What Can you Expect? —A Data Collection Project Using Linux
Now that I had the port cleared and set to the right communication parameters and the cable hooked up, I was ready to talk to the meter.
However, when I hooked it up to the serial port on the Linux box, I got no outputs. Luckily, I have a serial breakoutbox, a piece of test equipment that has a two-color LED for each signal, switches to disconnect signals and sockets to jump them together. This is plugged in between the computer port and the piece of equipment you are trying to diagnose. My inexpensive box lights the LEDs red for negative voltages and green for positive voltages.
After much probing and watching the serial breakout box, I discovered that the meter depended on having the RTS (request to send) signal stay low to provide the negative voltage for the meter's output drive circuit. Without RTS low, the meter's TXD (transmitted data) line wouldn't work. Normally when you open a port, both the RTS and the DTR (data terminal ready) lines go high.
Now, how do you control the modem control lines on a serial port? This is where having access to the source code for the serial drivers, and other utilities truly helped. If this were just a DOS application (single user, single tasking), it would be simple to read the ACE's (asynchronous communication element) control register, set the right bit, and write the data back out to the port. Since user-space programs can't write directly to system devices, I had to figure out how to tell the device driver to manipulate the RTS line. After much searching, I found a UNIX serial support site, which led to a serial utility site, which had a utility that I could hack to do what I wanted. I'm not a super C programmer, but this was just what I needed to give me the clues on how to operate the ioctl function of the serial driver. I hacked up a couple of programs: clrrts.c to clear the RTS line, and modctl.c, which can either set or clear RTS or DTR on a serial port. The source of clrrts.c and modctl.c can be found in the archive file ftp.linuxjournal.com/pub/lj/listings/issue68/3357.tgz.
During my earlier sessions with Expect, I discovered a little hitch with Expect and cron. The Expect version 5.25 delivered with Debian 2.0 stable (libc6) will not spawn processes when run by cron. The Expect 5.19 on Debian 1.3 (libc5) works fine. I reported a bug to the Debian maintainers to learn that it might be a while until the libc6 issues were fixed. I worked around the problem by manually installing the Expect 5.19 executable and Tcl 7.4 support libraries, from my Debian 1.3 system to the 2.0 server, which already had the general libc5 support libraries, to support another libc5 package I was running.
The meter has an auto-shutoff feature, which can't be disabled. It shuts the meter down if more than ten minutes go by with no activity. Clearly, this wasn't very good for long-term data collection. To fix this, I added some code to the Expect script to define how many times per hour I wanted the data logged, and set cron to read the meter once per minute. This keeps the meter on, but avoids having a huge log. The crontab line that runs rddmm.exp is:
* * * * * /usr/bin/expect5.19 /root/rddmm.exp
A couple of things showed up after a reboot. I discovered the Expect script was timing out, since the meter was not responding. Two things came out of this. The first was some interesting things that happen when you attempt to change certain stty parameters, and the serial port cable does not have connections to the modem control input lines: CTS (clear to send), DSR (data set ready) and DCD (data carrier detect). Basically, the port gets stuck. Since the cable that came with the meter left CTS, DSR and DCD open, and I did not want to modify the cable, I figured out which stty parameters not to use: hupcl and crtscts. I had placed hupcl in the original stty settings for the port while sorting out the RTS low requirement. The port had accepted the hupcl setting, because at the time the command was issued, I had the serial breakout box on the port and used the jumpers to wrap around the modem control signals. But when the meter cable alone was connected to the port, the lack of the feedback signals CTS, DSR and DCD caused hupcl to hang the port. This didn't show up until reboot.
Second, I needed to set “raw” mode on the serial port, the default as booted parameters are set to “cooked” which translates returns to newlines. This prevented the Expect script from seeing the \r at the end of the response. These changes were also incorporated into the Expect script.
The Expect script, rddmm.exp, with the lines numbered for reference, is included in the archive file along with a line-by-line explanation of the code.