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If there is an IC with multiple CS (chip select) lines, like the (old) MOS 6540 (for example):

6540 pinout

Source: cbmhardware.de

Warning: This pinout may not be 100% correct, please take a look at this website: dasarodesigns.com

Is it correct that you need to enable all of the available CS lines to enable the chip (with enable I mean high for high-active CS and low for low-active _CS inputs)?

Not meant to be a 6540-specific question!

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  • \$\begingroup\$ Something is wrong with the image. \$\endgroup\$ Aug 7, 2017 at 10:12
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    \$\begingroup\$ @BenceKaulics Looks OK on my computer (even after refreshing site). Or are you referring to an error in the document? If so, please let us know, what is incorrect. \$\endgroup\$
    – RhinoDevel
    Aug 7, 2017 at 10:17
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    \$\begingroup\$ 1: Off topic. 2: How is this not directly answered in the datasheet right where you'd expect to find it!? \$\endgroup\$ Aug 7, 2017 at 10:50
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    \$\begingroup\$ @OlinLathrop Read the comments below and you shall find the answers to your questions. :) \$\endgroup\$
    – RhinoDevel
    Aug 7, 2017 at 11:17
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    \$\begingroup\$ May be you can ask about it on retrocomputing.stackexchange.com \$\endgroup\$
    – user13267
    Aug 8, 2017 at 0:44

3 Answers 3

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Typically, yes. A chip is not selected unless all its chip select lines are activated. Sometimes the chip selects have different polarity, one is negated and the other is not.

This usually simplifies the chip select logic, and does no harm because if you only need one, you can tie the others to whatever level necessary.

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    \$\begingroup\$ Yeah. If you had only one CS line you'd need to use a binary decoder to select one of N chips. But with multiple lines and a dash of cleverness/insight you can simply use "logic" in the printed circuit wires to do this. \$\endgroup\$
    – Hot Licks
    Aug 7, 2017 at 12:04
  • \$\begingroup\$ @pipe, the answer to this question is specifics for each IC. Every IC with multiple CS# lines which I've seen so far, had multiple features and each CS# would select a particular feature, or a combination of CS# lines would select a feature. So, the right answer is "There is no common behavior. Read the datasheet.", rather than "Typically, yes." On that note, -1 (no hard feelings). \$\endgroup\$ Aug 7, 2017 at 17:05
  • \$\begingroup\$ The question itself lacks the link to the datasheet, which makes the question unclear. Despite missing the datasheet, the question got promoted to hot network questions. \$\endgroup\$ Aug 7, 2017 at 17:05
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    \$\begingroup\$ @NickAlexeev Here: 74138. Now you have seen an IC with three CS-lines. Same goes for the ROMs I've seen, though I can't find good datasheet for them. Maybe you're talking about WE and OE, because I can't really think of something called a "chip select" unless it actually selects the whole chip instead of a particular function. \$\endgroup\$
    – pipe
    Aug 7, 2017 at 17:36
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    \$\begingroup\$ the datasheet of the 6540 has its 40th anniversary. should go to retrocomputing SE. here is as much as I could find: amiga-stuff.com/hardware/6540.html, including the reasoning for having five chip selects: mask options provide user specification of chip select equations, allowing addressing anywhere within a 65K memory space without external decode circuitry \$\endgroup\$
    – dlatikay
    Aug 7, 2017 at 21:39
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Sometimes the answer is yes. But this is a chip design specific thing and requires that you look at each part's data sheet. There is NO general answer for this.

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  • \$\begingroup\$ Sure true, unfortunately the datasheet for (e.g.) the 6540 is not available (or at least I haven't found it..). \$\endgroup\$
    – RhinoDevel
    Aug 7, 2017 at 10:15
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    \$\begingroup\$ @RhinoDevel - Then change your question to be specific about that one chip if that is what you are having problems with. You asked a broad question with only vague reference to one chip so you get a broad answer. \$\endgroup\$ Aug 7, 2017 at 10:18
  • \$\begingroup\$ No, that's fine. This was meant to be a "newbie" question. :-) \$\endgroup\$
    – RhinoDevel
    Aug 7, 2017 at 10:19
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    \$\begingroup\$ Don't just pass it off. If you want to move from "newbie" where you learn something valuable then stop it. My point stands!! \$\endgroup\$ Aug 7, 2017 at 10:21
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    \$\begingroup\$ Sorry, I did not mean to be rude (I am no native english speaker). I get your point, but I wanted to know, if this is something "everybody" knows about, that I just missed (search engine did not help). Maybe I will ask another 6540 specific question. Thank you! \$\endgroup\$
    – RhinoDevel
    Aug 7, 2017 at 10:26
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I can't find a datasheet for the 6540 online anywhere, but I did come across a design for an adaptor that allows you to replace it with a different ROM such as a 2716:

http://www.dasarodesigns.com/product/6540-rom-adapter-rev-1/

In the accompanying text it states:

2) The 6540 has five chip select lines. CS3, CS4, and CS5 are active low meaning that they must be held low (+0v) for the chip to be on (its data lines to be low impedance). CS1 and CS2 are active high, meaning that they are must be high (+5v) for the chip to be on.

As this information appears to have been used to produce a working design it's likely to be correct.

As others have stated, it's important to check the datasheet for the exact answer but certainly in my experience it's usual to have to activate all the chip select lines in order to enable a device.

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