Invented by Giovanni Blu Mitolo Originally published: 20/11/2017, latest revision: 9/11/2018 Related implementation: /src/strategies/ThroughSerial/ Compliant versions: PJON v10.0 and following Released into the public domain
TSDL (Tardy Serial Data Link) is a simplex or half-duplex serial data link that supports both master-slave and multi-master modes. It supports collision avoidance, reliable frame separation and optional synchronous response to frame transmissions.
______ TX RX ______ | |-------| | |DEVICE| |DEVICE| |______|-------|______| RX TX
TSDL can be used to establish a point-to-point link between two devices if used with a bare serial link, or to support one or many to many communication using radio or RS485 transceivers.
Medium access control
TSDL operates in master-slave mode applying the request-response procedure. Being impossible to implement a carrier-sense procedure over a serial port TSDL uses variation of slotted ALOHA medium access method for multi-master mode. Before a frame transmission the serial buffer is read, if not empty ongoing communication is detected and collision avoided, if empty for a duration longer than the response time-out plus a short random time, frame transmission starts in which the packet is entirely transmitted. Of all contention based random multiple access methods, slotted ALOHA, which maximum data throughput is only 36.8% of the available bandwidth, is one of the least efficient, therefore TSDL in multi-master mode should not be used in networks where many devices often need to arbitrarily transmit data.
Before a frame transmission the communication medium is analysed, if any data is received communication is detected and collision is avoided, if logic 0 is detected for a duration longer than the response time-out plus a small random time, data is transmitted encapsulated in a SFSP (Secure Frame Separation Protocol) v1.0 frame.
A frame transmission in both master-slave and multi-master modes can be optionally followed by a synchronous response of its recipient, all devices must use the same response time-out to avoid collisions. The acknowledgment reception phase must be shorter than the response time-out to be successful.
Transmission Response _______ ______ ______ _____ _____ | START || BYTE || BYTE || END | CRC COMPUTATION | ACK | |-------||------||------||-----|-----------------|-----| | 149 || H || I || 234 | LATENCY | 6 | |_______||______||______||_____| |_____|
The required response time-out for a given application can be determined practically transmitting the longest supported frame with the farthest physical distance between the two devices. The highest interval between packet transmission and acknowledgement measured plus a small margin is the correct time-out that should exclude acknowledgement losses.
This document is automatically generated from the github repository. If you have noticed an error or an inconsistency, please report it opening an issue here
The PJON protocol handbook
The PJON 12.0 big box is a limited edition (only 500 units). It contains the PJON protocol handbook, the PJON protocol source code in CD and the stickers. This product contains everything you need to master the PJON protocol and its implementation, although it is also designed to nicely fit in your big box collection. Choosing this handy kit, you will always have PJON with you in different formats, also when not being connected to Internet. Buying PJON products you support its further development and enhancement.