You are quite right... however, times are a changing. When data rates were low one could mostly ignore transmission line effects, but now not so much. Secondly, devices are switching (edge rates) much faster than in the past and so this also creates the need for transmission line considerations (i.e. switching harmonics). Will an Oscilloscope be the magic bullet and solve your problems? Nope, but if you know when and how to use one, it can greatly assist the diagnoses and understanding of what is going on with a problem communication bus.I have never seen any controls guy ever have one of these, though I have seen some IT guys have one of those fancy Flukes that has the huge screen and the ability to perform oscilloscope functions.
Indeed, scopes can be very expensive (over $50K), but also relatively inexpensive ... a few hundred dollars. Does one need the big high end scope to troubleshoot RS485 issues? Nope... even the least expensive ones would be suitable, as the bandwidth of the RS485 is relatively modest.It just seems a bit overkill to have on the shelf at the office such a high dollar tool that will only rarely be needed.
As for its need... Would one use it every time? No, but it wouldn't hurt... If you are proficient with its use it is as simple to use as multimeter in RS485 applications. Why would I use it on fresh installs? For me it only takes a few moments and I can then see what the waveforms look like as well as ensure that minimal AC coupled noise is present. Where the device shines are in cases of weird/abnormal issues with a bus. For example I was called to a site where all was working fine up to one day... no devices were added or removed and no config/software changes where made. The problem was that one controller's (out of a total of 70) step down power transformer partially failed where one of the secondaries shorted to Earth ground. Because of how this device was designed this created an electrical path to the RS485 bus and the result was that the entire bus was acting like a huge 60 Hz antenna...and a good one at that. The result the communication signals were being heavily distorted and so retries, dropped packets, etc were causing havoc.
Your testing a approach is a good one, however, the signals are actually time varying and your DC multimeter will not provide too much information on this important content. Measuring the static voltage levels (i.e. no RS485 communication) gives you only a bit of information. Measuring the DC level on an active RS485 bus will give you an average DC level which is not very helpful. Measuring an AC voltage is also not very helpful, as this is also an average. One must remember that these signals are not strictly DC, but are time varying. It is the time varying aspect that creates issues, as electrical characteristics that could be ignored at DC can be quite influential outside of DC. An example of this is that I can connect one of my test instruments to a RS485 cable that is unconnected at the other each end (wires are open circuited). My test device will tell me within 1-3 feet where the end of the wire... whether the wire being on a spool or installed. It can also tell me where I have kinked the wire. Remember all of this is done with the opposite end unconnected or terminated... How can this be the case when the DC current flow is zero? The reason is that I am looking at the time varying content of the signal that I am injecting (via the test instrument) and the transmission line characteristics of the signal create responses (reflections) that my test instrument analyzes. If I go one step further, and connect the test device to to a bus that has RS485 devices connected, but are un-powered (i.e. no communication) I can tell you within 1-3 feet where each of the devices sit on that RS485 bus.Most of my checks on RS-485 are easily done with a Multimeter on DC volts. When working with new equipment/controllers:
- I find a good working controller and start measuring
- I take note of the voltage level(s)
- I take note if there is a rhythmic fluctuation
- If possible I get the voltage level on short runs as well as long runs to note how much drop there is
Anyway my point is that there is a lot of information in the non DC content of RS-485 and it is these issues that I see causing people the most grief.