Taken directly from the Saleae Support website.
Decode RS-232, RS-485, & RS-422
The new Saleae Logic products (Logic 4, Logic 8, Logic Pro 8, Logic Pro 16) can be used to read and decode RS-232, RS-485, and RS-422 data up to +/- 25V.
Our older Logic products (Original Logic, Logic16) have a 0V to 5V absolute maximum range. Therefore, neither product can be used to measure signals outside of this range. That limits its usage for directly recording RS-232, RS-485, and RS-422, which exceed this range in many cases
Precautions to Avoid Damage to Saleae Logic
These protocols typically operate at higher voltages, sometimes up to +/- 25V. For this reason, there are several precautions and setup requirements to consider beforehand, which you can read more about below.
Decode Differential and High Voltage Data
Use the Async Serial Analyzer
RS-232, RS-485, and RS-422 transmit data via asynchronous serial communication. For this reason, you can use our Async Serial Analyzer to decode the data. You can view our setup guide below.
Async Serial Analyzer - User Guide
RS-232, RS-485, and RS-422
- All four of the new Saleae devices include over-voltage protection to +/- 25 volts. It’s perfectly safe to connect any signal up to this range directly to its inputs
- The original Logic and Logic16 cannot be directly connected to these signals. They also have over-voltage protection, but it was not designed to be used with these signals continuously. Either use a voltage divider or a dedicated line transceiver/receiver to convert these signals to CMOS/TTL levels.
- When recording any of these signals, it is important to properly connect the ground from the logic analyzer to either the ground of the transmitter or the receiver. Do not connect ground to one of the signal wires, as this could damage your equipment. Neither RS-232, RS-485, nor RS-422 are isolated, which means that all transmitters and receivers on the bus must share the same ground to operate. In most cases, a ground wire is included in the bus wiring, which could be tapped with Logic. Otherwise, you will need to find another ground connection nearby to connect to.
- For RS-422 and RS-485, it's generally not necessary to record both the + and - signals. In most cases, recording only one-half of the differential pair is sufficient. However, it's usually a good idea to record both sides, at least at first, to evaluate the differences in the recording quality of the two signals. Because the threshold voltage of the logic analyzer is not matched properly for differential signals, it's likely that one side of the differential pair will have a cleaner recording than the other.
And no, this is far from being a foolish question.