Different ROMs for Lambda 8300

[1024MAK]

The 74LS05 can only drive the databus to a low state (so the Z80A sees the data as a NOP instruction) as it has only open collector outputs. The 470 ohm resistors “isolate” the memory from the 74LS05.

Mark

[mrtinb]

I assumed ULA pin 39 would always be HIGH or LOW, which would make CPU D0-D7 always HIGH or LOW. There is no resistor between 74LS05 and CPU D0-D7. That's what I don't understand. Please teach me.

[1024MAK]

A 74LS05 can only drive it’s outputs low, meaning logic low. The only other state for its outputs is effectively ‘open circuit’. It is not capable of driving it’s outputs to a logic high stage.

Think of the output stages of a 74LS05 as being a NPN bipolar transistor collector, with no other circuitry. When the NPN transistor is ON, current can flow from the collector (data bus) to the emitter (0V/GND/ground). Hence forcing the data bus to logic zero.

When the NPN transistor is OFF, no current can flow from the collector (output) to anywhere in the chip. Hence as far as the data bus is concerned, it’s like the 74LS05 is not present.

Mark

[mrtinb]

Sorry. So that's what open collector means.

Looking at the datasheet for 74LS05 https://www.ti.com/cn/lit/ds/symlink/sn ... 3774032386

The truth table is:

Code: Select all

IN OUT
H   L
L   H

What you are describing sounds like:

Code: Select all

IN OUT
H   L
L   Z

I know I'm the one not understanding. I better read up on open collector.

It's just me that can't see the difference between 3-state and open collector.

[mrtinb]

In the meantime I've come to suspect that my issue really has more to do with the 74LS05, as it looks like the data bus lines behave weird, while the address lines look normal.

Well changing the 74LS05 shoud be a cheap fix. So it's worth trying out. Requires you have desoldered an IC before though.

[1024MAK]

The truth table would be correct if external pull-up resistors (to the +5V supply) are used.

The difference between 3-state and open collectors is this: with 3-state (or tri-state) there are two inputs, one for the logic function, and one for the 3-state control. Hence the truth table may be:
A B C
0 0 1
1 0 0
0 1 high impedance
1 1 high impedance

Where A is the logic input, B is the 3-state input and C is the output, assuming the logic gate is an inverter/NOT gate and the 3-state control input is an active low input.

An example is the 74LS240 octal buffer

But with a inverter/NOT gate that has an open collector output, you only get the logic gate input(s), so it’s more like:
A B
0 high impedance
1 0

Where A is the logic input, B is the output if there is nothing else driving the output.

What this means is that gates with open collector outputs can be paralleled to form wired-OR (or wired-AND depending on how you look at it) logic systems.

In the Lambda, when any other device is driving the data bus, if the inputs to the 74LS05 are low, it’s NPN transistor outputs will be off. Hence it will not affect the data on the bus. Or if nothing is driving the data bus, the 10kΩ pull-up resistors will cause a (weak) logic high level on the data bus lines.

If the inputs to the 74LS05 are high, then regardless of logic level the memory chips are trying to drive the data bus to, the NPN transistor outputs in the 74LS05 will drive the relevant data bus lines low. The 470Ω resistors preventing the 74LS05 from shorting out the memory chips outputs. The Z80A sees the value on the data bus as a NOP.

This is very similar to what IC14 and IC15 do in a ZX80 (IC14 and IC15 are both 74LS05).

Mark

[frankc64]

In the meantime I've come to suspect that my issue really has more to do with the 74LS05, as it looks like the data bus lines behave weird, while the address lines look normal.

Well changing the 74LS05 shoud be a cheap fix. So it's worth trying out. Requires you have desoldered an IC before though.

Indeed. Bought a couple of 74LS05s yesterday that I am going to put in the machine tonight. No soldering needed. My machine is entirely socketed!

/Frank

[frankc64]

BTW, what is the correct PSU voltage for the Lambda. The one that came with it was 15V, which worries me. On the schematics it seems that voltages are internally regulated to 5V and 9V, but even so. I use a 12V supply instead, as it appears to me to be a safer solution?

[mrtinb]

I change my power supply between 6V, 9V and 12V depending on the noise on the TV. Somehow the TV signal changes during use, depending on how warm the computer is. I just change to a different voltage, to get a better TV signal. There is no voltage, that is clean all the time.

[1024MAK]

It depends partly on if the PSU you are using is a regulated unit or an unregulated unit. Regulated types hold their output at typically within +/-5% (or better) of their rated output voltage. Whereas unregulated PSUs have a much higher output voltage when they are not fully loaded (actual output current/power is less than their rated output current/power).

Certainly I found that mine needs a minimum of +12V, as detailed in this post.

A 15V DC supply is fine, it will not cause any damage (the 7805 voltage regulator is good for up to 25V maximum) and the 9V power circuitry is also good for up to 25V. The disadvantage is that the heatsink on the 7805 will get hotter as the input voltage goes up.

Mark