Yeah, it would kind of defeat the purpose. Without re-reading 24 pages of this topic, I was thinking you were using a power supply that already included a negative voltage.


I have one of the FTDI USB-to-RS232 adapters, and I have it set up to do the handshaking on CTS and RTS. The FTDI unit I have is very clumsy softwarewise compared to using a real COM port on my dedicated DOS PC though. I know BDD gave a link somewhere to a product that gives a set of RS-232 ports to modern PCs that came without them, and has the driver software. It might have been startech.com or sealevel.com . It probably needs a desktop PC that you can put cards into, rather than a laptop.

I've never used RTS/CTS in any of my projects. If it's used as a command line interface, then the prompt works as natural flow control. If it's a packet based interface, I make sure I can receive a full packet in a single burst.

I see you have some slotted holes. Have you checked if the PCB manufacturer can make those, and if there's an extra cost ?

In most cases, you can substitute them with big round holes instead.

EDIT: OH, I see what you mean, I'm not sure, I don't think the manufacturer will charge extra for it, but we'll see when I get it quoted (it's already dirt cheap)

Also make sure that the manufacturer gets the correct data. If you're sending gerber files, you need to include the slots in the milling or outline layer. Also, they need to understand that you want them plated (I assume you do). Some manufacturers automatically assume you want plated slots if you put them in a pad, but they may not all do that. It can be helpful to add some text in the appropriate layer with an explanation what you want, e.g. "3x plated slot, length * tool diameter".

There are free gerber viewers that you can download to inspect the gerber files before you send them away.

I saw ABT and FCT devices mentioned in the big thread on 816 timing. I looked them up and they both look pretty fast, the ABT seems to be a bit faster and cheaper. Is there a catch with these devices? If not, why aren't they recommended in places like the primer?

The primer is intended to be just that—a primer—so it's kept rather simple. The faster logic families are more likely to spell trouble for first-time builders. I would like to add more info about the various 74__xx logic families (probably in a separate article); but IIRC, ABT cannot pull up high enough to guarantee a valid CMOS logic '1'.

You'll notice that the resistor+capacitor in line for RDY are oriented differently. The resistor stands off the board while the capacitor lays on it. I laid the capacitor down so the current flow would be relatively parallel, and I'm considering doing the same for the resistor, is this necessary? Or should I just make them both vertical?

Ah, that's probably the catch with it, I assume that's probably why the "T" is tacked onto the name, for TTL levels like the HCT. I'll check out the data sheet and report back what it's logic 1 output is.

The resistor's resistance will dwarf the small added inductance you get from standing it up; so don't worry about it.


The "T" normally refers to the input thresholds, not the output. So for example 2.5V at the input of a 74HCTxx part will be in solid '1' territory, whereas for 74HCxx, it would be undefined. WDC's processors' data sheets say they need 70% of Vcc to get a valid '1', which at 5V is 3.5V. Some here have challenged that assertion, saying their observation is that it can handle lower-voltage 1's ok. I have not tested it myself, and even if I found it to be true in the unit(s) I test, I might not want to trust that all their units will always perform that way.