Test equipment TTL inputs/outputs

[richard.cs]

I've always blindly connected "TTL level" sync and reference inputs/outputs together on sig-gens, spec-ans, etc, without thinking too much and it's always just worked. I find myself wondering how these are actually implemented.

Are the 50 ohm lines terminated at each end so the receiver sees half the TX voltage? If so is the driving end "5V TTL" giving a very marginal logic level at the other end or 10V giving a good logic level into a terminated line (and potential damage at an unterminated one)? Or maybe only terminated at the TX end like VGA sync, so it can be driven with 5V and the RX sees a 5V pulse?

Is there no consistent standard? Or maybe inputs are more sensitive than normal TTL so a 2.5V received pulse is fine? I really have no idea. It's never mattered to me before but now that I find myself interested in building test equipment it's a rather useful thing to know.

[edavid]

Yes, there is no consistent standard, but an output marked TTL will not exceed TTL voltage limits even if open circuit, and an input marked TTL will have TTL-compatible thresholds, and will not exceed reasonable TTL loading.

Just because it's a BNC, doesn't mean it's a real 50 ohm input or output.

[SeanB]

TTl levels are more like 330R terminated at the transmit and receive end, and at the receive end it is capacitively coupled into an inverter biased to half the supply. Generally either a regular 7414 inverter fed from half the supply via a 330R resistor or a single transistor inverter. I have seen some using a 7474 as a receiver, with it being used to both recover a clock and a data signal from the received data.

[richard.cs]

Useful information thanks.