POS to DVR or NVR Integration: A Serial Splitter – Part 2
We’ve had a number of POS system installers wanting to take output from a POS system to a serial printer, and also send it to a DVR (Digital Video Recorder) or NVR (Network Video Recorder) over an IP connection. Integrating POS and video surveillance is a growing business, and some people are finding it a key to success with their customers. Business Solutions Magazine (June 2012) had a cover story on just this topic, pointing out how Datatek was making this type of installation a part of their business. Business Solutions covers this topic in some depth here.
In a previous blog post, we diagrammed this sort of setup and spoke in general terms about how simple this was to do — merely a matter of taking the serial TX line and the GND line from the cable running between printer and POS, tapping into them, and sending that input to the LAVA Ether-Serial Link (ESL). The ESL would send that along to the video recorder. Since the voltages involved for the ESL’s serial port (+10/-10 VDC) are enough, and the data rates for the POS printers are low (usually operating at 19200bps or 9600bps), there are no major electrical issues given normal cable lengths. The layout is like this:
“Merely a matter of taking the serial TX line and the GND line from the cable running between printer and POS, tapping into them, and sending that input to the LAVA Ether-Serial Link (ESL)” sounds damn easy. But it’s not. There are two big obstacles in the way: first, the serial lines are not clearly identified on most POS systems, and are not standard from system to system in any case; and second, getting the ground connection wrong could introduce a ground loop that could fry the Ether-Serial Link, the POS station, the printer, or any combination of these, depending on how lucky you are.
POS vendors are not particularly helpful on this, as you’ll find when you try to pry from them the pinouts of the devices they sell. Rather than coming back with the information, they will try to sell you a pricey cable, but that cable will still not split the signals in the way you need. It also doesn’t help that when the serial connectors are RJ45, the serial line designations are even more sketchy.
So here’s what you need to do:
1) Set up grounding between the ESL and the POS/printer. This is an essential first step to prevent damaging your hardware and must be left in place through this process. Since the chassis of both the printer and the POS system will be grounded, this is a good place to start. It also helps that the pinouts on the ESL are marked, right on the enclosure. You can start by taking the ground from the ESL’s serial port to the chassis of either or both of the POS system and the printer. If the POS system has a DB9 connector, the shell of that will be grounded too.
2) Set up a terminal application on the ESL to be able to monitor the serial port activity that you will be generating in Step 5.
3) Attach a wire to the RX connector in the ESL’s serial port. Again, this line is identified on the enclosure of the ESL.
4) Now you need to EXPOSE (not cut) the wires in the POS cable running from the POS to the printer. You will be tapping onto these wires one at a time to look for signal activity between the POS and the printer.
5) One by one test the connection between a wire in the POS cable and the RX signal line on the ESL by watching the terminal application you have running as you send data to the printer. The TX line coming from the POS will send data to the ESL, and that data will appear on the terminal application. This is the data line that you want.
6) At this point you want to find the GND line among those running between the POS station and the printer. Having eliminated one of the lines as the POS TX line, attach a voltmeter to the remaining lines one at a time, looking for the line with zero resistance. This is the second line that you will tap onto. Power off the devices in the system and complete the attachments, that is tap from the POS TX (printer RX) line to the ESL RX line, and from the POS/Printer GND line to the ESL GND line. You’re done.
NOTE: LAVA is considering creating a simple interface board to make this process a lot less painful. It will be essentially a breakout board that will let you easiliy plug between the POS station, printer, and ESL, and configure the correct GND and TX-RX connections without cutting wires and patching connections. It will look something like this:
Let us know what you think.