Connecting to the RS Media Linux Console PortPosted by milw on Monday, 17 March 2008
Here's a quick how-to for patching in to the Linux console port on the RS Media. Thanks to Nocturnal's careful dissection and documentation of the boards inside the RSM, we have easy access to a 3.3 V serial port on the media board, which is located under the panel behind RSMs head. Before you get started, you need to decide on what type of interface you would like to use between your computer and the RSM. Since this port is run at 3.3V on the RSM, you can't simply hook up an RS-232 cable to your computer's serial port - the 12 volt signals are guaranteed to fry your RSM! Nocturnal's approach was to use a Bluetooth wireless serial port module, which avoids the need for voltage adjustments. By the way, I highly encourage you to read Nocturnal's account, and pay heed to his warnings!
Since I have been working with the Parallax Propellor chip (a 3.3V microcontroller), I already had on hand the Prop Plug, a $24 USB to 3.3V serial adaptor.
To use this with RSM, only 3 wires need to be soldered- Rx, Tx and VSS (ground). For convenience, use a bit of 0.1" pin header material and some 3 conductor wire in your RSM; this will allow easy attaching to the Prop Plug.
Start by disassembling the RSM, removing the 8 screws from the back of the torso shell. Gently pry the shell apart, taking care not to yank on the cable connecting the media board to the main board. Gently detach the connectors and the ribbon cables; the ribbon cables are held in by a small plastic bail that can be pried loose to release the cable.
Arrange the media board so the SD card slot is to your right, and the usb connector is to your left, with the processor side down. The serial port connections are a set of 6 solder pads to the left side of the board labelled J37, each pad is ~1 mm wide by ~3 mm long.
The top most pad (closest to edge of board) corresponds to pin 6 in Nocturnals pinout, this the ground (VSS; my green wire). Pin 1 corresponds to the pad closest to you, this is VDD (3.3V) but not needed for this connection. Pin 2 is the RSM's Rx, which needs to be wired to the Prop Plug Tx (my yellow wire); and Pin 3 is the RSMs Tx which goes to the Prop Plug Rx (my red wire). Here's how I laid out my connections, using 3 stranded wire:
And here's the other end of the cable soldered to the pads, and in place behind RSM's head. (yes, I actually soldered with the board in place; it is tight but doable if you are careful!). Double check your soldering and wire positions before connecting anything, lest the magic smoke escape!
Now it's time to configure your communication program. I'm using HyperTerminal on WinXP, so you'll have to alter these instructions for your system. First, figure out what COM port your USB adaptor is on. Open the Device Manager and expand the Ports (COM and LPT) node:
Mine is on COM7, so now fire up HyperTerminal and create a new connection. Under the 'Connect Using' picker, choose your USB com port, then hit Configure:
Set your port as shown above- 115200 baud, 8 bits, No parity, 1 Stop, Flow Control None.
Now for the moment of truth: Connect your cables to the RSM and open the connection in your terminal program. While watching your terminal, turn RSM on. You should immediately see the Linux boot sequence, it starts like this (I cropped the image after capturing a screenshot):
If you see similar text appearing on your screen, congratulations! You can now explore the inner workings of your RSM!
If your screen is blank, it is time for some troubleshooting. First- tap a few keys in your terminal window while looking at the Prop Plug- you should see a blue LED flash for every character. If you don't see anything, you first need to check that your terminal program is talking to the correct port that the Prop Plug is on.
If your Plug is OK and flashing, then double check all of your wiring- no shorts, all wires going to the proper places? Try booting the RSM again, and watch for a red LED flashing on the Prop Plug- this is the data coming from the RSM. If you see the red flashing but still don't have any text appearing on your terminal screen, double check the communication parameters.
If after all that, you are still having trouble, just drop a line on the forums and ask for help- we'll pitch in and get you up and running!