Timing issues and resistor R100 on the Amiga 1200

[Sandro]

Trying to solve some timming issues which my A1200 have I found that
there is a resistor called R100 in the clock part of the A1200 which is present only in Rev 2b motherboards but not in the others revisions ie 1D4 1D1 etc

according to the A1200 schematics based on 1D revisions this resistor exists and is 470 ohms
you can see R100 in the schematics here




however is not there in a rev 1.D1 motherboard




is not on rev 1.D4 neither




but is present in 2B motherboards




This resistor seems vital for proper clock and timing of the A1200
seems this resistor is a terminator,fact is it connects the clock vital line to 5V

so my questions for the experts only:

1: why is not there on the oldest A1200 revisions ?
2: what could happen if I connect this resistor in my A1200 rev 1.D1 ?

[ajk]

That acts as a pull-up resistor. My guess is that the oscillator already has an internal pull-up, so the external one is not necessarily needed. Perhaps it depends on the particular type of oscillator and they just either put those in in the factory, or didn't, as needed.

[Sandro]

Originally Posted by ajk

That acts as a pull-up resistor. My guess is that the oscillator already has an internal pull-up, so the external one is not necessarily needed. Perhaps it depends on the particular type of oscillator and they just either put those in in the factory, or didn't, as needed.

the oscillator of my motherboard doesn't have any internal pull-up resistor
there is not any connection from 5v to the vital clock line
I just tested it using a multimeter

edit

do you think that adding a resistor could be safe ?
please consider I just checked and there is not any type of resistance from E131 to 5v

[Daedalus]

It's also possible that some revisions of custom chip need the pull-up due to extra loading or something whereas others didn't. I know that the Budgie revision has a bearing on some timing issues related to the purity of the clock signal so maybe that explains the difference?

Edit: Also, there are some vector remakes of the schematics available here, meaning they're searchable and easier to read than those old fax-looking ones.

[Sandro]

Originally Posted by Daedalus

It's also possible that some revisions of custom chip need the pull-up due to extra loading or something whereas others didn't. I know that the Budgie revision has a bearing on some timing issues related to the purity of the clock signal so maybe that explains the difference?

Edit: Also, there are some vector remakes of the schematics available here, meaning they're searchable and easier to read than those old fax-looking ones.

well, A1200 mobo 2b uses budgie rev A , all others 1.D versions uses budgie rev 0

but I'm sure this is nothing related to this because the R100 is present on commodore 1D schmematics
for some reason R100 not was included in the 1Dx motherboards, I will insert this R100 on my A1200 1D1 mobo to check what happens

[Daedalus]

I'm pretty some sure the Budgie revision was mixed with motherboard revision. I'll have to check, but I think one of my revision 1D4 motherboards also used Budgie rev. A. But either way, if it's just a terminating resistor there's no harm in trying it, and it might improve things. Worst case scenario the machine doesn't start due to a missing clock signal.

[Sandro]

Originally Posted by Daedalus

I'm pretty some sure the Budgie revision was mixed with motherboard revision. I'll have to check, but I think one of my revision 1D4 motherboards also used Budgie rev. A. But either way, if it's just a terminating resistor there's no harm in trying it, and it might improve things. Worst case scenario the machine doesn't start due to a missing clock signal.

yep is a terminator resistor or pull up resistor
please check your 1D4 with budgie rev A and tell me if such motherboard have R100 inserted
thanks

[Sandro]

update:

I inserted R100 in my A1200 and the computer don't boot
removed the resistor and everything was fine

perhaps the member ajk was right about the crystal having internal pull-up resistor, I Checked it again there is a 232 ohms resistance in clock line with 5v, I was measuring incorrectly before

[Daedalus]

Interesting... Next time I have my Amigas open I'll make a note of the Budgie revision versus the board revision. In the meantime I did find this:

Rev 1D.1
Rev 1D.3 (Budgie Rev -01 Release, E123C and E125C have been removed from motherboard)
Rev 1D.4 (Budgie Rev -02 Release, additionally XR358 470Ohm resistor removed, 470Ohm resistor added to pin 43 of Alice)
Rev 2B (Both budgie Rev -01 and Rev-02 were used. Resistor 118 was changed from 470Ohm to 220Ohm)

From Big Book of Amiga Hardware, so maybe it was the Rev. 2 motherboards I was thinking of with the mixed revisions.

[Sandro]

Rev 1D.1
Rev 1D.3 (Budgie Rev -01 Release, E123C and E125C have been removed from motherboard)
Rev 1D.4 (Budgie Rev -02 Release, additionally XR358 470Ohm resistor removed, 470Ohm resistor added to pin 43 of Alice)
Rev 2B (Both budgie Rev -01 and Rev-02 were used. Resistor 118 was changed from 470Ohm to 220Ohm)

this is incorrect data because E123c and E125c were not removed until version 2b
you can inspect 1d4 motherboards and 1d3 you will see they have inserted those caps

also note that only rev 2b have R118 installed so it seems nothing was changed because older motherboards don't have this resistor

http://www.bigbookofamigahardware.co...uct.aspx?id=13

I wonder where is XR358 on the motherboards because I can't found it