diff --git a/Transceiver-Comparison.md b/Transceiver-Comparison.md index 289141a..837c253 100644 --- a/Transceiver-Comparison.md +++ b/Transceiver-Comparison.md @@ -27,4 +27,101 @@ Other parts - Linear Regulator (for active terminator) - [LM1117SX](https://www.mouser.com/ProductDetail/Texas-Instruments/LM1117SX-ADJ-NOPB?qs=X1J7HmVL2ZFVdHO2uaroKA%3D%3D) - LDO Voltage Regulators 800mA LDO Linear Reg - - Cost: $1.51 \ No newline at end of file + - Cost: $1.51 + +## Discussion + +### Raspberry Pi +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). + +The documentation on the Raspberry Pi's GPIO pins [is here](https://www.raspberrypi.org/documentation/hardware/raspberrypi/gpio/README.md). +> 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. + +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). +> 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. + +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). + +### SCSI-2 Bus +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. + +[This reference](https://www.ti.com/lit/an/slla035/slla035.pdf) on SCSI terminators also states +> ... (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) + +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). + +See [section 5.4 of the SCSI-2 standard](https://www.staff.uni-mainz.de/tacke/scsi/SCSI2-05.html) + + 5.4.1 Single-ended alternative + All signals not defined as RESERVED, GROUND, or TERMPWR shall be terminated at both ends of + the cable. The implementor may choose one of the following two methods to terminate each end + (see figures 9 and 10): + + a) The termination of each signal shall consist of 220 ohms (+/-5%) to the TERMPWR line and + 330 ohms (+/-5%) to ground. Using resistors with +/-1% tolerance improves noise margins. + b) The termination of each signal shall meet these requirements: + 1) The terminators shall each supply a characteristic impedance between 100 ohms and + 132 ohms. + 2) The terminators shall be powered by the TERMPWR line and may receive additional power + from other sources but shall not require such additional power for proper operation + (see 5.4.3). + 3) The current available to any signal line driver shall not exceed 48 mA when the driver + asserts the line and pulls it to 0.5 V d.c. Only 44.8 mA of this current shall be + available from the two terminators. + 4) The voltage on all released signal lines shall be at least 2.5 V d.c. when the + TERMPWR line is within specified values (see 5.4.3). + 5) These conditions shall be met with any legal configuration of targets and initiators + as long as at least one device is supplying TERMPWR. + + + 5.4.1.1 Output characteristics + All signals shall use open-collector or three-state drivers. Each signal driven by an SCSI + device shall have the following output characteristics when measured at the SCSI device's + connector: + VOL (low-level output voltage) = 0.0 to 0.5 V d.c. at 48 mA sinking (signal assertion) + VOH (high-level output voltage) = 2.5 to 5.25 V d.c. (signal negation) + + 5.4.1.2 Input characteristics + SCSI devices with power on shall meet the following electrical characteristics on each + signal (including both receivers and passive drivers): + VIL (low-level input voltage) = 0.0 V d.c. to 0.8 V d.c. (signal true) + VIH (high-level input voltage) = 2.0 V d.c. to 5.25 V d.c. (signal false) + IIL (low-level input current) = -0.4 mA to 0.0 mA at VI = 0.5 V d.c. + IIH (high-level input current) = 0.0 mA to 0.1 mA at VI = 2.7 V d.c. + Minimum input hysteresis = 0.2 V d.c. + Maximum input capacitance = 25 pF (measured at the device connector + closest to the stub, if any, within the + device) + + It is recommended that SCSI devices with power off also meet the above IIL and IIH + electrical characteristics on each signal. + To achieve maximum noise immunity and to assure proper operation with complex cable + configurations, it is recommended that the nominal switching threshold be approximately 1.4 V. + + +### RaSCSI +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: + +> DP, D0, D1, D2, D3, D4, D5, D6, D7 + +Similarly, the following signals are **from** the RPi **to** the SCSI bus: + +> BSY, MSG, C_D, REQ, I_O + +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.) + +Finally, these are the signals **from** the RPi **to** the SCSI bus: + +> ATN, ACK, RST, SEL + +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. + + 2.85v Typical SCSI Open-Circuit Voltage + 2.5v SCSI Minimum Output Voltage for Deasserted Signal + 2v SCSI Minimum Input Voltage for Deasserted Signal + ... + 0.8v Maximum Input Voltage for Asserted Signal + 0.5v SCSI Maximum Output Voltage for Asserted Signal + 0.2v Typical Driver-Asserted Signal + +(1) [Comment by `saybur`](https://68kmla.org/forums/index.php?app=forums&module=forums&controller=topic&id=30399&page=7&tab=comments#comment-646496)