Board Options

I wanted to have physical controls for the emulator and when I last discussed this on the list I also had a request to have the ability to have control and status so I added a connector for expansion. The loading of the 74F07 chips limited what pins they could be connected to which prevented using some of the BeagleBone's more flexible pins for an expansion connector.

If the 74F07's in my original version are replaced with CMOS input parts they won't excessively load the LCD/Boot configuration pins. The only CMOS open collect part capable of driving 20+ mA that I could find was the 74LVC07 that is only made in surface mount.

I have made two layouts, available here. Mfm is with all through hole parts and mfm.3v has three surface mount parts.

The advantages of the partial surface mount version are:

• Pins that can be SPI, I2C, and serial go to the expansion connector.
• It is 3.3V only so the battery option discussed in the board description can be used without modification to the board. USB sticks can't be used for storage if the battery is used.
• It uses about 1.2W less power so the holdup caps will run longer.
• 74LVC07 chips are cheaper ($.45 vs $.92 to $2.90 each)

• It has three surface mount chips though SO14 .05" lead spacing so not that hard to solder.
• The LVC chips are also significantly faster than 74F07/7407 so it is possible they could cause crosstalk or reflection problems driving the cable.

The Thevenin equivalent of the 220/330 ohm resistors on the through hole board is 132 ohms to 3V. The closest option for the partial surface mount board is 130 ohms to 3.3V or 150 ohms to 3.3V. 130 ohms keeps the termination impedance the same and 150 ohms keeps the low current the same. Since ESDI drives switched to 150 ohm terminators I use 150 ohms on the partial surface mount version.