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.
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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.
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