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Centralized Preamble Configuration on a Bus Transmission system

IP.com Disclosure Number: IPCOM000014860D
Original Publication Date: 1999-Dec-01
Included in the Prior Art Database: 2003-Jun-20
Document File: 2 page(s) / 67K

Publishing Venue

IBM

Abstract

Today, a number of bus interfaces use collision detection as a method of determining when a bus is busy or when two stations have started a transmission simultaneously. This is true for Ethernet adapters and for the Advanced Server Management interface (IEEE RS-485). When a collision is detected or when a station wanting to transmit sees a message transmitted on the bus, that station backs off a random amount of time before it can start a new transmission. In the case of transmissions on an RS-485 interface, a fixed number of sync characters plus a random time in between the transmission of those sync characters (preamble) is used before the start of a message. Since this fixed number of sync characters must cover the worst case, eight sync characters was chosen as the number of sync characters in which no collision would occur in the worst case scenario of data transmission. However, the transmission of sync characters and the random delay between those sync characters uses bandwidth and adds a latency delay on all data transmissions. This is especially true for small messages. A new algorithm assigns a single sync character to each active station on the bus in a centralized manner to minimize the number of sync characters and thereby, maximize the bandwidth and minimize the latency. Rs-485 bus T T

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Centralized Preamble Configuration on a Bus Transmission system

   Today, a number of bus interfaces use collision detection as a method of determining when a bus is busy or when two stations have started a transmission simultaneously. This is true for Ethernet adapters and for the Advanced Server Management interface (IEEE RS-485). When a collision is detected or when a station wanting to transmit sees a message transmitted on the bus, that station backs off a random amount of time before it can start a new transmission. In the case of transmissions on an RS-485 interface, a fixed number of sync characters plus a random time in between the transmission of those sync characters (preamble) is used before the start of a message. Since this fixed number of sync characters must cover the worst case, eight sync characters was chosen as the number of sync characters in which no collision would occur in the worst case scenario of data transmission. However, the transmission of sync characters and the random delay between those sync characters uses bandwidth and adds a latency delay on all data transmissions. This is especially true for small messages. A new algorithm assigns a single sync character to each active station on the bus in a centralized manner to minimize the number of sync characters and thereby, maximize the bandwidth and minimize the latency.

Rs-485 bus

T

T

Secondary

SP

Server

Secondary

SP

Server

Primary

SP

Secondary

SP

Server

Server

Please, refer to Figure 1 below for an illustration of the discussion that follows. The stations on the bus are assigned a station number from 0 to N-1, where N is the total number of stations (N=4 in Figure 1). The primary station is assigned station number 0. The primary station sends out a sync packet every T ms consisting of a special code (1 byte) followed by a byte with an integer value 1 through N-1 to identify a station nu...