embedded - CAN总线中SRR和RTR位的区别

Question

我对 CAN 11 位和 29 位协议(protocol)(11 位帧中的 RTR 和 29 位帧中的 SRR)感到困惑。谁能解释一下为什么我们在 29 位 can 协议(protocol)中使用 SRR 位 而不是 RTR 位 ...

Answer 1

此消息解释了原因:http://www.microchip.com/forums/m376934.aspx

CAN bus considers the 11 bit indentifiers to have a higher priority than the 29 bit identifiers

(因为“11”和“29”这两种“协议(protocol)”可以共存于同一总线)。因此:

In the case of a 29 bit extended frame, the RTR bit has been moved to the end of the message so it doesn't play in the priority bitwise arbitration of the 29 bit IDs. That's why the Standard frame RTR now defined as SRR is transmitted as recessive. It prevents a 29 bit RTR message from having a higher priority than a non RTR message.

...此页面也是如此:http://hem.bredband.net/stafni/developer/CAN.htm

A Substitute Remote Request (SRR) bit is included in the Arbitration Field. The SRR bit is always transmitted as a recessive bit to ensure that, in the case of arbitration between a Standard Data Frame and an Extended Data Frame, the Standard Data Frame will always have priority if both messages have the same base (11 bit) identifier.