RF Module

Serial RF Link

last updated: March 2, 2008

With this link, information that is sent to the robot's display is also sent to a nearby laptop. From the laptop, commands can be sent to the robot. The two units are almost identical. The only difference is how they're powered. On the PC side, power comes from 2 AA batteries and supplies the Laipac module, PIC and level shifter. On the robot side, 5V comes from the robot, passes through the RS232 cable, to the 3V regulator and the Laipac module and PIC

I'll be getting some boards made soon (aiming for summer of 2008). They'll be about 0.8" X 1.2" and should cost somewhere from $3 to $5 each. Let me know if you're interested getting 1 or more.

Range: (applies to version 8 only)

Outdoors:
In a parking lot with no obsticles, I had no problems at 100m. If I point the units at each other and make sure I'm not standing in the way, they could be up to 200m from each other.

Indoors:
Can't get them to fail while they're in the house. I even tried putting one unit in a metal filing cabinet!

Configuration:

Payload structure:

In versions 1 and 3, only the first byte is used

byte 0 - user data
byte 1 - user data (used only if there's data left in the UART rx buffer)
byte 2 - user data (used only if there's data left in the UART rx buffer)
byte 3 - left nibble: payload type, right nibble: # of user bytes used
byte 4 - sequence # to be acknowleged
byte 5 - sequence # being acknowleged

The source code

(same code in both units)
written in C for the CCS compiler
version UART buffers speed packet loss link indicator range bytes sent per packet lost packet
management
1 none 1Mb/s 10% no much less than version 8 1 none
3 rx 1Mb/s 10% no much less than version 8 1 none
8 rx & tx 250kb/s 0% yes 100m-200m
outside, no obsticles
1 to 3 re-sent when ack not received

This unit plugs into the serial port of a PC

This unit connects to the robots main controller

A rough wiring diagram

Parts list
Laipac TRF-2.4G 2.4GHz RF module (known as TXRX24G at Quality Kits)

This module is made by Wenshing in Taiwan. It's based on the nRF2401A RF transceiver chip from Nordic Semicondutor. Laipac imports them and covers the manufacturers name with a black blob.

top view without cover, bottom view

Quality Kits
(Kingston)
$13
breakout board

I didn't use these because I didn't know about them at the time. They would have made wiring much easier.

Spark Fun

(By the way, their website is worth spending some time at)

$1 US
PIC 16F628 (I'll upgrade to a 16F688 soon) Digikey
RS232 converter
(DCE version)
SKU#30000
HVW Tech $12
3V regulator * Digikey
4MHz resonator Digikey, Active
22uF capacitor * Digikey, Active
2.2uF capacitor * Digikey, Active
PC board ADD Electronics 85
.1uF surface mount capacitor - 0805 size Digikey
2 - 2 pin KK headers Digikey, Active
4 pin KK header Digikey, Active

* For voltage regulation. Not needed if powered by 2 AA batteries


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