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The unit will drive up to just 5 instruments. I suppose it is fair value?compared with some other offerings of this function."
Reason being that each instrument sources 5V through 3k. You need to pull that below 0.8V. Say that you need .5V to be on the safe side. That means you need to sink 1.5mA per instrument on the bus.
So, how many you can handle depends on the chosen microcontroller as none of the cheap GPIB interfaces, including mine, use the dedidated SN75160/75161 chips.
Anders