Hardware/Software Serial port¶
Medium: Hardware/Software Serial port | Pins used: 1 or 2
With ThroughSerial data link layer strategy, PJON can run through a software emulated or hardware Serial port. Thanks to this choice it is possible to leverage of virtually all the arduino compatible serial transceivers, like RS485, radio or infrared modules, still having PJON unchanged on top. Take a look at the video introduction for a brief showcase of its features.
Why PJON over Serial?¶
Serial communication is an hardware integrated or software emulated data communication that can reach very fast communication speed but it includes only the data link layer; adding PJON on top of the Serial data link layer it is possible to leverage of the PJON protocol layer features like acknowledge, addressing, multiplexing, packet handling, 8-bit CRC and traffic control.
How to use ThroughSerial¶
Pass the ThroughSerial type as PJON template parameter to instantiate a PJON object ready to communicate through this Strategy.
PJON<ThroughSerial> bus; // 2 pin over-sampled data link layer
Call the begin method on the Serial or SoftwareSerial object you want to use for PJON communication and pass it to the set_serial method:
/* Set 100 milliseconds is the maximum timeframe between every receive call
in any of the connected devices. If this timeframe is in average exceeded
by some of the connected devices, communication reliability could drop
or be disrupted. (before PJON.h inclusion) */
#define TS_MAX_BYTE_TIME 1000000
/* Set 0.5 milliseconds as the minimum timeframe of free port before transmitting
(before PJON.h inclusion) */
#define TS_FREE_TIME_BEFORE_START 500
/* This timing configuration is ok for a master-slave setup, but could lead to
collisions if used in a multi-master setup.
If using ThroughSerial in a multi-master setup with synchronous acknowledgment
NEVER set TS_FREE_TIME_BEFORE_START < TS_MAX_BYTE_TIME or a device could start
transmitting while a couple is still exchanging an acknowledge */
#define TS_MAX_BYTE_TIME 100000
#define TS_FREE_TIME_BEFORE_START 110000
/* Above is shown multi-master compatible setup able to receive a synchronous
acknowledgment with a maximum delay 100 milliseconds. Channel analysis before
transmission is set to 110 milliseconds to avoid collisions.
Which is the correct value for your setup depends on the maximum average time
interval between every receive call in your system. TS_MAX_BYTE_TIME
should be around the same duration. So in a sketch where there is only a
delay(10) between every receive call at least 10000 should be the correct
value for TS_MAX_BYTE_TIME.
If your tasks timing are long and a satisfactory setup can't be reached
consider to drop the use of the synchronous acknowledge and start using the
asynchronous acknowledgment instead. */
/* Set the back-off exponential degree */
#define TS_BACK_OFF_DEGREE 4
/* Set the maximum sending attempts */
#define TS_MAX_ATTEMPTS 20
/* The values set above are the default producing a 3.2 seconds
back-off timeout with 20 attempts. Higher TS_MAX_ATTEMPTS to higher
the back-off timeout, higher TS_BACK_OFF_DEGREE to higher the interval
between every attempt. */
#include <PJON.h>
PJON<SoftwareBitBang> bus;
void setup() {
Serial.begin(9600);
bus.strategy.set_serial(&Serial);
}
For a simple use with RS485 serial modules a transmission enable pin setter has been added:
bus.strategy.set_enable_RS485_pin(11);
All the other necessary information is present in the general Documentation.
Known issues¶
- Being PJON not an interrupt driven, its communication can be affected and potentially disrupted by long delays added in the user’s sketch. Try to reduce as possible the interval between every
receivecall. A delay between every receive call higher than 100 millisseconds can disurpt the synchronous acknowledment transmission phase, higherTS_MAX_BYTE_TIMEinThroughSerial.hif necessary.