uLan Driver 0.7 reviewDownload
uLan provides 9-bit message oriented communication protocol, which is transferred over RS-485 link. Characters are transferred sam
uLan provides 9-bit message oriented communication protocol, which is transferred over RS-485 link.
Characters are transferred same way as for RS-232 asynchronous transfer except parity bit, which is used to distinguish between data characters and protocol control information. A physical layer consists of one twisted pair of leads and RS-485 transceivers.
Use of 9-bit character simplifies transfer of binary data and for intelligent controllers can lower the CPU load, because of the CPU need not to care about data characters send to other node. Producers of most microcontrollers for embedded applications know that and have implemented 9-bit extension in UARTs of most of today's MCUs. There is the list below to mention some of them:
all Intel 8051 and 8096 based MCUs with UART
members of Motorola 683xx family ( 68332, 68376, ... )
Hitachi H8 microcontrollers
The driver is implemented as relatively independent layers and subsystems. Messages are prepared and received in the driver dedicated memory. This memory is divided into blocks with uniform size with atomic allocation routines.
When message is being stored into blocks, head of message with couple of data bytes is stored in the first allocated memory block. If all data cannot be stored in the first block, next blocks are allocated and linked together.
The message heads are linked in bidirectional linked lists of messages prepared for sending, processed messages and messages prepared for client notification. These lists or queues are main mechanism for transferring of messages between subsystems.
Link protocol is programmed as finite state automata with state stack, which state routines are executed by interrupt handler. State routine can return positive integer information, negative error notification or zero, which leads to wait for next interrupt. Information or error is used as input parameter when state routine is called.
When the state routine wants initiate transfer to another state routine it changes pointer to the actual state routine. If previous state routine returns nonzero value new routine is called immediately, in other case next interrupt invokes new state routine.
There is stack of callers of actual state routines which enables to constructs automata subsystems, which can be used in more places in main automata loop. Main purpose of this automata is to send or process messages coming in list of messages prepared for sending and if specified, move these messages onto list of messages prepared for client notification.
Received messages are put onto this list too. Subsystem is supervised by timeout handler, which can revitalize communication in case of die of other node. The interrupt and timeout handlers are fully SMP reentrant.
The automata subsystem uses pointers to chip driver routines for hardware port manipulation. This is only part dependent on used chip, today 82510, 16450 and OX16C950PCI. These routines can send and receive 9 bit character, connect to RS-485 line by the arbitration sequence, wait for specified time for character and initialize and close port.
File operation subsystem makes interface between OS kernel VFS and client message queues. It enables to prepare single or multi-frame messages and stores notifications of received or processed messages in clients' private state structures. This part is heavily operating system dependent.
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