Posted By
Retroshire
on 2020-10-10
12:57:43
|  Rom (Funcion Lo Hi) problems
Problem with the ROM recognition in my Plus/4. Function Roms don't work. Strange: Diag264 says the Basic rom is identic with the Funcion Lo rom. It does the same with fresh burned Eproms. I then removed the Basic rom, Diag264 says Basic Rom and Function Lo rom are skipped (absent), while Function Lo is not removed. Function Hi identified as Bad, while it is a freshly burned Eprom (Original Function Hi gives the same fault)
What part is directing the roms?
> I read in the Ultimate Plus/4 ROM chips map:
HARDWARE FDD0-FDDF * Roms latch: select the configuration of ROMs viewable by O.S. - On schematic, this is the CLK/IN for 4 D-FlipFlops located in the 74LS175 chip ADDR CLK ROM low ROM high $8000-$BFFF $C000-$FFFF
Am I possibly dealing with a failing 74LS175?
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Posted By
Retroshire
on 2020-10-12
12:53:23
| Re: Rom (Funcion Lo Hi) problems
No HW experts in the house? Or a stupid question?
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Posted By
MCes
on 2020-10-12
13:20:01
 | Re: Rom (Funcion Lo Hi) problems
In your shoes... I could try to replace U20 (74LS139) and U21 (74LS175).
But for the first step I could try to place original ROMS: on PLUS4 motherboard the ROMs pin1 are "not connected", but it has to be connected with pin 28 (Vcc) for a normal EPROM use.
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Posted By
Retroshire
on 2020-10-13
06:25:25
| Re: Rom (Funcion Lo Hi) problems
MC, I don't understand what you mean by pin 1 and pin 28. You suggest that they should be connected for normal eprom use. You mean a wire between those pins?
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Posted By
MCes
on 2020-10-13
10:03:57
| Re: Rom (Funcion Lo Hi) problems
---YES---
http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/plus4/index.html
On this site you can found the schematic of C16 and PLUS4, into C16 the pin1 of each rom si connected with Vcc, but into PLUS4 this connection is absent.
Original ROMs don't need it, but (E)EPROMs yes, they need it!
Before other test I suggest to place a little wire over each EPROM chip that connect pin1 with pin 28 (Vcc), but remember that during the (E)EPROM burning this wire has to be removed!.
Otherwise the little wire can be soldered on back side of the PLUS4 motherboard.
Perhaps the situation won't change, or maybe yes...
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Posted By
Retroshire
on 2020-10-13
11:36:08
| Re: Rom (Funcion Lo Hi) problems
OK, I understand. Is it possible one of the 74LS is effected by placing an eprom before (jiffydos) without connecting pin 1 and 28? Because it is since placing the Jiffydos kernal, the function roms don't work anymore. Also after placing original roms back they don't work.
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Posted By
MCes
on 2020-10-13
12:05:03
| Re: Rom (Funcion Lo Hi) problems
I don't think that jiffydos modify could be the problem, but something could be happened during the modify....
Which kind of jiffydos hardware implementation did you use? (pics, please..).
Did you modify something on motherboard?
Did you use the solderer on the motherboard?
Two times repairing 1541 drive I was driving me crazy, so I found that the 1541 owner pulled out the chips (and reposioned them) with a screwdriver making force on the board with the blade of it.....
A PCB track was cutted by this operation and the cut was hidden under the chip......
Repair a manipulated hardware is sometimes hard.....
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Posted By
Retroshire
on 2020-10-17
03:00:38
| Re: Rom (Funcion Lo Hi) problems
Problem solved: it seemed to be a faulty PLA. Swapped it from a C16, all roms recognised and working again. A not so common fault. But thanks for the suggestions.
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