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Brainboxes Guide to RS232 & RS422/485 Serial Communications
RS232 StandardRS232 is the best-known industry standard providing asynchronous serial communication using � 12 Volt logic levels over distances up to 100 feet, at speeds up to 20,000 Baud.
RS422 StandardRS422 allows one transmitter and up to 10 receivers with data transmission rates up to 10 Megabits per second for distances up to 40 feet and up to 100 Kilobits per second for distances up to 4,000 feet (1,219 metres).
To achieve good long distance noise immunity, 2 wires are used to carry each signal, configured as a twisted pair of cables. The TXD pair and RXD pair are used to carry the data whilst the RTS pair and CTS pair lines are used for handshaking. Thus 2 twisted pairs are used without handshaking and 4 twisted pair cable is used with handshaking.
RS485 StandardRS485, based on the RS422 standard allows up to 32 driver/receivers pairs on a party line data bus. Whilst only one of these should be transmitting data at any time, the rest can all simultaneously listen to the data. Handshaking is performed by a software protocol. Two twisted pairs form a full duplex system. Often only one twisted pair cable is used as the TXD and RXD lines are tied together; this is known as half duplex mode. Traditionally, the RTS signal is used as the party line control to tristate the PC's transmitter, but in multitasking operating systems the only reliable method is hardware autogating.
New Universal PCI Serial ConnectivityBrainboxes 'New Universal PCI Serial Range' meets the new slot standard forPCI serial cards that is rapidly being adapted by many PC manufacturers.
PCI 2.3 requires that all plug-in cards support 3.3V signalling, howevermany systems still use 5V signalling. To accommodate both signalling typesour new range of Universal PCI is compatible with both 3.3V and 5V. Whatthis means is that Brainboxes Universal PCI serial cards can be used invirtually any PC system available.
What makes our 'New Universal PCI' range unique is that all our new cardsare using Brainboxes own BB16PCI958 industry leading UART, which is a near1-chip solution that only requires line drivers.New Universal PCI Serial Connectivity
Brainboxes 'New Universal PCI Serial Range' meets the new slot standard forPCI serial cards that is rapidly being adapted by many PC manufacturers.
PCI 2.3 requires that all plug-in cards support 3.3V signalling, howevermany systems still use 5V signalling. To accommodate both signalling typesour new range of Universal PCI is compatible with both 3.3V and 5V. Whatthis means is that Brainboxes Universal PCI serial cards can be used invirtually any PC system available.
What makes our 'New Universal PCI' range unique is that all our new cardsare using Brainboxes own BB16PCI958 industry leading UART, which is a near1-chip solution that only requires line drivers.
All our Universal PCI cards have Microsoft signed drivers (Windows 2000,Windows Server 2003 and Windows XP).
Universal PCI Serial Card Features:
* Works in both 3.3V and 5V PCI slots
* PCI 2.3 Compliant
* Speeds up to 921, 600 Baud
* PCI Power Management 1.1 compliant
* Fully Plug and Play
* Microsoft Signed Drivers - Windows 2000,Server 2003 and XP
In addition to the above, most products in the new range have greatercontrol of FIFO trigger levels, readable FIFO levels giving improvedtransfer efficiency. These features combined with greater flexibility inchoosing the baud rates available deliver improved overall performance.
AutogatingAutogating is implemented in a card's hardware to automatically detect the start and finish of data transmission. This guarantees a rapid "Receive turnaround" in half duplex operations, therefore preventing data loss. When the gating of transmitter and receiver lines are controlled through software (using RTS) in a multitasking operating system, it can sometimes take too long for a transmitter to become a receiver because the transmitter can not set RTS false quick enough. This means that when the transmitter finishes transmitting data it can potentially lose part of the response because it is unable to become a receiver in time. To overcome this problem Brainboxes RS422/485 devices have a hardware feature called autogating. This feature automatically detects that transmission is about to commence and gates the twisted pair to transmit. When the last stop bit has been transmitted the line is automatically gated to receive. This process happens very quickly at the hardware level (requires no software) and ensures that the transmitter is ready to receive a response as soon as it has finished transmitting. In detail, the driver will write bytes of data into the transmit holding register (THR) on the UART. When the driver has finished, the UART will remain in the same state for 1.5 bits then the transmit holding register empty (THRE) bit will be set, this triggers the card to gate the ouputs TX+ and TX- to become RX+ and RX-.
16550 UART16550 UARTs provide a 16 byte input and 16 byte output FIFO hardware buffer for each serial port allowing high data rates without data loss or overrun errors in todays multitasking operating systems.
FIFOA FIFO (First In First Out) is a type of buffer, where the first byte to arrive is the first to leave. Brainboxes Serial Devices all have Hardware FIFOs. FIFOs reduce the chances of data loss by 'buffering' the data. This way the device driver can then read all of the data from the FIFO in one go, whilst communication is still continuing. If you imagine someone asking you to load apples on to a lorry. If they hand you one at a time they are likely to be ready with the next apple before you have got back from the lorry. If you ask them to fill a box however, whilst you take a box they can be filling the next box. This is how a FIFO works. You use what is called a trigger level to determine at what point (how full) the buffer should be emptied by the driver; or in the analogy how full do you let the box get before taking it to the lorry. Some Brainboxes cards have the standard 16 byte FIFO on it, but some have a 64 byte FIFO and Photon cards have a 128 byte FIFO. In an application where it may not be possible to implement flow control a very large buffer with a modest receive trigger level can significantly reduce data loss.
(c)Brainboxes Limited 2003 - Acknowledgements to Brainboxes of Liverpool, who compiled these FAQs on Brainboxes Bluetooth Products. Applicable to Brainboxes Bluetooth Products.
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