Why Ethernet?
The Linux PC Robot uses standard TCP/IP networking over Ethernet for remote control. A separate workstation running the joysender client sends joystick commands across the network to the robot's control daemon. Because the robot carries a full desktop PC with a standard NIC, there is no special wireless module needed for wired testing — just a cable.
Making your own cables lets you create exactly the length you need (no coiled slack on a mobile robot), replace damaged connectors in the field, and understand the wiring — useful for debugging connectivity problems.
What You Need
- Cat 5e or Cat 6 unshielded twisted-pair (UTP) cable — bulk spool
- RJ-45 modular plugs (buy extras; practice makes perfect)
- RJ-45 crimping tool
- Wire stripper
- Cable tester (optional but strongly recommended)
T568B Wiring Standard
There are two EIA/TIA wiring standards for Ethernet — T568A and T568B. T568B is the most common in North America. For a straight-through patch cable (robot to switch or router), wire both ends identically to T568B. For a crossover cable (robot directly to workstation without a switch), wire one end T568A and the other T568B.
Most modern network equipment including the NICs on typical desktop PCs support Auto-MDI/MDIX — they detect the cable type and adjust automatically — so a straight-through cable works in virtually all situations today.
| Pin | T568B Color | T568A Color | Signal |
|---|---|---|---|
| 1 | White/Orange | White/Green | TX+ |
| 2 | Orange | Green | TX− |
| 3 | White/Green | White/Orange | RX+ |
| 4 | Blue | Blue | Unused (PoE) |
| 5 | White/Blue | White/Blue | Unused (PoE) |
| 6 | Green | Orange | RX− |
| 7 | White/Brown | White/Brown | Unused |
| 8 | Brown | Brown | Unused |
Step-by-Step: Crimping an RJ-45 Connector
- Cut the cable to the desired length with a clean, square cut.
- Strip approximately 1.5 inches (38 mm) of the outer jacket using the wire stripper. Be careful not to nick the insulation on the inner conductors.
- Untwist the four pairs and straighten the individual wires.
- Arrange the wires in T568B order (left to right with the clip facing down): White/Orange, Orange, White/Green, Blue, White/Blue, Green, White/Brown, Brown.
- Hold the wires flat and parallel, then trim them to approximately 0.5 inches (13 mm) from the jacket edge — all at the same length.
- Slide the wires into the RJ-45 plug, maintaining the correct order. Each wire should reach the front of the plug and be visible through the clear plastic.
- Confirm all eight conductors are properly seated, then crimp firmly with the RJ-45 crimping tool.
- Repeat for the other end.
Testing the Cable
A basic cable tester lights up LEDs 1 through 8 in sequence on both the local and remote units. If all eight LEDs light in order on both ends, the cable is wired correctly. Common failures:
- Open: a wire is not making contact — re-crimp or re-cut and start again.
- Short: two wires are touching — caused by nicked insulation or wires crossing inside the plug.
- Miswire: wires are in wrong positions — check your color order against the T568B standard.
- Split pair: wires from different pairs are intermixed — causes crosstalk and degraded performance at higher speeds.
Cable Considerations for a Mobile Robot
On a moving robot, cables take more stress than in a static installation. A few tips:
- Use a short tether (2–5 metres) during initial development so the robot stays within reach of an emergency stop.
- Strain-relieve the cable where it enters the robot chassis — a simple cable tie anchor prevents the connector from being yanked out during movement.
- Cat 5e is flexible enough for most robot applications. Avoid Cat 7 or shielded cable as it is stiffer and harder to route.
- For truly untethered operation, add a Wi-Fi adapter (USB or PCI) to the robot PC and use the same joysender TCP protocol over a wireless LAN.