If you connect a 1kΩ resistor in series with your multimeter (*1), with your multimeter switched to the 20mA (or equivalent) DC current range, you can measure the ability of a logic circuit to source or sink current. And therefore work out if the voltage on an otherwise unconnected input is due to a pull-up (or pull-down) or is just a bit of small leakage current via the internal circuitry.
Useful tip, thanks. I'll give it a go soon.
It either works or catches fire. Either way is fun. Zolatron 64 project (on Medium)
Having tried many bodges, making up a fresh copy of the serial card and even respinning the serial card (hasn't arrived yet) I discover that it's not a hardware problem. It's a software problem. -=sigh=-
And it's an extremely weird software problem ... but that's for another day.
It either works or catches fire. Either way is fun. Zolatron 64 project (on Medium)
Use any two small diodes (preferrably Schottky, like BAT54 or similar) and two resistors like 4k7. Pull up both RxD inputs with resistors to their own power supplies, connect diodes' anodes to RxD inputs, cathodes to the other parties TxD outputs. That's all.
Use any two small diodes (preferrably Schottky, like BAT54 or similar) and two resistors like 4k7. Pull up both RxD inputs with resistors to their own power supplies, connect diodes' anodes to RxD inputs, cathodes to the other parties TxD outputs. That's all.
Like I said, it turned out to be a software issue. I changed one line of code - actually just one character in the line, and moved a jumper on the serial board and normality was restored with the original hardware setup. Basically, I had to change the address of the serial board.
It either works or catches fire. Either way is fun. Zolatron 64 project (on Medium)
