This limitation is due to the fact that the transmitter is active all the time, and the line pair is set so that the TD(B) line is high during Mark state, and the TD(A) line is set low. If another transmitter output is connected to the same wire pair, and attempts to send data by setting the line pair to Space state, the first transmitter is still trying to hold the opposite state and neither can communicate. To overcome this limitation, the RS-485 mode was established, where each transmitter tri-states to a high impedance when not transmitting. See following.

RS-485 Operation

In RS-485 mode the transmit driver must be enabled during transmit (TX SD) and tri-stated to a high impedance when the data has been sent. In the 2-wire (half-duplex) mode, the receiver is enabled when not transmitting, and disabled (RX SD) during transmit (echo off).

The 3PCI2 provides Send Data (SD) Control for the RS-485 Driver and Receiver. This is hardware controlled based on the contents of the UART output buffer. When data is present, the driver is enabled, when the output buffer becomes empty, it is disabled. This automatically handles whatever baud rate is used.

The RS-485 transmitter and receiver have separate settings for 2- wire modes (TX SD, RX SD) or 4-wire (TX SD, RX ENABLE).

Example - Settings, DTR Operation, RS-485 Mode selected.

(Windows 98 shown)

Documentation Number 3PCI2-0903 Manual

Chapter 6

47

B&B Electronics Mfg Co – 707 Dayton Rd - PO Box 1040 - Ottawa IL 61350 - Ph 815-433-5100 - Fax 815-433-5104 B&B Electronics Ltd – Westlink Comm. Pk. – Oranmore, Galway, Ireland – Ph +353 91-792444 – Fax +353 91-792445

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B&B Electronics 3PCI2 manual RS-485 Operation, Example Settings, DTR Operation, RS-485 Mode selected
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3PCI2 specifications

B&B Electronics has long been a leader in the field of industrial networking solutions, and their 3PCI2 serial communication card exemplifies this expertise. The 3PCI2 is a dual-port PCI serial communications interface designed to provide PC systems with reliable and efficient connectivity to serial devices. This card is particularly useful for applications requiring multiple serial device connections, making it a favorite among developers and engineers.

One of the main features of the 3PCI2 is its ability to support two independent, programmable serial ports. These ports can handle RS-232, RS-422, or RS-485 communication, allowing for flexible connection options based on specific application needs. This versatility is essential in industries such as factory automation, transportation, and telecommunications, where different devices may require distinct communication standards.

The 3PCI2 utilizes advanced technology that ensures high-speed data transfer rates, reaching up to 115.2 kbps. This speed, combined with the card's reliable architecture, minimizes latency and ensures smooth communication between the PC and connected devices. Additionally, the use of direct memory access (DMA) capabilities enhances performance by allowing data to be transferred directly between the serial ports and memory without involving the CPU, thereby reducing system overhead.

Another notable characteristic of the 3PCI2 is its plug-and-play compatibility. Installation is straightforward, as it can be easily inserted into any available PCI slot on a standard PC motherboard. The card is also supported by a comprehensive driver package compatible with various operating systems, ensuring seamless integration into existing systems.

Furthermore, the 3PCI2 features high-quality signal integrity and noise immunity, which are critical in industrial environments where electromagnetic interference can impact performance. The card is designed with robust components to withstand harsh conditions, making it reliable for long-term use in demanding applications.

Overall, B&B Electronics’ 3PCI2 serial communication card stands out for its combination of features, versatile connectivity options, and robust performance. Its ability to provide reliable, high-speed communication in a dual-port configuration makes it an excellent choice for industries that rely on effective serial communication solutions. As technology continues to advance, the 3PCI2 remains a relevant and dependable option for developers looking to enhance their systems with reliable serial connectivity.
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