48 lines
2.1 KiB
Markdown
48 lines
2.1 KiB
Markdown
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---
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title: Hardware Design
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---
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# Hardware Design
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## Basic Hardware Version
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The hardware uses an Arduino to write data and to toggle control lines with the
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appropriate timing to access the PROM. A pair of 74LS164 serial to parallel shift
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registers latch the address lines. Use of the shift registers allows the Arduino to
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control up to 16 address lines using only 3 output ports.
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The basic circuit is as follows:
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* Pins D2..D9 are wired to the data lines on the target PROM.
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* Pins A0..A2 are wired to the WE, CE, and OE control lines on the target PROM.
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* Pins A3..A5 control shift registers to produce the address lines.
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* Pins D10..D12 control A16..A18 for chips larger than 64K bytes.
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Note that the existing design uses 74LS164 shift registers, but another 8-bit parallel out
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shift register, like the 74LS594 or 74LS595, could be used instead with some pin changes.
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The two shift registers can produce a sixteen bit address, although the 28C256 only needs
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15 addresses. Chips larger than 64K are supported by using the shift registers for A0..A15
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and connecting Arduino pins D10..D12 to the chip's A16..A18
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**NOTE:**
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The schematic does not show the Vcc and ground pins for the 74LS164 shift registers.
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These must be connected to +5 and ground, respectively.
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## Intel 8755A Hardware Version
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The Intel 8755A uses a multiplexed data and address bus, plus 3 additional address lines.
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Most Arduino hardware has enough pins to support this directly, so no additional hardware
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is needed for addressing. An Arduino pin is also used to drive the ALE latch pin.
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The 8755A requires the Vdd pin to be be switched between 5V and 25V during the programming
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of each byte. A simple transistor circuit is used for this. No voltage regulator circuit
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is present here for the programming voltage. Because this is designed as a quickly
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assembled hardware design, a triple output bench supply was used to provide the 25.5V,
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5.5V and 5V outputs.
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