Page 2.59
CIRCUIT DESCRIPTION
As described before, if the CPU attempts to read or write
the onboard memory during this time, MDENB will go high
and force RDY low, generating a wait state. After REFWAIT
goes low, RDY goes high, allowing a normal bus cycle to
occur.
Note that the refresh circuits do not generate a refresh request
every time the CPU is not accessing memory. Refresh hap-
pens only
once every 16 ps. The CPU is running about 80
times faster than this and can perform many instruction cycles
between refreshes. Since the RAMs can go for about 2 ms
before requiring refresh, there is no danger of losing memory.
Parity Circuits
The parity circuits consist of U153, U101, U117, U137, and
U152. These circuits maintain the parity status for each byte
in the 192K of RAM. If a memory location's parity is in error,
then the parity circuits send an error signal to the CPU.
U101, U117, and U137, are 64K x 1 R AMs and store 1 bit
of parity information for each address location of RAM. These
RAMs are addressed by RAS and CAS in the same way as
the other RAMs. However, data transfers take place through
U153 instead of the data bus. U153 is a 9-bit odd or even
parity generator and checker that processes and maintains
the parity status.
During a memory write, the data written into RAM is present
at pins 1 through 8 of U153. Pin 14 of each parity RAM is
in a high impedance state, so U153 pin 4 is logic 1 through
R107.
The following truth table show the levels of the odd and even
outputs for the number of high inputs:
Содержание Z-100 Series
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