[Coco] Interfacing 3.3V Logic to the Coco

[Michael R. Furman]

I’m working on a little hardware project and the device I want to connect to the CoCo cartridge bus runs at 3.3V.  I’ve done a bunch of searching around on this topic and not being an EE I’m a bit overwhelmed on all the different options I’ve found on various bus transceivers and level shifters.

To me it seems like the TI TXB0108/TXS0108 bi-directional level shifters would do the job for me.  There are a bunch of modules on eBay for a bit less than $1… The eBay listings say they’re TXB0108(YE08) but the pictures in every single auction show the TXS0108(YF08)   Not sure what the difference between these are and if it makes any difference?  I’ve got a partial hand-wavy understanding of Push-Pull but Open-Drain?

http://www.ti.com/lit/ds/symlink/txb0108.pdf <http://www.ti.com/lit/ds/symlink/txb0108.pdf>

9.1 Application Information The TXB0108 can be used in level-translation applications for interfacing devices or systems operating at different interface voltages with one another. It can only translate push-pull CMOS logic outputs. If for open-drain signal translation, please refer to TI TXS010X products. Any external pulldown or pullup resistors are recommended to be larger than 50kΩ.

http://www.ti.com/lit/ds/symlink/txs0108e.pdf <http://www.ti.com/lit/ds/symlink/txs0108e.pdf>

9.1 Application Information The TXS0108E can be used in level-translation applications for interfacing devices or systems operating at different interface voltages with one another. The device is ideal for use in applications where an open-drain driver is connected to the data I/Os. The device is appropriate for applications where a push-pull driver is connected to the data I/Os, but the TXB0104 device, (SCES650) 4-Bit Bidirectional Voltage-Level Translator might be a better option for such push-pull applications. The device is a semi-buffered auto-direction-sensing voltage translator design is optimized for translation applications (for example, MMC Card Interfaces) that require the system to start out in a low-speed open-drain mode and then switch to a higher speed push-pull mode


I also took a look at Zippster’s proposed design for the Coco FPGA Experimenter’s Board and he  seems to be using a LCX16245 which is not a level shifter but seems to be a 5V-Tolerant bus transceiver.  I guess this looks appropriate for sticking on the Data or Address Busses where you already know which direction things are going via the R/W line?
https://sites.google.com/site/thezippsterzone/coco-fpga-experimenter-s-board <https://sites.google.com/site/thezippsterzone/coco-fpga-experimenter-s-board>
http://www.onsemi.com/pub/Collateral/74LCX245-D.pdf <http://www.onsemi.com/pub/Collateral/74LCX245-D.pdf>
http://www.onsemi.com/pub/Collateral/74LCX16245-D.pdf <http://www.onsemi.com/pub/Collateral/74LCX16245-D.pdf>


—

Michael R. Furman
Email: n6il at ocs.net
Phone: +1 (408) 480-5865