The job was simple: I needed an RS232 interface on a Raspberry Pi so that I could use it to log console output from a misbehaving router (don't ask). I could have picked up one of the millions of USB-to-RS232 converters on eBay (and run the gauntlet of FTDI's clone-bricking updates). However, I have read mixed reports of the stability of the drivers for these adapters in the Raspberry Pi kernel. Besides, there actually is a UART baked into the Pi SoC. The problem is that it only works at 3.3V logic levels so it can't be connected directly to a device that uses RS232 voltages. That's OK...I just picked up one of these and wired it to the appropriate GPIO pins on the Raspberry Pi's I/O connector
Simple, eh? No. It worked just fine talking to a PC, but when I connected it to a Cisco 1841 router, very strange things happened.
When I sent a character from the Raspberry Pi to the router (I think I might have just hit carriage-return), this absolute explosion of characters was the result
Wow...what's going on here ! Out with the oscilloscope. Here is an 'A' being sent from the Pi to the router:-
The blue trace is the Pi's TX line and the yellow trace is the Pi's RX line. So far so good. However, a few moments (about 68ms, to be exact) later, this happens:-
As far as I can see, what is happening is that when the Cisco echos back the character it received (yellow trace), there is crosstalk onto its RX (blue), so the router is seeing an "echo" of its own character (but corrupted). The router echos that character back also, sees another "phantom" character on its RX etc etc.
As I mentioned, Raspberry Pi talking to a PC works just fine. Sure enough, looking at that on the oscilloscope...
...there are little "spikes" evident on the Raspberry Pi TX line, but nothing like as bad and apparently not enough to cause a problem.
So the question was, where (and how) was the crosstalk happening ? A few possibilities:-
- Within the router itself (highly unlikely, or the router wouldn't work with a PC either and would be basically useless)
- Within the console cable (that was my first theory when was using a patch-cable, but I replaced that with a flat-8 cable which keeps the RX and TX lines separated by a few mm and that made no difference)
- On the adapter board somewhere (most likely, but why, then, did I see hardly any crosstalk at all with the RPi talking to a PC or to a switch ?)
To test the theory that the crosstalk was happening on the adapter board, I disconnected the RX from the router from the TX line on the adapter board and used to oscilloscope to watch (a) the (now-disconnected) RX line from the router and (b) the TX line on adapter board separately.
I think it is a question of voltage. The router uses a relatively high voltage on its TX line...around +/-10V. Still perfectly within the RS232 specifications (which allows for +/-12V) but higher than either a Cisco switch I tested or the PC use (both about +/-6V). It looks like the higher voltage breaks down the line-driver. Sure enough, conducting the same experiment with a switch instead of a router...
So it looks like the MAX3232 line-driver doesn't like +/- 10V on the RS232 side. It should - according to the data-sheet it should be good for +/- 25V:-
I did one more test to confirm the theory: I put a couple of back-to-back 3.3V zener diodes in between the router TX and the adapter RX. This has the effect of dropping the voltage that the adapter sees by about 4V.
Sure enough, the problem goes away (I didn't bother saving an oscilloscope trace but - trust me - it was perfect: no signal at all leaking onto the adapter TX). Its really looking like its the voltage that is causing the problem.
So at what voltage does the problem kick in? If I feed a square wave to the RX pin on RS232 side while watching what appears on the TX pin (nothing should) and gradually crank up the voltage, we should be able to find out. In each of the screen-captures below, the yellow traces is the signal being fed to the RX pin and the blue trace is what is coming out of the TX pin.
Starting with 19.2V peak-to-peak...
No problem so far. Increasing to 19.4V peak-to-peak...
...and 19.6V peak-to-peak...
...by now we would probably be seeing problems. Increasing the voltage to 20.6V peak-to-peak...
...and we would definitely be seeing problems.
If a real MAX3232 IC (mis)behaves like this, I'll eat my hat. I have looked closely at the IC on the adapter and - to my untrained eye - it looks fine (decent enough quality moulding, quality of laser etching looks OK) but I'm willing to bet that these are fakes.
I would really love to get a known-genuine MAX3232 IC from a reputable supplier and use it to replace the suspect one on the adapter, but they only seem to be available in quantities of 2500 (at about €1 each). Sorry, but I'm just not that interested. However, if someone wants to send me a few genuine MAX3232s (you will need to pinkie-swear that they are definitely the real thing) I'll be happy to do the test and report the results here.
PS: An interesting thing I learnt in the course of doing this project is that the Raspberry Pi (the Raspbian image, anyway) runs a serial console on its built-in UART. A serial terminal emulator connected at 115,200 baud will receive a login prompt through which you can log in to the Raspberry Pi. You never know when that will come in handy.