Q: The ydoc data logger has terminals on X2 connector, to connect Modbus (RS-485 half duplex) sensors. What RS-485 network topology is supported?
A: Reading the official documentation available on modbus.org will provide detailed information about the supported network topology.
Modbus design
RS-485 half duplex or full duplex bus: Grounded shield, one or two twisted pairs, bus termination with 120 Ohm resistors, stub or input /output terminals (daysi-chain) to connect the master and devices.
- Maximum bus (cable) length: 1200 meter. (With a maximum speed of 19200 Baud).
- Maximum speed: 115200 Baud. (With maximum 12 meter bus length).
- Number of devices: 32 (31 devices and 1 Master).
- Master and Devices positions are free of choice.
RS-485 (un)supported bus topology
*Termination on both bus (or daisy-chain) ends or termination mounted in first and last device on the bus (or daisy-chain).
*Master and Device positions on the bus are not obligatory.
Topology details explained
- FULL SUPPORT: Bus topology, High speed (115200 Bd), Full bus length (1200 meters), Ground shielded twisted pair(s), Stub lenght (< 1meter), Termination on both (bus) ends.
- FULL SUPPORT: Daisy-chain topology, High speed (115200 Bd), Full bus length (1200 meters), Ground shielded twisted pair(s), Termination on both both (bus) ends or both end devices.
- PARTITIALLY SUPPORTED: Low speed (<19200 Bd), Limited length (<25 meters), Limited number of sensors (<10), Daisy-chain, (Grounded) Shielded twisted pair, No termination.
- NOT SUPPORTED: Star topology, (Unshielded cabl, No twisted pairs, No termination).
- NOT SUPPORTED: Cluster topology, (Unshielded cable, No twisted pairs, No termination).
Best practice
Typically, “Example 3” (Limited daisy-chain) will work for small systems with an ydoc data logger.
- If problems like sensor reading time-outs occur, check and eliminate limitations (if possible) or upgrade to a full supported topology.
- Use a grounded, twisted pair cable, to eliminate noise on the Daisy-chain.
- If the device has no separated input and output terminals (to support daisy-chain), keep the Stub cable as short as possible.
- Maximum length 12 meter, maximum speed 9600 Baud
- Reduce number of sensors on the chain.