1024MAK wrote:So it has no trouble driving the /TUBE line with the 4.7k resistor wired as shown. Indeed if needed, such an output could drive a LED.
Where would such an LED be put? Something like this?
My earlier post was written to try to explain that a 74LS138 output pin was powerful enough to "supply" enough current to light a LED when the output was low.
This is the circuit: IC1 (74LS138N) is the chip in the Beeb. Resister R2 is included to limit the LED current. In this case, if a red or green standard LED is used the current will be about 7.7mA.
I have put the word supply in quotes because in practice the 74LS138 output pin is actually providing a path to ground rather than producing a "positive" output current.
If a standard LED is used, as the 74LS138 output pin is rated for 8mA in this configuration, it can affect the logic level (by logic level, I mean the defined voltages as per the TTL specifications, logic low is between 0V and 0.8V, and logic high is between 2V and 5V). This is not a problem if the only function required is a LED.
If however, the 74LS138 output pin is required to control a LED and
provide a good logic level to another chip, then either an additional circuit known as a buffer (also known as a driver) is needed, or alternatively a high efficiency low current LED can be used. Now resistor R2 is 1.5k, so with a red or green high efficiency low current LED, the current is about 2mA.
Now I presume you are after an indicator to show if the second processor is active?
If a LED is added to the /TUBE control line, it will only light when the 6502 CPU in the Beeb reads from or writes to the second processor. Depending on how much data is being transferred, you may see it flicker on, but without having tried this, the results may be disappointing.
One other thing that I really should have mentioned earlier. The TubeSilencer chips are of the CMOS type. These can give unpredictable results if an input line is left floating (disconnected without a defined logic level) as the wire to them acts as an aerial (CMOS inputs are high impedance - a very, very low current can affect them when an input is floating). And yes I know this is hard to understand.