Add Serial Port
- 1 Getting Started
- 2 Attach the serial port
- 3 Single or Bi-directional Traffic?
- 4 Level shifter
- 5 Serial Port Settings
- 6 External Links
Locate J1 on the Mainboard after removing it from the case. This should be a 4 pin unpopulated header. The easiest way to find this is to first find the CPU first, and then look near the corners for J1.
Attach the serial port
The manufacturer of the Kuro filled the jumper holes for the Serial port with solder. Removing the solder from the jumper holes on the Kuro board is simple.
- Press the tip of a 15 watt soldering iron with a good sharp tip (if the tip isn’t sharp use a file to sharpen it and tin the tip) into the hole from the back of the circuit board.
- When the solder is flowing nicely, get your mouth about 1" from the front of the board and blow hard into the wet solder while at the same time removing the solder iron. You will blow the solder out of the hole leaving a hole that you can easily put a wire through.
Single or Bi-directional Traffic?
To allow one-way serial port traffic (read only) you can just add the 4 pin header and be done. To enable bi-directional serial port activity, you must also add a 10K resistor as described below.
Enable Write Mode (optional)
This step is optional, but you might find that the serial port is more useful with bi-directional traffic. To enable write support across the serial port, remove the 10K ohm pullup resistor from R75 and solder this, or another 10K ohm pullup resistor (SMD package size is 0603 if you want to buy one), to R76.
An RS232/TTL level shifter needs to be added. You can buy one or as an alternative you can make the level shifter cable yourself:
The Interface should be attached as close as possible to the board. The signals are very weak and wires as short as 2" (5 cm) have been reported to cause data loss across the serial port.
If you use the Compsys level shifter you should remove the 9 pin D connector from the shifter and solder the transmit and receive pins directly to the new header. This makes the shifter sit directly above the circuit board and makes the wires as short as possible. You will need to use 2 short (about 3/4", 2 cm) pieces of wire to connect the power and ground connections on the level shifter to the header. Next you should connect 6" (15 cm) pieces of wire to the transmit, receive and ground connections on the RS-232 side of the level shifter and route them to the 9 pin D connector which can be mounted on the rear of the Kurobox. Here's a photo of the level shifter and a photo of the connector on the back of the Kuro.
Note that the Compsys level shifter is designed to be used on a client device as opposed to a host device. As such it has the wrong gender 9 pin D connector. it also has the transmit and receive pins switched on the serial connector. This may not be an issue depending on what you plan to do with the serial port. If you plan to connect it to your computer for monitoring the console messages, you can just use a straight through cable. It will become an issue when you start connecting peripherals to it as the connector will not mate with your peripherals. You have two options in this case. the first is to buy a null modem connector to sit between the Kurobox and your peripherals. The second option is to replace the connector with the correct connector and switch the wires on pins 2 and 3.
Once you have the level shifter installed you should be able to see console messages via the serial port.
The serial port signals from the processor are only 3.3V. For proper RS-232 12V signaling, an RS-232 level shifter needs to be added. These are very common in PDA serial cables also, but can be purchased
Build your own LVTTL/RS232 or LVTTL/USB interface
- LVTTL/RS232 - Building a Custom Serial Interface
- LVTTL/USB - Use a Nokia Serial Cable on an ARM9 Linkstation
- LVTTL/USB - Use a cheap phone sync cable with the serial port
Buy your own TTL Level Shifter
- LVTTL/RS232 - MAX232 based from Futurelec - MINIRS2323V
- LVTTL/USB - FTDI chip based cable - TTL-232R-3V3
Usage considerations for the TTL-232R-3V3
A working/tested pinout/wire-color scheme is:
- Solder the 4-pin header to the board. Make sure you don't have shorts.
- You will need to switch wires on the TTL-232R-3V3 cable. Use a sharp object to lift the plastic tabs and carefully pull the wires out. Rearrange them according to the table above (black, empty, orange, yellow, empty, empty) and slide those wires back in. Tape the other wires to make sure they don't short anything.
- When plugging in the cable, make sure black aligns with GND, yellow with TXD, and orange with RXD.
- Connect the USB cable to your computer, start a terminal program with the right settings.
- Turn on the device, you should see output from the bootloader in couple seconds.
Serial Port Settings