FRC OI

The OI is cool, mainly because it has lots of switches and flashing lights and can be connected to even more switches and lights. It's a useful input box since it has many analog and digital inputs and is easy to connect to anything with a serial port.

In order to use the OI for your own nefarious purposes, you need to build a tether adapter. This consists of two female DB-9 connectors with pins 2, 3, and 5 wired straight through and pins 8 and 9 connected together on the OI side (it's safe to connect them on both sides since they are inputs on the computer, but don't connect them if you make a null-modem or connect them to any device that drives those pins). All other pins should be left unconnected. The tether uses normal RS-232 signal levels so no conversion circuitry is required.

My tether adapter

The two connectors on my adapter look different because they were bought from different places and have different plating. Otherwise they are identical. I used two one-inch hex standoffs to hold it together.

Tether pinout:

Tether 8 (RC tether sense) is 220 ohms to Vcc. Tether 9 (OI tether sense) is 10k to GND, diode clamp to Vcc/GND (no Ri), U3 /RE, MCU RD3/PSP3.

When tether is not present (low), U3 receiver is enabled. When tether is present (high), tether pin 9 is driven high to disable U3 receiver.

The tether data is just the 26-byte packets that would have been sent to and received from the radio if it were plugged in.

Packet formats

The dashboard spec describes only the packets sent from the RC, and it labels the CRC bytes as "Reserved". The following packet formats are based on actual observations of a 2001(?) OI and RC.

I number bytes from 0 instead of 1 (like the IFI docs) because I usually program in C and it's easier this way.

Packets sent from the OI to the RC:

Packets sent from the RC to the OI:

The unlabelled bytes in the RC packet are probably as documented by IFI, but I haven't checked them.

The RC sends back several OI inputs. I guess this is done so you can monitor both OI and RC activity from the dashboard port.

The packet number always increases by one from one packet to the next. This can be used to detect dropped packets.

The mapping from battery voltage to the value of RC byte 16 appears to be nonlinear.

The OI to RC packet format appears to be unchanged as of 2006, but the CRC has changed (see below).

The CRC

All packets sent by the OI and RC have a 16-bit CRC for error detection. Each device will ignore any packet with the wrong CRC. If you are just using the OI for input and you have a hard connection to the OI (no radios), then you can ignore the CRC. If you are using radios, then you should check the CRC to make sure the data was not corrupted in transit.

If you want to send data to the OI, you must set bytes 15 and 17 (counting from 0) of the packet to the lower and upper 8 bits of the CRC respectively.

There are many CRC implementations on the Internet, and some of them are not mathematically correct (they don't actually calculate a CRC). The OI CRC is a correct LSB-first CRC which can be calculated with this code:

#include <stdint.h>
 
 #define CRC_START	0xffff
 #define CRC_POLY	0xc6f6
 
 uint16_t crc_packet(uint8_t *data)
 {
     int i, bit;
 
     uint16_t crc = CRC_START;
     for (i = 0; i < 26; i++)
     {
         if (i == 15 || i == 17)
             continue;
 
         crc ^= data[i];
 
         for (bit = 0; bit < 8; bit++)
         {
             if (crc & 1)
             {
                 crc = (crc >> 1) ^ CRC_POLY;
             } else {
                 crc >>= 1;
             }
         }
     }
 
     return crc;
 }

The CRC was different in 2006, and was probably changed when the control system changed in 2004.

Code

Here's an example of code that talks to the OI: xmms_oi.tar.gz.

This is an XMMS plugin that uses an OI joystick port to control playback and volume. The robot feedback LEDs indicate volume. Three of the robot controller LEDs indicate playback status:

This code shows how to interpret packets received from the OI and how to build correct packets to send to the OI.

Code to dump and generate OI packets in Linux: tether.c
This is another good example. This program is useful for monitoring data from the OI or RC.