From 71ea8a1f9f7295bf58b8859c6bd4dfa62c1f48fb Mon Sep 17 00:00:00 2001 From: thomasabishop Date: Mon, 15 Dec 2025 18:29:19 +0000 Subject: [PATCH] notes on serial devices --- zk/Bus.md | 16 +++------------- zk/mosquitto_mqtt_client.md | 8 ++++++++ 2 files changed, 11 insertions(+), 13 deletions(-) diff --git a/zk/Bus.md b/zk/Bus.md index cb3ec1a..ed0825f 100644 --- a/zk/Bus.md +++ b/zk/Bus.md @@ -4,8 +4,6 @@ tags: - computer-architecture --- -# Bus - A bus is a communication system that transfers data between components inside a computer, or between computers. @@ -35,19 +33,11 @@ can mean: There is a distinction between buses that use **serial** and buses that use **parallel transmission**. They are distinguished by the way in which the bits -are transferred from one point to another, along the bus. - -Serial Transmission is the type of transmission in which a single communication -link is used to transfer the data from an end to another. On other hand Parallel -Transmission is the transmission in which multiple parallel links are used that -transmit each bit of data simultaneously. - -![Serial transmission](../img/serial-transmission.jpg) - -![Parallel transmission](../img/parallel-transmission.jpg) +are transferred from one point to another, along the bus. See +[Serial transmission](./Serial_transmission.md) - Serial buses are cheaper to implement than parallel buses -- Serial buses operate at greater [latency](Bus.md#latency) than parallel buses +- Serial buses operate at greater latency than parallel buses ## Latency diff --git a/zk/mosquitto_mqtt_client.md b/zk/mosquitto_mqtt_client.md index 5bfc802..f49c3a7 100644 --- a/zk/mosquitto_mqtt_client.md +++ b/zk/mosquitto_mqtt_client.md @@ -65,3 +65,11 @@ This registers on the broker device: Client (null) received PUBLISH (d0, q0, r0, m0, 'test_topic', ... (11 bytes)) hello world ``` + +## Topics are inherently ephemeral + +Once you exit from `mosquitto_sub` the topic ceases to exist. You can't then +relisten to the topic later as it hasn't persisted. + +Topics are ephemeral routing labels, not persistent database entities. Think of +them more like radio frequencies. They only exist whilst they are transmitting.