Unslotted Csma/Ca - Mantracourt T24-BSi Technical Manual

Unslotted CSMA/CA

Most of the T24 range of devices will allow you to turn off the CSMA (Carrier Sense – Multiple Access).
This is recommended to be turned on but in some circumstances turning it off will increase data rate and reduce
latency. For example a single deice transmits at 200Hz. With no other device on this channel CSMA can be
disabled giving a more accurate 200Hz transmission.
CSMA is implemented to reduce the collisions between packets from different devices.
With CSMA Disabled
When a device wants to transmit it checks the channel to see if another device is transmitting. If not then the
transmission takes place immediately. If the channel is busy then the transmission will occur as soon as the
channel has been detected as clear.
With CSMA Enabled
When a device wants to transmit it checks the channel to see if another device is transmitting. If the channel is
busy then we wait until it is clear.
Now we back off for a random period.
The first time we back off one of the following periods is randomly selected:
320uS 640uS 960uS
If the channel is now clear then the transmission takes place.
If the channel is busy then a new random backoff period is selected from the following:
320uS 640uS 960uS
If the channel is now clear then the transmission takes place.
If the channel is busy then a new random backoff period is selected from the wider range and the procedure
repeated.
Certain packets are transmitted as if CSMA is disabled regardless of the device setting. These include the
sleep/wake packets and responses to requests.
Data Provider Control Interface DPCI (
When acquisition modules are operating in low power mode it is not easy to communicate using the full
read/write packets as most of the time the module is asleep.
Also in some cases the consumer of the data only knows the Data Tag from the Data Provider packet and does
not know the ID of the sending module.
Therefore there exists a control interface within the Data Provider packet scheme whereby modules such as a
handheld or a PC can perform rudimentary control on an acquisition module while knowing no more than the
modules Data Tag.
There are two ways to utilise this interface. Manual Implementation where you control sending the Data
Provider packets yourself or Base Station Assisted whereby the base station handles the sending of the packets.
Manual Implementation
Data received in a Data Provider packet has a Data Tag. We reuse that Data Tag to send a Data Provider packet
back to the data provider module to perform a function.
So to control the provider module we simply send a Data Provider packet using the same Data Tag but containing
data of type 0xFF and the data consists of a single Function byte which has fixed functionality depending on its
value. The status byte is not used and may be left at zero.
Function Byte Values
Value Function
0 None
1 SLEEP
2 PAUSE
3 STAYAWAKE
4 CONTINUE
5 DOSYSTEMZERO
6 REMOVESYSTEMZERO
79 Mantracourt Electronics Limited T24 Technical Manual
1.28mS 1.600mS 1.920mS 2.240mS
)
Advanced