invoking ReadBytesAsync method sometimes never return

[yushu3166]

code: if (!await ConnectAsync()) { return null; } Stopwatch stopwatch = Stopwatch.StartNew(); stopwatch.Start(); try { var cts = new CancellationTokenSource(1000 * 3); byte[] bytes = await _Plc.ReadBytesAsync(DataType.DataBlock, DB, start, length, cts.Token); stopwatch.Stop(); if (bytes == null || bytes.Length == 0) { _LogUtil.Error($"S7Plc error:{stopwatch.Elapsed.TotalMilliseconds}，IP：{_Plc.IP}，DB：{DB}，start：{start}，length：{length},MaxPDUSize:{_Plc.MaxPDUSize}"); } return bytes;

} catch (Exception ex) { stopwatch.Stop(); _LogUtil.Error($"S7Plc Exception:{stopwatch.Elapsed.TotalMilliseconds}，IP：{_Plc.IP}，DB：{DB}，start：{start}，length：{length},MaxPDUSize:{_Plc.MaxPDUSize}, ex:{ex.Message}"); } return null;

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byte[] bytes = await _Plc.ReadBytesAsync(DataType.DataBlock, DB, start, length, cts.Token); // sometimes here never return or throw exception,why?