Multi-Drop RS232

David Thomson, Original Code Consulting - 1990’s

The reason I started this blog was because I recently came across several very interesting examples of communication protocols in commercial instruments I had to deal with. But this first post reaches way back several decades, to when I was fairly new to LabVIEW. At the time, I knew that RS232 had no electrical support for multi-drop configurations, but I found several instances where this had been done anyway. One situation was that on numerous research aircraft, it was common to broadcast navigation data on a RS232 line and connect as many instrument computers as listeners as desired. I'm not sure how many listeners would take to cause problems, but this was generally pretty successful. Luckily, Ethernet is now replacing serial in many instances of this situation. The other place I ran across this was when dealing with a particular brand of flow controllers. (If other people know this to be common with other brands and other similar types of industrial instrumentation, please let us know.) In any case, these flow controllers could be purchased with RS232 or RS485 interfaces. But in both cases, they were intended to support multi-drop networks of devices. In fact, the company sells break-out boxes for the RS232 version of their devices that allow one to plug as many as nine flow controllers into the same RS232 bus. Since the devices are addressed, there is no real problem from a software perspective, but electrically, the RS232 spec has no guarantee that this will work. The "approximately" 50 foot RS232 cable limit can also come into play. In one instance a customer needed 3 to 5 flow controllers on the same bus, but they were not physically close together. Luckily I was aware of the potential issue and gave them a heads-up early in the project. We were able to get 3 of the flow controllers working on the same bus, each with 10 to 20 feet of cable, but adding a fourth was unsuccessful, necessitating a second bus for the final units.