Modbus Master/Slave Architecture
Key Takeaway
How Modbus master/slave communication works — request-response cycle, polling design, timeouts, retry logic, and gateway/concentrator architectures.
Quick Answer
Modbus uses a strict master/slave model. The master initiates all transactions; slaves only respond when polled. One master per RS-485 bus segment; multiple masters allowed on Modbus TCP.
Request-Response Cycle
- Master sends request (slave address + function code + register address + quantity)
- Slave processes request
- Slave sends response (data or exception code)
- Master processes response and moves to next poll
Polling Design
- Group consecutive registers to minimize transactions
- Poll critical registers more frequently
- Calculate minimum cycle time: ~10-50ms per RTU transaction at 9600 baud
- 20 slaves × 1 transaction = 200-1000ms minimum cycle
Timeout and Retry
Configure master timeout (how long to wait for response) and retry count (attempts before declaring slave offline). Typical: 500ms timeout, 2-3 retries.
Gateway Architecture
Modbus TCP gateways bridge serial RTU slaves to Ethernet masters. The gateway translates TCP requests to RTU frames and routes responses back.
Frequently Asked Questions
No. One master per RS-485 segment. Use a gateway or data concentrator for second master access.
At 9600 baud, each transaction takes ~10-50ms. For 20 slaves with one transaction each, minimum cycle is 200-1000ms.