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Dear electronics enthusiasts,
I'm working on a rather daring project—for me.
I'd like to build a peripheral for the classic Atari ST computer. This computer has a SCSI-like port called ACSI (Atari Computer System Interface). It's essentially a bus that communicates directly with the ST's DMA chip, a fairly fast interface even 40 years ago. Theoretically, up to eight peripherals can be connected. The bus operates at 5v TTL level. The ST is the master on the bus and determines which device it wants to communicate with. Five signals go from the ST to the peripherals. Two signals can be pulled low by any of the peripherals on the bus (the ST predetermines which device). If these signals are not pulled low, the peripherals must maintain their high-impedance mode (high-z). In addition, there are eight data lines that allow data to be sent or read byte by byte. The data lines are therefore bidirectional. These lines are normally also kept in high-z or input mode by the peripheral device unless it's its turn to write data.
I'm designing a printed circuit board and have made some progress, but I'm still unsure about some things. I want to connect a Raspberry Pico W to the board. This Pico W operates on 3.3 volts. Power is supplied via the Pico's USB connector. My current schematic is attached.
Questions:
- The Pi operates on 3.3 volts on the pins. The Atari ST on 5 volts. I've solved this by using two LVC245A transceivers: one for the eight data lines, which are bidirectional, and one for the five input lines, which is essentially used as a level shifter since I've hard-wired the DIR and OE ports to VCC or GND. My question is: Should I power these 245s with 5 volts or 3.3 volts? (DIR and OE are also 3.3 volts controlled)
- There's an SN74LS07 between the two outgoing lines. These lines should be pulled HIGH-Z or LOW. However, I'm unsure about its effectiveness because I'm measuring either 5 volts (via pullup) or 0.03 volts separate from the ST, which isn't 0 volts. (P.S. They're from Aliexpress, so there might be a problem there.)
- The third question is about pullup/pulldowns. I'm wondering if, besides the two outgoing lines, it might be better to place pullups or pulldowns on the other lines as well. I know from another project (acsi2stm) that the lines are simply connected directly to the 5V-tolerant pins of an STM without external pulls. Much easier, of course, but I want to use a Pico W for its Wi-Fi capabilities.
Advice and constructive criticism are very welcome!
For those who want to read it:
https://docs.dev-docs.org/htm/search.php?find=ACSI for more information about the ACSI bus
https://github.com/retro16/acsi2stm/tree/stable The acsi2stm project
I'm working on a rather daring project—for me.
I'd like to build a peripheral for the classic Atari ST computer. This computer has a SCSI-like port called ACSI (Atari Computer System Interface). It's essentially a bus that communicates directly with the ST's DMA chip, a fairly fast interface even 40 years ago. Theoretically, up to eight peripherals can be connected. The bus operates at 5v TTL level. The ST is the master on the bus and determines which device it wants to communicate with. Five signals go from the ST to the peripherals. Two signals can be pulled low by any of the peripherals on the bus (the ST predetermines which device). If these signals are not pulled low, the peripherals must maintain their high-impedance mode (high-z). In addition, there are eight data lines that allow data to be sent or read byte by byte. The data lines are therefore bidirectional. These lines are normally also kept in high-z or input mode by the peripheral device unless it's its turn to write data.
I'm designing a printed circuit board and have made some progress, but I'm still unsure about some things. I want to connect a Raspberry Pico W to the board. This Pico W operates on 3.3 volts. Power is supplied via the Pico's USB connector. My current schematic is attached.
Questions:
- The Pi operates on 3.3 volts on the pins. The Atari ST on 5 volts. I've solved this by using two LVC245A transceivers: one for the eight data lines, which are bidirectional, and one for the five input lines, which is essentially used as a level shifter since I've hard-wired the DIR and OE ports to VCC or GND. My question is: Should I power these 245s with 5 volts or 3.3 volts? (DIR and OE are also 3.3 volts controlled)
- There's an SN74LS07 between the two outgoing lines. These lines should be pulled HIGH-Z or LOW. However, I'm unsure about its effectiveness because I'm measuring either 5 volts (via pullup) or 0.03 volts separate from the ST, which isn't 0 volts. (P.S. They're from Aliexpress, so there might be a problem there.)
- The third question is about pullup/pulldowns. I'm wondering if, besides the two outgoing lines, it might be better to place pullups or pulldowns on the other lines as well. I know from another project (acsi2stm) that the lines are simply connected directly to the 5V-tolerant pins of an STM without external pulls. Much easier, of course, but I want to use a Pico W for its Wi-Fi capabilities.
Advice and constructive criticism are very welcome!
For those who want to read it:
https://docs.dev-docs.org/htm/search.php?find=ACSI for more information about the ACSI bus
https://github.com/retro16/acsi2stm/tree/stable The acsi2stm project
