Posted: 2013-12-0201:19 PM. Last Modified: 2021-11-0710:55 PM
PicoScope Basics (Including Serial Decoding)
I originally recorded and posted this video for our Advanced BACnet Troubleshooting students, but figured some folks here may also be interested in the video.
This 11 minute video covers the basics of using a PicoScope to inspect an MS/TP segment.
For those already familiar with the PicoScope, you may still want to check out the last 4 minutes of the video. I briefly demonstrate the PicoScope software's built-in serial decoding. This allows you to decipher data from packets, and ultimately determine which controller each message came from (assuming no repeaters are in use...otherwise, you'll need to know if a controller is on your segment or on the other side of the repeater).
Token messages are the most common on a typical MS/TP channel, and the makeup of a token packet is as follows:
55 FF 00 DD SS ...
- 55 FF is the preamble--all BACnet MS/TP packets should start with these two bytes (possibly after a short pad)
- the next byte is 00 - this is the MS/TP frame type, and 00 indicates this is a token
- DD is the destination: this is the MAC address / "Infinet ID" that's going to receive this token
- SS is the source: this is the MAC address / "Infinet ID" that's sending the token
Any abnomalities in a packet as observed using the scope may indicate an issue with a specific controller (or the bus wiring between your scope and the controller), so it's particularly valuable to use serial decoding to figure out the source MAC address / "Infinet ID" of some packets. In past classes, we'd talked about trying to determine this by hand, but the software'd built-in serial decoding makes this a much easier task!
For example, if there's a break in one conductor somewhere on the bus, you may see some controllers with normal waveforms on a scope, and packets from others may show both the red and blue trace moving together during the packets. Using serial decoding, you can determine which controller addresses are on your side of the bus and which controller addresses are on the far side of the break. Using a network diagram, you can determine which span of the bus to inspect.
It has been discontinued, and the 2204 is a suitable replacement.
You don't need a super high speed scope since we're just concerned with viewing low speed serial (76.8K is low speed for o-scopes). It is important, though, to ensure you buy a scope with 2 analog channels.
UPDATE: Newark / Element14 now carries these scopes, so you can now get them shipped domestically (to the US and some other countries) without long shipping times and high shipping prices. The 2204A also includes a pair of probes, which is fantastic.