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Add some research about the electrical signals involved
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@ -28,3 +28,100 @@ Other parts
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- [LM1117SX](https://www.mouser.com/ProductDetail/Texas-Instruments/LM1117SX-ADJ-NOPB?qs=X1J7HmVL2ZFVdHO2uaroKA%3D%3D)
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- [LM1117SX](https://www.mouser.com/ProductDetail/Texas-Instruments/LM1117SX-ADJ-NOPB?qs=X1J7HmVL2ZFVdHO2uaroKA%3D%3D)
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- LDO Voltage Regulators 800mA LDO Linear Reg
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- LDO Voltage Regulators 800mA LDO Linear Reg
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- Cost: $1.51
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- Cost: $1.51
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## Discussion
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### Raspberry Pi
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The following descriptions are sourced from the RPi's official documentation, but may vary between different versions of the RPi (0, 1, 2, 3, 4, etc).
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The documentation on the Raspberry Pi's GPIO pins [is here](https://www.raspberrypi.org/documentation/hardware/raspberrypi/gpio/README.md).
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> Each of the three banks has its own VDD input pin. On Raspberry Pi, all GPIO banks are supplied from 3.3V. Connection of a GPIO to a voltage higher than 3.3V will likely destroy the GPIO block within the SoC.
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While the drive current of the GPIO pins are configurable up to 16mA, the 3.3v supply that powers them [cannot reach the maximum current for all pins](https://www.raspberrypi.org/documentation/hardware/raspberrypi/gpio/gpio_pads_control.md).
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> The Raspberry Pi 3V3 supply was designed with a maximum current of ~3mA per GPIO pin. If you load each pin with 16mA, the total current is 272mA. The 3V3 supply will collapse under that level of load.
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Simultaneously, [at 3.3v the pins are rated to sink at least 18mA and still meet spec](https://www.raspberrypi.org/documentation/hardware/raspberrypi/gpio/README.md).
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### SCSI-2 Bus
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The SCSI protocol suggests drivers capable of sinking 48mA. [(1)](#ref-1) You can see in [this example](http://www.interfacebus.com/Glossary-of-Terms-SCSI-interface-schematic.html) of a SCSI interface circuit.
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[This reference](https://www.ti.com/lit/an/slla035/slla035.pdf) on SCSI terminators also states
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> ... (i)t is the terminator’s job to source as much current as possible during deassertion. This role is restricted by the SCSI specification, however, which limits each terminator to supplying a maximum of 24mA to prevent the line current from exceeding the 48mA current sink limit of the open-collector drivers. (pg 8)
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There is also a discussion of the role of passive and active SCSI termination in [this reference](https://www.microsemi.com/document-portal/doc_download/14643-an-1-understanding-the-single-ended-scsi-bus) (see pg 10).
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See [section 5.4 of the SCSI-2 standard](https://www.staff.uni-mainz.de/tacke/scsi/SCSI2-05.html)
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5.4.1 Single-ended alternative
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All signals not defined as RESERVED, GROUND, or TERMPWR shall be terminated at both ends of
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the cable. The implementor may choose one of the following two methods to terminate each end
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(see figures 9 and 10):
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a) The termination of each signal shall consist of 220 ohms (+/-5%) to the TERMPWR line and
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330 ohms (+/-5%) to ground. Using resistors with +/-1% tolerance improves noise margins.
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b) The termination of each signal shall meet these requirements:
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1) The terminators shall each supply a characteristic impedance between 100 ohms and
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132 ohms.
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2) The terminators shall be powered by the TERMPWR line and may receive additional power
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from other sources but shall not require such additional power for proper operation
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(see 5.4.3).
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3) The current available to any signal line driver shall not exceed 48 mA when the driver
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asserts the line and pulls it to 0.5 V d.c. Only 44.8 mA of this current shall be
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available from the two terminators.
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4) The voltage on all released signal lines shall be at least 2.5 V d.c. when the
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TERMPWR line is within specified values (see 5.4.3).
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5) These conditions shall be met with any legal configuration of targets and initiators
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as long as at least one device is supplying TERMPWR.
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5.4.1.1 Output characteristics
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All signals shall use open-collector or three-state drivers. Each signal driven by an SCSI
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device shall have the following output characteristics when measured at the SCSI device's
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connector:
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VOL (low-level output voltage) = 0.0 to 0.5 V d.c. at 48 mA sinking (signal assertion)
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VOH (high-level output voltage) = 2.5 to 5.25 V d.c. (signal negation)
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5.4.1.2 Input characteristics
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SCSI devices with power on shall meet the following electrical characteristics on each
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signal (including both receivers and passive drivers):
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VIL (low-level input voltage) = 0.0 V d.c. to 0.8 V d.c. (signal true)
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VIH (high-level input voltage) = 2.0 V d.c. to 5.25 V d.c. (signal false)
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IIL (low-level input current) = -0.4 mA to 0.0 mA at VI = 0.5 V d.c.
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IIH (high-level input current) = 0.0 mA to 0.1 mA at VI = 2.7 V d.c.
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Minimum input hysteresis = 0.2 V d.c.
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Maximum input capacitance = 25 pF (measured at the device connector
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closest to the stub, if any, within the
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device)
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It is recommended that SCSI devices with power off also meet the above IIL and IIH
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electrical characteristics on each signal.
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To achieve maximum noise immunity and to assure proper operation with complex cable
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configurations, it is recommended that the nominal switching threshold be approximately 1.4 V.
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### RaSCSI
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From [the v1.6 schematic](https://github.com/akuker/RASCSI/blob/master/hw/rascsi_1p6/rascsi_din.sch.pdf), the following signals need to be bidirectional:
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> DP, D0, D1, D2, D3, D4, D5, D6, D7
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Similarly, the following signals are **from** the RPi **to** the SCSI bus:
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> BSY, MSG, C_D, REQ, I_O
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For SCSI-2, it seems that the drivers should be open-collector/open-drain drivers (ie: they can pull the bus down to ground, and the termination on the bus will pull back high.)
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Finally, these are the signals **from** the RPi **to** the SCSI bus:
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> ATN, ACK, RST, SEL
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They should probably be Schmitt-trigger inputs on the SCSI bus side. See the "Single-Ended SCSI" section of [this document](https://www.ti.com/lit/an/slla035/slla035.pdf); specifically page 8 of the PDF.
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2.85v Typical SCSI Open-Circuit Voltage
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2.5v SCSI Minimum Output Voltage for Deasserted Signal
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2v SCSI Minimum Input Voltage for Deasserted Signal
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...
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0.8v Maximum Input Voltage for Asserted Signal
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0.5v SCSI Maximum Output Voltage for Asserted Signal
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0.2v Typical Driver-Asserted Signal
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<a name="ref-1">(1)</a> [Comment by `saybur`](https://68kmla.org/forums/index.php?app=forums&module=forums&controller=topic&id=30399&page=7&tab=comments#comment-646496)
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