J058 SCRAM
The board base address is selected by IC6, a 3 to 8 line
decoder, and a link connects one of the outputs to an input of
IC7, a custom PAL chip. This chip decodes the CM lines as well,
and sends out a trigger to start a timing chain consisting of
IC10 and IC11. The correct RAM socket is enabled by another PAL
chip, IC8, which also deals with the 8K/2K option. Links from
each
The board I/O address is determined by the linking on IC9, an octal comparator, which then signals to IC7. If the CM lines are correct for I/O, IC6 starts the timing chain. The RTC chip used on the Arcom SCRAM has a multiplexed address/data bus, so IC7 transfers address then data either to or from the RTC chip, IC15, asserting DATACK* onto the bus when this operation is complete.
IC18 is a voltage-sensing chip. When the supply voltage is below the working limit, (e.g. during power-up or power-down) it immediately turns off the LEDs in the opto-isolators IC19, 26, 27, 28. This turns off the phototransistors, thus preventing IC8 putting any of the chip selects low. On power-up it keeps the chip selects disabled until the power supply reaches operating voltage and is stable.
This system gives the board a very high degree of protection against data corruption during switch-on and switch-off.