An other important decision was: buy every tool you need, in the quality you need.
This is a bit philsophical: as I passed my 50th birthday, I realized that my remaining life time would grow more precious each folowing year, while the value of my remaining money would get less important every year.
I use these tools to repair
LA
The most important (and the most epxensive) tool is a logic analyzer.
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Logic analyzer have 3 core parameters: channel count, time resolution, and memory depth.
I only focused on USB based LA's, which need a PC to display the signals.
- I hated it to spent desktop space and money for a stand alone device, when I already had a PC sitting at my working place.
- For documentary purposes, I need a easy transfer of screen shot images and recoreed sample data to the PC: for post processing (see my UNIBUS analyzer <link>).
After much research, I think in 2014 you have basically have these options
- Build your own LA, there are tons of do-it-your-self projects.
- Then there's the "toy" class, target to micro controller developers. These have 8 or 16 channels, and are available for $100 or so from many vendors.
- Then there's a device class, which apparently was founded by the famous "Intronix" LA: FPGA based, 34 channels, max 500MHz. These cost a few hundred dollars. Also many vendors, all much better than the original Intronix LA now.
- Then there's the ultimative "profi" class: Agilent or Tektronix, every count of channles available, Multi GHz resolution, tons of Megabytes. Typically starting at $10000 ...
- Strangely there seems to be a big gap between the "profi" and the "post-Intronix" class. I found only ZEROPLUS offering an LA with > 34 channels and below $10000.
Minimal requirements
Channel count: To repair old computer CPUs, you need many channels in parallel. Just a calculation: to monitor the UNIBUS alone you would need 56 probes, which can barely be reduced to 34.
Optimally, while your problem CPU is running, you want to see the micro program counter (MPC). All DEC CPUs have extensive documentation ofr the micro programs of their CPUs, you must use this information.
The MPC is typically 10 bits widht. So you need at least 34+10 44 probes, before your work even should begin.
Bottom line: Buy nothing below 32 channels!
Time resoluting for old CPUs is not so critical. If build from 72Sxx chips, 5ns resolution are enough, resulting in 200MHz capture frequency. 100MHz resolution are the minimuml.
Memory depth specifies how many samples fit into the LAs memory. For example: "16KSamples" means at 100MHz you can record a time of 16000 * 10ns = 160microseconds. In this time a PDP-11 executes about 100 opcodes, so it seems sufficient for errors in short test programms. However it requires precise triggering.
From my experiences, 256kByte are enough.
Many LAs have a "compression" feature, they record not at a fixed clock but record only actual signal changes. This will multiply usable memory, but is often coupled with some constraints on trigger features or sampling frequency. I'd say: Don't make it a buying decision.
Triggering: You must be able to trigger on certain words on parallel data busses, combining several data lines to one trigger condition. Triggering on the nth occurence of a signal is fine ("trigger counter"). Triggering on time constraints ("find a loss of signal A for 10 microsconds") is fine. Having at least two trigger levels to catch a "find B only after A happend" condition is sometimes necessary. I'd say: read the trigger spec carefully.
What's not an issue: support for serial protocols. Today digital components communicate in a serial manner with each other, there are lot of protocols: RS232, I2C, SPI, 1-wire, CAN, USB, LIN, ... Offering protocols is a major goal for LA vendors, but for retrocomputing, you need only RS232 decoding.
My LA's
In 2009 I bought the Intronix. In 2014 I finally spent $x000 for a 70chanel x 200MHz x 2Msample LA: the ZEROPLUS LAP-B 702000X. It's a chinese vendor. Cost/performance ratio is very good, but the manual is the worst I saw ever. The control software is a bit better, but I still can operate only the basic features.
Trigger lsvels: