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PostPosted: Sun Jan 07, 2018 4:14 am 
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Joined: Sun Jan 07, 2018 2:23 am
Posts: 6
So I have been looking at these boards, ANROM/AN1ROM and BNROM and I see that the D pins on the '161's (pins 03 - 06) are wired a bit differently than on CN/UN-boards.

Here are how they are wired on the boards I have:

Code:
ANROM:
                                  .---\/---.
                            +5V - |01    16| - +5V
            /ROMSEL & '02 11 4B - |02    15| - NC
                         PRG D0 - |03    14| - PRG A15
                         PRG D1 - |04    13| - PRG A16
                            +5V - |05    12| - NC
                         PRG D4 - |06    11| - CIRAM A10
                            GND - |07    10| - GND
                            GND - |08    09| - CPU R/W & '02 03 1B/'02 02 1A
                                  `--------'

                                     '161


AN1ROM:
                                  .---\/---.
                            +5V - |01    16| - +5V
            /ROMSEL & '02 11 4B - |02    15| - NC
                         PRG D0 - |03    14| - PRG A15
                         PRG D1 - |04    13| - NC
                         PRG D3 - |05    12| - NC
                         PRG D4 - |06    11| - CIRAM A10
                            GND - |07    10| - GND
                            GND - |08    09| - CPU R/W & '02 03 1B/'02 02 1A
                                  `--------'

                                     '161


BNROM:
                                  .---\/---.
                            +5V - |01    16| - +5V
              /ROMSEL & PRG /CE - |02    15| - NC
                         PRG D0 - |03    14| - PRG A15
                         PRG D1 - |04    13| - PRG A16
                         PRG D2 - |05    12| - NC
                             NC - |06    11| - NC
                            GND - |07    10| - GND
                            GND - |08    09| - CPU R/W
                                  `--------'

                                     '161


When I have looked at other board, it seems that unused D-pins are wired to +5V.
The BNROM connects '161 05 to PRG D2 and leaves '161 06 floating, while Q2/Q3 are floating.
The AN1ROM board connects unused '161 04 D1 to PRG D1 and '161 05 D2 to PRG D3 but leaves Q1 & Q2 floating.

So my question is, what difference does these connections make on the boards?
From what I understand is that the correct way is to connect unused D-pins to +5V, and not leave them floating, while Q-pins can be left floating, is this correct?

And what is the purpose to connect PRG D's to the '161 but leaving the Q's floating? Do they act like +5V on the '161?


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PostPosted: Sun Jan 07, 2018 1:37 pm 
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Joined: Sun Apr 13, 2008 11:12 am
Posts: 7676
Location: Seattle
chimaera wrote:
So my question is, what difference does these connections make on the boards?
Because these boards only use the 74'161 as a latch ... it's only which bits of the value written are available for bankswitching.

If any boards used the 74'161's ability to count up, then the order could matter, but none do.

Quote:
From what I understand is that the correct way is to connect unused D-pins to +5V, and not leave them floating, while Q-pins can be left floating, is this correct?
Unused D pins (inputs) on HC parts should be connector to something, because latching a floating pin could cause the part to suddenly heat up problematically.

LS part inputs can be left floating, because they care about current rather than voltage.

Quote:
And what is the purpose to connect PRG D's to the '161 but leaving the Q's floating?
None. I'd guess it's a carry-over from some previous board design; the connection of D2 on BNROM implies to me it was made by modifying an UNROM schematic.


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PostPosted: Sun Jan 07, 2018 2:24 pm 
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Joined: Sun Jan 07, 2018 2:23 am
Posts: 6
Quote:
Unused D pins (inputs) on HC parts should be connector to something, because latching a floating pin could cause the part to suddenly heat up problematically.

LS part inputs can be left floating, because they care about current rather than voltage.


I checked the BNROM board and it does indeed use an 74LS161 part, so I guess thats why they left pin 06 floating.

So there is really no special trick about this kind of wiring. I was just curious because none of the other boards I looked at (CNROM/MHROM/GNROM/UNROM/UN1ROM/UOROM) has this kind of configuration. I guess it is unique to the AxROM series.

Thank you for your answer lidnariq.


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