RS485 Modbus RTU读取异常：请求帧长度超61无响应

Let's break down your problem and walk through the most likely fixes:

1. First, Check Your CRC Byte Order (Critical!) #

Looking at your working frame 01 03 00 00 00 61 84 22:

• The first 6 bytes are the request header, and the CRC for this header is 0x2284 (calculated via standard Modbus RTU CRC-16).
• In the frame, this CRC is sent as 84 22 — meaning low byte first, high byte second.

Now look at your non-working manual frame 01 03 00 00 00 62 C4 23:

• The CRC for the header 01 03 00 00 00 62 is actually 0xC423, which should be sent as 23 C4 (low byte first), not C4 23. Your manual frame has the CRC bytes reversed, so the device rejects it as invalid.

Check your code's CRC handling:

var crc = BitConverter.GetBytes(ModRTU_CRC(items, items.Length));

On little-endian systems (most Windows machines), BitConverter.GetBytes will return the CRC as [lowByte, highByte], which is correct for Modbus. But double-check that you're concatenating crc to your items array in the correct order (don't reverse them!).

2. Verify the Device's Maximum Register Read Limit #

Most Modbus RTU devices enforce a maximum number of registers you can read in a single request (common limits are 64, 96, 100, or 125 registers). Your working request uses 97 registers (0x61), and anything above that fails — this strongly suggests your device's hard limit is 97 registers per request.

Fix: Split Your Request into Batches #

Instead of requesting 120 registers (0x78) in one go, split it into two smaller requests:

• First request: 01 03 00 00 00 61 84 22 (get registers 0-96)
• Second request: 01 03 00 61 00 17 [CRC] (get registers 97-119, which is 23 registers — 0x17)

Calculate the CRC for the second header 01 03 00 61 00 17 (it should be 0x9E75, so the full frame is 01 03 00 61 00 17 75 9E). This will let you fetch the remaining forward/reverse reactive power data without hitting the device's limit.

3. Adjust Serial Port Timeout Settings #

When requesting more registers, the device needs more time to assemble and send the longer response. If your ReadTimeout is too short, the Read operation will return 0 bytes before the device can respond.

Update your code to set a longer timeout:

port.ReadTimeout = 2000; // 2 seconds, adjust based on your device's response time

4. Check RS485 Half-Duplex Control #

RS485 is half-duplex, so you need to make sure your serial port switches from transmit to receive mode immediately after sending the request. If you're not controlling the RTS pin correctly, the device's response might not be detected.

Add code to handle RTS (exact logic depends on your hardware):

// Before sending: enable transmit mode
port.RtsEnable = true;
port.BaseStream.Write(dataItems, 0, dataItems.Length);
port.BaseStream.Flush();
// After sending: switch to receive mode
port.RtsEnable = false;
// Now read the response
receivedBytes = port.BaseStream.Read(buffer, 0, (int)buffer.Length);

5. Confirm Register Address Validity #

Double-check your device's manual to confirm that the forward/reverse reactive power registers are indeed located at addresses 0x0061 and beyond. If you're requesting registers that don't exist, the device might either send an exception response (which you might miss due to timeout) or ignore the request entirely.

To catch exception responses, increase your buffer size and ensure you're reading all incoming bytes. An exception response for invalid registers would look like 01 83 02 [CRC], where 02 means "Illegal Data Address".

内容的提问来源于stack exchange，提问作者Moeez