Revisions
Revisions
Thanks to all for the responses.
Here are some updated schematics using
tri-state buffers and
schottky diodes. I also moved the switch to the +5V side of the power supply and went with a DPDT toggle switch.
From Chuck(G)
Absent an n-pole switch You could go with a CMOS switch or even a TTL tristate buffer to terminate the resistors. Just set it to high-Z when you don't need it.
You might find a more elegant solution in Pericom's lineup.
Another idea is to isolate the terminating resistors from each other with schottky diodes. Then switch the +5.
@Chuck: It seems like all 3 options would work (CMOS switch, tri state buffers, schottky diodes). IMHO, the CMOS analog switches look to be the most reliable, yet the most expensive (aside from the Pericom solution.) The tri-state buffers seem like a great solution. Are there any drawbacks to this approach? According to what I found on the web, the schottky diodes lose only 0.2V or less, so that seems like the most practical solution. Which way would you go?
From antiquekid3
What about those DB-25 parallel switches that people used to use to select printers? You could definitely use a switch like the one in there for your application, I would presume. The wafer switch has to switch a lot of wires, so it'd be great to connect/disconnect those resistors. Definitely not as elegant as two octal tri-state buffers, though.
@antiquekid3: I had initially thought of a wafer switch too, but I was worried about arcing and the fact that it's a mechanical switch. I was hoping to go with as little mechanical as possible, if for no other reason than it's a good science project for me to try to implement it with circuits. I agree with you about the tri-state buffers being an elegant way to go, which is kind of what I'm shooting for. My concern about wafer switches may be unjustified, but I'm just erring on the side of caution to protect my SCP drive controller board.
From MikeS
In my experience it's not usually a problem if the cable isn't terminated at the end (as long as it's terminated somewhere). Why not try it as is, i.e. only put the terminating resistor pack in the last drive inside the cabinet and just extend the cable to the (unterminated) external drive (as short as possible).
Switching the grounds as per your diagram doesn't look like a solution; besides, the grounds would be connected through the drives' power ground.
@MikeS: I hadn't considered terminating any other drive than the last drive in the chain. I'll have to try that out.
Also, I updated the diagram to switch on the +5V side vs the ground side for the "enabled" mode. Please help me understand why sharing the drive ground is a bad thing. Do I need to look at a completely isolated +5V source? I was hoping to use the +5V source already in the enclosure. I thought I would be using the same ground that the chip resistor terminators normally plugged into the drive were using. I'll take another look at the drive schematics.