CY545 Stepper System Controller
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In addition, the CY545B will only issue an output message and take control of FPL if there is a
query or quoted message in the memory program. If there is no such command, the CY545B
will always accept the immediate command from the host, with no chance of changing FPL to
the output mode during the first handshake of the immediate command.
In this case, the host handshake is less complex, because the host is not required to test the
state of FPL during the first handshake with no data. FPL should always be high. However,
during the first handshake the data bus must still be left floating, so the CY545B can continue to
read from memory, if required, to finish the currently executing command.
When using the serial command interface, it is also possible to issue immediate commands to
the CY545B while it is executing a program from the local external memory. As with the
immediate parallel commands, the CY545B will stop the memory execution, process the
immediate command, then continue execution from the memory, unless the immediate
command was the "0" command.
Since the CY545B may be in the middle of a command when the host decides to send the
immediate serial command, it is best to use the Clear To Send ("CTS") protocol to help control
this environment. This will prevent overflow of the CY545B serial buffer, since it cannot read the
new command in the middle of executing the command read from memory.
The CTS signal is enabled by setting MB5 of the mode register to one, using the Mode ("O")
command.
The CY545B will drive CTS high (off), if enabled, during the execution of any memory
command, then drive it low again as the next memory command is being read. If the host uses
the CTS signal to indicate when it is safe to send a command letter, the CY545B will accept the
letter, and save it in the serial buffer until the current memory-based command is finished. It will
then read the immediate serial command letter, and wait for more serial data, to complete the
command. CTS will stay low during this time.
Once the immediate command has been received, the CY545B will execute it, with CTS driven
high. External program execution will then resume after the immediate command.
As with the parallel command interface, the CY545B does not mix the command read from
memory with characters received over the serial interface. Once the first letter of a serial
command has been received, execution from the external memory is suspended until the
immediate command is finished.
The Serial Interface
The CY545 serial command interface uses two primary signals, RxD and TxD. Serial data is
received on RxD and is sent on TxD. Note that when the parallel interface is not used,
IO_REQUEST and BUSY may be tied to select a fixed baud rate for the CY545. Otherwise, the
adaptive mode will be used, or a parallel mode command can select a fixed baud rate (either
from the local external memory, or from a parallel-connected command source). In the adaptive
case, the CY545 must receive two carriage return codes before receiving or sending any other
serial data. It will adjust the baud to match that of the received carriage return codes. With an 11
MHz crystal, the CY545 can adapt to standard baud rates between 300 and 19,200 baud.
© 2002 Cybernetic Micro Systems
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Chapter 16 - Timing and Control
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