Dr Jefyll wrote:
drogon wrote:
Given the general lack of parallel ports on modern PCs these days then that leaves a microcontroller and so if using a µC, then I'm struggling to see why there is a need for a 6502 at all. [...] You can build up a device uing an Arduino Uno and a pair of 74x595 shift registers
I agree it's questionable whether the proposed EPROM programmer ought to be 6502-based. But I don't feel that the lack of parallel ports on modern PCs forces us into microcontroller territory.
74x595 shift registers can also be driven from an asynch ("RS232") port. Besides the 595's, what would be on the receiving end is an oscillator and
four 74xx chips, as shown below (one each of 'hc4520, 'hc74, 'hc4015 and 'hc132). I like some of the other approaches that were mentioned, including the FTDI FT245, but let's have a closer look at my Dumb Asynch® approach.
While not my intention to detract from Jeff's very cool discrete Async' Receiver concept... I believe sending four bytes at ~86-us intervals (115,200 baud) for each single byte+address+control 'frame' may preclude the use of EEPROM and Flash ROM targets. Please correct me if I'm wrong but I don't see how you can meet the timing limits for the multi-byte command sequences on those devices.
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Since we're discussing tools and barriers to participate in the hobby (necessary tools, costs, ease of construction, etc.), I'd like to suggest that the Arduino Uno and the Arduino Nano clones are a worthwhile investment. I'm necessarily frugal (financially challenged?) and a commercial programmer, while not entirely out of reach, would have used up several months of my meager $20/month hobby budget. Instead I enjoyed designing and building a FLASH/EEPROM programmer shield for an Arduino Uno. I bought five Arduino Uno clones with micro-USB connector ($3.03 each, including shipping) one month, ordered ten Flash Programmer PCBs ($18.20, including shipping) the next month, and I ordered 32-pin ZIF sockets ($1.29 each) the following month. Drawing & details below... I used a pair of latches which can be loaded with 16 address bits much faster than a pair of shift registers (while also using less pins on the Arduino board).
I believe newcomers might enjoy building simple and relatively inexpensive tools like a Flash/EEPROM Programmer shield, especially with the gratification of seeing results immediately after a couple hours of assembly. A relatively simple set of instructions could guide them through installing the Arduino IDE and a 'sketch' (program) into the Arduino board as well as guiding them through the programmer shield assembly process (some 100+ solder connections). Program Flash or EEPROM chips using a terminal program with x-modem at 115200 baud.