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IC substitutes

falter

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A question that arose as I have been fixing things I broke -- I am assuming over the course of time many same-design products have been made over the years in the IC department and that there might be equivalents to and earlier type in a later form. I'm wondering if that's true and if so, how you go about most easily determining compatibility?

For example, on my Rev I Apple II keyboard, I need possibly another 7400, which I did order. However, in doing some reading it would appear that 74LS00 might also be suitable as a replacement, the LS having something to do with the power requirements, which on reading appear to be the same as the original 7400. Can I assume, for example, that a 74LS02 is equivalent to an older 7402 and so on? How do you know?
 

SomeGuy

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There are timing differences between the LS, non-LS, and other variety. See http://en.wikipedia.org/wiki/7400_series

Using a different one may appear to work in some cases, but could cause subtle issues or errors in edge cases. It is possible that in some cases manufacturers may have used one type when another would have sufficed due to supply, but you would need to be familiar with the specific circuits to be sure.

Or you could just try it and see if it works well enough :)

The Apple IIs are kind of sensitive to timing, but I would think something in the keyboard wouldn't be too much of an issue if it were a little off.
 

MikeS

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"TTL-compatible" essentially means that a device is designed to generate/respond to a signal between 0V and 5V where < 0.7V represents a zero and > 2.0V represents a one (although the meanings of one and zero are sometimes reversed within a given system).

The differences among the various TTL-compatible device families (aside from temperature range, mil spec etc.) are:

Speed - the time it takes to switch from one to zero and vice versa
Power consumption - the total power drawn under specified conditions.
Input current - the input current required to switch from one to zero
Output current (Fan Out) - the output current it can supply (and number of inputs it can drive) and still remain < 0.7V in its low (zero) state.

In the beginning there was TTL (after RTL, DTL, etc. ;-) ), usually 74 (or 54) with a number following that defines the particular logic elements in it.
Then there was Low Power (74Lxx): Lower power, but slower.
Then Schottky (74Sxx): Faster, but requiring more power.
Finally Low power Schottky (74LSxx): Lower power and faster; probably the most common family in our vintage systems.

Since all TTL families are essentially compatible with regard to signal levels the only issues are speed and Fanout, and if you're replacing a 74xx with a (faster) 74LSxx then speed is probably not an issue. Because the LS's output drive is lower, the only time replacing a 74xx with a 74LSxx may matter is if the 'LS chip's output is driving more than one 74xx chip or something other than a TTL IC.

At least that's how I see it...

Of course there were also various other later TTL-compatible families with higher output drive, lower power consumption, faster speed, CMOS-compatibility, etc. etc.
 
Last edited:

falter

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"TTL-compatible" essentially means that a device is designed to generate/respond to a signal between 0V and 5V where < 0.7V represents a zero and > 2.0V represents a one (although the meanings of one and zero are sometimes reversed within a given system).

The differences among the various TTL-compatible device families (aside from temperature range, mil spec etc.) are:

Speed - the time it takes to switch from one to zero and vice versa
Power consumption - the total power drawn under specified conditions.
Input current - the input current required to switch from one to zero
Output current (Fan Out) - the output current it can supply (and number of inputs it can drive) and still remain < 0.7V in its low (zero) state.

In the beginning there was TTL (after RTL, DTL, etc. ;-) ), usually 74 (or 54) with a number following that defines the particular logic elements in it.
Then there was Low Power (74Lxx): Lower power, but slower.
Then Schottky (74Sxx): Faster, but requiring more power.
Finally Low power Schottky (74LSxx): Lower power and faster; probably the most common family in our vintage systems.

Since all TTL families are essentially compatible with regard to signal levels the only issues are speed and Fanout, and if you're replacing a 74xx with a (faster) 74LSxx then speed is probably not an issue. Because the LS's output drive is lower, the only time replacing a 74xx with a 74LSxx may matter is if the 'LS chip's output is driving more than one 74xx chip or something other than a TTL IC.

At least that's how I see it...

Of course there were also various other later TTL-compatible families with higher output drive, lower power consumption, faster speed, CMOS-compatibility, etc. etc.

Thanks Mike!

Well, I did finally remove the suspect dead 7400, and then tried the last remaining 7400 I had here, but no dice. So then I swapped to 74LS400, and voila.. we have life again. But, not getting the correct keys. So I'm not sure if this is an issue related to the LS chips being slightly different in some way, or some other issue.. here's a screenshot:

20140305_140648.jpg

The first line is the QWERTY row.. (the bunch of Ps at the end are when I hit the repeat key -- when I hit that, it does a random smattering of the last key pressed (sometimes just 1, usually 4 or more). Next is the row below it, and then finally the numbers above. 1 2 3 and 4 produce nothing, then 5 6, then no 7 or 8 then 9 etc.. I'm trying desperately to understand the schematic to see if there's any sense to there. But maybe I have to wait for those replacement chips I ordered. Hoping it's not the encoder. I know for sure it's not motherboard logic problems -- I changed to a different motherboard and got exactly the same result.
 

MikeS

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Is there a repeatable pattern; e.g. is the 'E' consistently replaced with a 'U', an 'I' with a 'Y', etc.?

Where's the schematic?
 

falter

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Is there a repeatable pattern; e.g. is the 'E' consistently replaced with a 'U', an 'I' with a 'Y', etc.?

Where's the schematic?

http://mirrors.apple2.org.za/Apple ...Gayler - The Apple II Circuit Description.pdf

In the appendices for single piece keyboard.

Just for an experiment, I removed the 555 timer chip. Suddenly the entire top row of keys started working. The lower rows remain problematic. And yes, the incorrect keys result in the same characters each time, consistently. I'm prepared to lose the Repeat key if I have to.. the 555 unfortunately that area was in bad shape to begin with and just didn't desolder well.

I've swapped around different 74SL00s and 74SL04s but no change. I'm concerned the issue is something to do with the traces. When the 555 blew I removed it and installed a socket, but during that process some of the pads came off. I also have one or two iffy pads on the 74SL00/7400 at U1. What I wish I had was a really good hi-res shot of the front and back of the PCB for that keyboard (single piece, brown PCB). To compensate for damaged pads I did my best to follow traces and reconnect with jumper wires. If I had a hint of where i'm going wrong I think I could nail it. I'm still just not figuring out how to read the darn schematic. :) Note: the bottom row was never working (bad switches). Hoping to get some replacements in soon.

Here's a pic with four of the keyboard rows typed:

20140305_162129.jpg20140305_162129.jpg
 

Chuck(G)

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If it's the fanin/fanout that's the issue, try using an 'S00 instead of the LS00. Same output current as the '00, but about twice as fast. Somewhat more power-hungry. Easer to find new than the original '00.

Should you ever run across any 74Lxx logic, be very careful--you could wire-OR the outputs and sometimes the pinout of th 74Lx differs from the 74x. Not common, but I believe that the front panel of the Altair 8800 used one such device.
 

falter

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Using the table gslick gave me, I looked at the outputs.. output 5 appears to be stuck high. E becomes U, I becomes Y, D becomes T, etc. with that one bit high. It's the same one for each. Now, I've rotated chips around and am feeling pretty confident it's not those. I just wish I could read the schematic better and understand how the whole input/output works.

I would assume if I had, say a pin on one of the chips that wasn't making a connection, that would be stuck 'low'? Still learning this stuff. :)
 

falter

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I did it!

I re-read the 'process' gslick had used to deduce which chip was giving me trouble on my Apple II+ and applied it here. Sure enough, output 5 runs through pin 2 of the 7404. When I re-examined it, I discovered that it was one of the pins where the pad had broken off and the pin was no longer making contact with the trace. I exposed a little bit of trace nearby and then ran a 'string' of solder up to it. Presto chango, it's back to working.

Unfortunately the Repeat key does not work.. I don't know if I can figure that one out without seeing a pic of the PCB and traces again. I don't know if I'd have much use for that key anyway..
 

falter

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It gives me hope that I can recover the dead clone keyboard I have here. Same thing as I started with on the real Apple II keyboard.. only ctrl-reset works. Going to socket the chips and do some swaps. Hoping the controller chip is as robust as the one on the real apple II keyboard has been
 
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