notes on serial devices

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
 

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.