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Morrow - 8K Dynamic RAM
This board was produced when Morrow was still known as Tinker Toys.

8K DRAM Board

The Thinker Toys "Synchrofresh" 8K memory unit was an early DRAM S-100 board. It contained two completely independent 4K blocks. Each of these two 4K blocks could be placed on any 4K address boundary. At the top of the circuit board just above position 2A, there is a three-position slide switch. The left and middle positions are both write enable states while the right position is the write protect state.  When the board is to be used as read/write memory, the slide switch should be to the left or in the center. In these positions, the CPU can write into or read from either 4K memory block. When the unit is to be used as "read only" memory, the slide switch should be in the right most position. In this position, write commands issued to the memory by the CPU or DMA devices are ignored. The write protect mode is especially useful when it is necessary to prevent a program or data from being accidently destroyed by another program or from the front panel.

Addresses to the memory from the S-100 buss and the refresh address counter are buffered and selected through three 74lS157/158/257/258 quad two-input multiplexors. The control input to the multiplexors comes from a D-flip-flop which switches data and refresh addresses to the array at the proper times. A careful examination of the timing of this flip-flop will reveal that addresses are switched in the middle of a memory cycle. This would be unacceptable were we using static RAMs. The situation with dynamic RAMs is somewhat different. The address inputs of a dynamic RAM chip are not only buffered but also latched. This latching occurs with the rising edge of CENBL (see manual below), the signal which initiates a cycle for the memory chip. The main concern with the address signals to the 2107B is that they are stable at the rising edge of CENBl and remain so for app rox imate Iy 150 ns.· In the present case, the MUX CNTL signal does not change for 250 ns after the rising edge of CENBl, giving a margin of 100 ns.

The manual for this board can be obtained here
Morrow S-100 Boards
24K RAM   64K RAM    Disk Jockey FDC     Multi I/O    SuperRAM     SwitchBoard    Z80 Board    Hard Disk Controller

DJ-DMA FDC  8K DRAM    HDC-DMA Controller


This page was last modified on 05/04/2016