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- # Asynchronous Communications Interface Adapters
-
- Machines talking to each other is generally useful and they
- often use ACIA devices to do so. Collapse OS has drivers for
- a few chips of this type and they all have a similar approach:
- unbuffered communication using RTS/CTS handshaking as flow con-
- trol.
-
- The reason for being unbuffered is simplicity and RAM. The logic
- to implement input buffering is non-trivial and, alone, doesn't
- buy us much in terms of reliability: you still have to signal
- the other side when your buffer is nearly full.
-
- Because we don't really need speed, we adopt a one-byte-at-once
- approach: The RTS flag is always high (signalling that it's not
- ready for communication) *except* when calling the ACIA driver's
- "read" word, which is blocking.
-
- That "read" word will pull RTS low, wait for a byte, then pull
- it high again.
-
- This slows down communication, but it's simple and reliable.
-
- Note that this doesn't help making communications with modern
- systems (which are much faster than a typical Collapse OS
- machine and have their buffer output faster than the RTS flag
- can be raised) very much. We have to take extra care, when
- communicating from modern system, not to send too much data too
- fast. But for COS-to-COS communication, this simple system
- works.
-
- # Broken hardware
-
- Some designs are broken with this scheme. For example, the
- RS2014 SIO module hard-wires CTS to GND because the FTDI
- connector doesn't have such a pin (modern computers can always
- handle the load).
-
- In these cases, a solution would be to use Break signals as a
- workaround, but I prefer avoiding complexity for now. So when
- you deal with broken design, you'll have to sidestep it either
- by implementing your own Break handling or by lowering com-
- munication speed.
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