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#1 2014-11-21 19:13:19

max1zzz
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Registered: 2014-05-18
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Whats -5v Used for In LC's?

Just wondering, whats -5v Used for in the LC's? And is there any chance of running a LC without it?

I'm putting together a small LC based Rackable file server for my old macs (Basically it's gonna be a LC logicboard, PDS Ethernet and a SCA Hdd in a small 1U Box) and the need for -5v is complicating things. It either requires me to use a DC-DC converter or a pre built PSU (Well, baring me compleatley building a PSU from scratch using a center tapped transformer. I Guess i could also take two identical Bricks, join them together and take the join as ground aswell)

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#2 2014-11-21 20:49:04

uniserver
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Re: Whats -5v Used for In LC's?

pretty sure it's for the serial ports.2

Last edited by uniserver (2014-11-21 20:49:26)


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#3 2014-11-21 20:51:49

max1zzz
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Re: Whats -5v Used for In LC's?

So will it run without -5v?

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#4 2014-11-21 20:54:06

uniserver
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Re: Whats -5v Used for In LC's?

that is a good question.


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#5 2014-11-21 20:55:48

uniserver
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Re: Whats -5v Used for In LC's?

so far in my google searches i see this.

The -5V rail was used for old-school floppy disk drive controllers and some ISA bus cards.


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#6 2014-11-21 20:57:16

uniserver
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Re: Whats -5v Used for In LC's?

here is a little more :  :-)

-12 V: This voltage is used on some types of serial port circuits, whose amplifier circuits require both -12V and +12V. It is not needed on some newer systems, and even on older ones not very much is used, because the serial ports require little power. Most power supplies provide it for compatibility with older hardware, but usually with a current limit of less than 1 A.

-5 V: A now archaic voltage, -5 V was used on some of the earliest PCs for floppy controllers and other circuits used by ISA bus cards. It is usually provided, in small quantity (generally less than 1A), for compatibility with older hardware. Some form factor power supplies such as the SFX no longer bother to supply it (systems using the SFX power supply are intended not to have ISA bus slots).


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#7 2014-11-21 20:58:30

uniserver
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Re: Whats -5v Used for In LC's?

hold on i am going to hit the schematic really quick .


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#8 2014-11-21 21:02:52

max1zzz
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Re: Whats -5v Used for In LC's?

Ok, so it looks like I should be good then, I won't be using a floppy drive and proabley won't be using the serial ports

Let me know what you find in the schematics.

Edit: Can you send me the LCII schematics - I see you have removed the links from your site due to bandwith costs

Last edited by max1zzz (2014-11-21 21:06:25)

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#9 2014-11-21 21:07:30

uniserver
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Re: Whats -5v Used for In LC's?

the schematics i have are from bomarc, and i don't see anything that identifies voltage.
but i did find this

23.2 Voltages

Voltage for a bit

At the EIA-232 serial port, voltages are bipolar (positive or negative with respect to ground) and should be about 12 volts in magnitude (newer ones are 5 volts but the 12 volt example is given here). For the transmit and receive pins +12 volts is a 0-bit (sometimes called "space") and -12 volts is a 1-bit (sometimes called "mark"). This is known as inverted logic since normally a 0-bit is both false and negative while a one is normally both true and positive. Although the receive and transmit pins are inverted logic, other pins (modem control lines) are normal logic with a positive voltage being true (or "on" or "asserted") and a negative voltage being false (or "off" or "negated"). Zero voltage has no meaning (except it usually means that the PC is powered off).

A range of voltages is allowed. The specs say the magnitude of a transmitted signal should be between 5 and 15 volts but must never exceed 25 V. Any voltage received under 3 V is undefined (but some terminals will accept a lower voltage as valid). One sometimes saw erroneous claims that the voltage is commonly 5 volts (or even 3 volts) but it's usually 11-12 volts on older PCs. If you are using a EIA-422 port on a Mac computer as an EIA-232 (requires a special cable) or EIA-423 then the voltage will actually be only 5 V. The discussion here assumes 12 V. There was much confusion about voltages on the Internet.

Note that normal computer logic normally is just a few volts (5 volts was once the standard) so that if you try to use test equipment designed for testing 3-5 volt computer logic (TTL) on a 12 volt serial port, it may damage the test equipment.


and yes i think the machine should still work w/o -5v,   does your psu for some reason not have -5v?


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#10 2014-11-21 21:16:36

max1zzz
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Re: Whats -5v Used for In LC's?

Well my plan was to use a high current 12V brick, coupled with a linear 5v regulator (Possibly with a big pass transistor - depending on how much current the board actually draws). And there isn't any easy way ti generate -5v from there

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#11 2015-03-06 14:59:19

max1zzz
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Re: Whats -5v Used for In LC's?

Finally got around to testing this, without the -5v the LC seems to run just fine. It happily fires up and boot's from floppy. I haven't tried serial, but i don't have any real need for it so i'm not bothered if it doesn't work

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#12 2015-03-07 01:05:08

techknight
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Registered: 2014-05-22
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Re: Whats -5v Used for In LC's?

Using a linear regulator for the 5V is just crazy. haha. Once that machine is fully loaded, with FDD access, etcc I bet it dumps alot of heat.

you could consider getting one of those el-cheapo DC-DC converters for 12V to 5V on ebay. Would run much cooler.

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#13 2015-03-07 13:15:37

max1zzz
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Registered: 2014-05-18
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Re: Whats -5v Used for In LC's?

lol yeah, I came to realize that smile  I'll probably get one of those converter's, the go for about £4 from the UK smile

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#14 2015-03-26 06:31:13

macdoogie
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Registered: 2015-03-26
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Re: Whats -5v Used for In LC's?

uniserver wrote:

so far in my google searches i see this.

The -5V rail was used for old-school floppy disk drive controllers and some ISA bus cards.

The above looks like information for IBM compatible PCs(ISA bus, anyone?), not necessarily applicable to Macs.

Negative voltages in legacy designs generally are used for either analog circuitry (Audio amplifiers, and maybe some Video stuff as well) or differentially signaled I/O Busses like RS232. Macs use RS422 for serial and I forget if that utilizes negative voltages for signaling.

Did your sound work in your LC without the -5V(Q for Original Poster, not uniserver)? From the specs on the side of the supply, the -5V rail has a relatively tiny current rating, so I doubt there's too much using it.

Using a linear regulator to reduce 12V down to 5V is really going to get hot! You will be dumping more than half your power output as heat! Build a little switcher on a perfboard, they're really not all that intimidating once you understand the basics.

Hope this helps! big_smile

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#15 2015-03-26 22:13:37

uniserver
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From: Sf, Mi
Registered: 2014-05-15
Posts: 956
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Re: Whats -5v Used for In LC's?

So far i do know that the LC475 and Q605,  … and what ever performa variants of this 25/33mhz 040 MB…
if your -5 rail is not working you get no sound.

With the LC-123, pretty sure you still get sound even if the -5 rail is dead.
RS-422's not sure...


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#16 2015-03-28 01:11:18

techknight
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Registered: 2014-05-22
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Re: Whats -5v Used for In LC's?

Woa, I didnt know that.

So theres a chance I did fix that LC475 sound problem, but didnt know that I did because I dont have -5V handy. Just +12v and +5V on my bench supply

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#17 2015-03-28 05:26:59

uniserver
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Re: Whats -5v Used for In LC's?

dammit! LOL


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#18 2015-03-28 10:55:40

max1zzz
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Registered: 2014-05-18
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Re: Whats -5v Used for In LC's?

I can confirm that sound works on the LC1 without -5v

Before i can do any more testing i need to figure out whats wrong with the logicboard, intermittently it refuses to power up, i'm betting there is a rotted via somewhere that makes connections sometimes and not others

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#19 2015-04-08 20:00:12

max1zzz
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Registered: 2014-05-18
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Re: Whats -5v Used for In LC's?

An interesting side note, the LC will in fact run on just the 5v line (I haven't tested it's functionality, but it at least boots)

This doesn't really help this project as i have no high current 5v supplies, but it's a interesting find.

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