Connecting Kern balances to PC

So we got an idea in the lab: Let's monitor how a sample dries out by continuously measuring its weight. Because all the balances were taken, we bought two new cheap balances from Kern. They were cheap but they also had a pretty decent resolution of 1 mg. Not bad for 300 €. We thought. But you know what they say, you get what you are paying for.
The balances (series PFB) have RS-232, aka serial port connector so we bought a RS-232 to USB adaptor based on one of the FTDI chips. We connected them, used Realterm for communication and guess what: nothing happened. We tried about five different adaptors we could find lying around the lab but none of them worked. Even more strange was that our analytical Ohaus balances were working fine with the adaptors. So the problem was somewhere within the new Kerns. The manual did not help much. The company is a german one but I suspect the work has been done far more to the east. The english in the manual somehow has a dodgy feeling. The manual did not help us at all. Especially the parts of menu which were documented in the manual as "not documented". Thank you very much Kern!

Partly desperate and partly angry we decided to buy the original Kern software and cable. For a hefty 175 €. We thought that if it wouldn't work, we return it all as defective and if it would work we have to find out why.
After many weeks, a CD and a cable arrived. Earlier, I said that you get what you pay for. But sometimes, you get far, far less. The Kern original balance software is a piece of shit. Honestly, it looks horrible as if some high-school guy made that over a weekend (after getting pretty high on Friday). And the cable was a simple RS-232 to RS-232 cable. Not even a USB we asked for. But it worked with our RS-232 to USB adaptor and now it was time to do a little hacking and find out why.

Well, have you ever heard of a null modem? In the times when emails were delivered by homing pigeons, there was a machine called a teleprinter. I have no idea what that was and I don't want to know but apparently, to connect them to modems, you had to use a null modem cable. Which is a crosslinked RS-232 cable. And for some nostalgic reasons, our Kerns are using this null modem thing. So the only trick was that the TX and RX lines had to be crosslinked in the cable. Damn you, Kern! I found it after disassembling the cable, which itself costs an outrageous 45 €. Finally, for the second balances, we ordered a cable and two connectors on Conrad and assembled the second cable for about 5 €.

But that shitty day wasn't over yet. We decided not to use the Kern software (which we practically wrote off) but rather Realterm which was now working. So we set Realterm to send weight command (ASCII "w") every 10 seconds and log the response so over a few days, we would get the drying curve. But every few hours, the balances with the non-original cable tared themselves. We had no idea why. The cable was the very same that Kern uses and the connectors too. The only limitation RS-232 should have is a 2500pF capacitance which we would hardly exceed with two meters of cable. But to be sure, I salvaged a CAT5 UTP ethernet patch cable and used one of the twisted pairs for the TX/RX and one cable for ground. Also, I trimmed the cable to 30 centimeters. The situation improved, the balances stopped taring themselves but every now and then, there is a missing measurement. The balances just do not respond sometimes. After analysing the file created by balances with the original cable, I found out that they too have some missing measurements. For me, it looks like the communication feature is really shitty. These Kerns are good as budget lab balances but only if you don't need the connectivity.


  1. RS232 interfaces can be a bit tricky if you don't have experience with them.

    The pinout is documented in the manual (at least for some of their scales). Just bad that they don't document if it's DTE or DCE...
    It can also be determined by measuring the pin voltages because on RS232 the sending line (TxD in DTE view) has the negative signal voltage -3..-15 V when not sending while RxD has 0 V.

    You might want to check the signal voltage with a multimeter or better an oscilloscope. If it is very low, some other RS232 to USB adaptor (or an amplifier) might make it work more stable.

    1. Yep, you're absolutely right. We could have investigated it more but we took the easy route and just paid some 160 € to get the original software and cable. Which definitely was and is a total ripoff. Later, we made our own cables for a fraction of the cost of the original… The Kern original software is crappy so we ended up with Realterm. We can live with the taring issue, the true weight can be calculated from the recorded data but yes, I might check the levels on an oscilloscope. Or maybe I get the Bus Pirate. The balances itself are excellent value for the money, even if it's just a strain gauge technology.
      Financial support from the Czech Science Foundation is gratefully acknowledged, sorry we wasted 160 € on shitty software and overpriced cables. It could have been worse though: http://arstechnica.com/staff/2015/02/to-the-audiophile-this-10000-ethernet-cable-apparently-makes-sense/