CY545 Stepper System Controller
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In the CY545B, the FPL signal is driven low when the CY545B has data to output, and it is
removed after the host has read the last character, and brought I/O Request high again, but
before the CY545B brings Busy high. This timing is compatible with hardware implemented
handshakes, requiring no time delays or complex logic to implement.
Notice that the FPL signal is removed at the end of the message, while the CY545B is still Busy,
but after the I/O Request has been driven high again.
Also, there is a fundamental handshake change required for the CY545B to accept immediate
commands from the host, while reading and executing commands from the local external
memory.
If the CY545B is to execute a program from the local external memory, and accept immediate
parallel commands at the same time, there is a resource conflict involving the data bus. This
bus is used to both read commands from the memory, and read commands from the parallel
host, while only one device is allowed to drive the bus at a time. Notice that the CY545B may
also drive the bus while outputting messages to the host or the local HP display! This makes the
data bus a very critical resource in CY545B designs.
In order to allow immediate commands to be issued at any time, even while the CY545B is
reading data from the local memory, we must implement a new handshake protocol for the
immediate commands, which prevents corruption of data on the data bus. In the CY545B, this is
done without using any more signals, so no other User Bits are given up to perform this more
complex handshake.
Instead of using additional signals, immediate commands are isssued with a modified first
handshake, involving no character! That is, the host drives I/O Request low, and waits for the
Busy signal to go low, but the data bus is not driven. This allows the CY545B to finish reading
and executing any command, from the local external memory, that is in progress when the host
decides to issue the immediate command.
After the memory command is finished, including possible output of a message or query
response, the CY545B will drive Busy low, with FPL high, to indicate that it is now accepting the
host's immediate command. This is similar to a normal command handshake, except that the
command letter is not put on the data bus.
Since the data bus is not driven during this first handshake, the CY545B reads a 0FFh data byte
(due to the pull-up resistors on the local data bus), and will ignore this byte.
This special, first character handshake is followed by the normal command format, starting with
the normal command letter, which the CY545B reads with a normal handshake. After executing
© 2002 Cybernetic Micro Systems
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Chapter 16 - Timing and Control
ETC [ ETC ]
