I just wasted an hour tracking down a problem with an application that controls Sharp projectors via a RS-232 (serial) connection. On sending the projectors a “POWR 1″ code to turn the projector on, we were getting “ERR” in response. The problem ended up being that the bulb on the projector was burnt out, so the projector was intelligently unable to turn itself on.
This is one case where a simple problem turned out to be a fairly annoying one.
My coolest project from work as of late has been to write a program to control the Sharp LCD projectors in RIT’s Software Engineering department through a serial interface. I really like writing code that interfaces with hardware, so this was one I enjoyed doing.
One thing I learned quickly is that debugging serial applications is hard. When my code wasn’t working, I didn’t know if it was because of the hardware, the software, or the command messages I was sending. If serial consoles or line printers were still widely available, it wouldn’t be hard to see what was going on. Since they aren’t, an alternate solution was devised utilizing GNU screen.
With two Linux boxes running Debian, I set one up with my code to send data from, and the other with screen attached to the serial device at /dev/ttyS0. Screen functions normally, but with the added bonus of displaying messages recieved from the serial device, and when entered, sending messages to the device. Screen makes it possible to visualize serial communications and send test messages without having to write or modify any code, making it an infinitely useful tool in debugging serial applications.
My bug turned out to be twofold: one hardware, and one software. The first was that the serial connection to the projector was not complete; there is a break somewhere in the wall. The second was that the manual states only that the command message be followed by a newline character. I was sending Unix newlines, like ‘\n’. The projector was written expecting Windows style newlines, which is a newline prepended by a carriage return, or ‘\r\n’. This simple fix, which I would have never stumbled over without the help of screen, was the source of my software based problems.
