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SerialPortStream Goals

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Jason Curl edited this page Feb 13, 2016 · 1 revision

Table of Contents

Goals

This project tries to achieve the following:

  • An implementation similar to the MS implementation of SerialPort. It's not meant to be 100% compatible, but instead provide similar functionality
  • Abstract the driver implementation and provide for a more reliable transport, by making writing serial data completely buffered. With the MS implementation, one can write data, but subsequently needs to check if all data is written or not. If it isn't written, then it needs to be retried. The SerialPortStream makes this easier
  • Provide for reliable and consistent behaviour. See the next section.

Issues with the MS Serial Port

The SerialPortStream tries to solve the following issues observed:

  • Zach Saw describes issues regarding behaviour of the fAbortError flag in the Serial DCB. The SerialPortStream defines this flag.
  • Closing a serial port, then reopening it generally causes problems. The SerialPortStream shouldn't have this issue.
  • The ReadTo() implementation can subtly change the byte stream buffer, when one switches from characters to bytes. This problem occurs because the MS implementation actually converts the characters back to bytes into its buffer. So if you have UTF8, decoded some invalid characters and then have a timeout, this results in the invalid characters being converted back to bytes, resulting in "lost" data. I take some care when decoding bytes to characters to ensure a seamless and accurate transition between bytes and characters.
  • Write() gives the data to the serial port. If the operation is asynchronous, the callback results in the number of bytes that were actually transferred to the driver. You need to check yourself if this is valid or not. In the synchronous case, the data is simply thrown away. The SerialPortStream method simply copies data to a local buffer and uses asynchronous writes in a different thread. It works hard in the background to send out the data you provided. If the data can't be sent, then you get a TimeoutException without any data being buffered at all. So you can implement reliable protocols and your code is simpler.

Extra Features

  • You can obtain the RingIndicator pin status.
  • The Read() and Write() buffers are completely independent of the low level Windows driver.

Reading and Writing - Buffering

Why is it interesting to perform buffering? A driver might be configured to be 4096 or 8192 bytes (which is quite typical). Testing with the PL2303 chipset, one can't write more than about 12KB with a single write operation.

A Write buffer may be 128KB, which one writes to. The thread in the background will write the data and issue as many write calls as is necessary to get the job done. A Read buffer may be 5MB. The background thread will read from the serial port when ever data arrives and buffers into the 5MB.

So long as the I/O thread in .NET an execute every 100-200ms, it can continue to read data from the driver. Your own application doesn't need to keep to such difficult time constraints. Such issues typically arise in Automation type environments where a computer has many different peripherals. So long as the process doesn't block, your main application might sleep for 10 seconds and you've still lost no data. The MS implementation wouldn't be so simple, you have to make sure that you perform frequent read operations else the driver itself might overflow (resulting in lost data).

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