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The buffer's implementation wasn't buying us much in exchange for its complexity. A modern machine was still too fast for it (copy/pasting text from a modern machine would send bytes too fast for the RC2014) and in the (theoretical so far) case of COS-to-COS communication, the buffer didn't help in cases where the baud rate was faster than the processing of each byte received (for example, if the byte was written directly to EEPROM). I'm scrapping it and, instead, use the RTS flag to signal the other side when we're ready to receive a new byte. Also, implement driver for channel B in SIO. I will need it to talk to my TRS-80 4P.
31 lines
1.3 KiB
Plaintext
31 lines
1.3 KiB
Plaintext
# Asynchronous Communications Interface Adapters
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Machines talking to each other is generally useful and they
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often use ACIA devices to do so. Collapse OS has drivers for
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a few chips of this type and they all have a similar approach:
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unbuffered communication using RTS/CTS handshaking as flow con-
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trol.
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The reason for being unbuffered is simplicity and RAM. The logic
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to implement input buffering is non-trivial and, alone, doesn't
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buy us much in terms of reliability: you still have to signal
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the other side when your buffer is nearly full.
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Because we don't really need speed, we adopt a one-byte-at-once
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approach: The RTS flag is always high (signalling that it's not
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ready for communication) *except* when calling the ACIA driver's
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"read" word, which is blocking.
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That "read" word will pull RTS low, wait for a byte, then pull
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it high again.
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This slows down communication, but it's simple and reliable.
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Note that this doesn't help making communications with modern
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systems (which are much faster than a typical Collapse OS
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machine and have their buffer output faster than the RTS flag
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can be raised) very much. We have to take extra care, when
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communicating from modern system, not to send too much data too
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fast. But for COS-to-COS communication, this simple system
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works.
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