchange data. The termination also includes pull up and pull down resistors to establish fail-safe bias for each data wire for the case when the lines are not being driven by any device. Two wire telephone cables will not function because in that case the ground signal levels on both sides will be floating. It is NOT necessary to introduce long delays in a network to avoid "data collisions." Because delays are NOT required, networks can be constructed, that will utilize the data communications bandwidth with up to 100% through put. Electronic data communications between elements will generally fall into two broad categories: single-ended and differential. The Electronics Industry Association (EIA) has produced standards for RS485, RS422, RS232, and RS423 that deal with data communications. While the standards bring uniformity to data communications, many areas are not specifically covered and remain as "gray areas" for the user to discover (usually during installation) on his own. Also, RS485 drivers are able to withstand "data collisions" (bus contention) problems and bus fault conditions.