Unibus Interface ICs

[pbirkel@gmail.com]

The DEC PDP-11 Unibus Handbook identifies three standard ICs for use when connecting to Unibus lines:

Bus Receiver - 8640 Quad NOR
Bus Transceiver - 8641 Quad
Bus Driver - 8861 Quad NAND

It seems to be generally agreed that the 8861 driver/transmitter can be substituted by the 7439 Quad 2 Input NAND Buffer O.C.

There seems to be no recognized physical substitute for the 8640, at least in part because it uses the 1 & 8 pins rather than 7 & 14 pins for GND/VCC, respectively (see Figure 1-25). Is that correct? What have folks been doing when needing to physically replace one of these -- substitute from a sacrificial module?

For new designs, or rewiring old ones (dead-bug?), is there a generally agreed logical substitute for the 8640?

Unibus handbook Figure 1-30 does specify the RC-equivalent input for this IC but I have not as yet tried to cross-walk it against well-known SN74xx-series chips. I imagine that I'm the umpteenth person to encounter this problem, so before I try to rediscover fire I'm hoping that someone else could share their torch of knowledge :->?

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paul

 

[DaveH]

The DEC PDP-11 Unibus Handbook identifies three standard ICs for use when connecting to Unibus lines:

Bus Receiver - 8640 Quad NOR
Bus Transceiver - 8641 Quad
Bus Driver - 8861 Quad NAND

It seems to be generally agreed that the 8861 driver/transmitter can be substituted by the 7439 Quad 2 Input NAND Buffer O.C.

There seems to be no recognized physical substitute for the 8640, at least in part because it uses the 1 & 8 pins rather than 7 & 14 pins for GND/VCC, respectively (see Figure 1-25). Is that correct? What have folks been doing when needing to physically replace one of these -- substitute from a sacrificial module?

For new designs, or rewiring old ones (dead-bug?), is there a generally agreed logical substitute for the 8640?

Unibus handbook Figure 1-30 does specify the RC-equivalent input for this IC but I have not as yet tried to cross-walk it against well-known SN74xx-series chips. I imagine that I'm the umpteenth person to encounter this problem, so before I try to rediscover fire I'm hoping that someone else could share their torch of knowledge :->?

-----
paul

I don't really know Unibus but if the 8640 is similar to the DEC6380 used as a bus receiver on Omnibus cards then you may be able to use a 7402. I regularly use either the 7402 or even the LS02 if the fan out is small to replace a failed 6380. Yes the pin out is total different so it was "dead bug" the first few times but when it became obvious that I might have to do this quite a lot, I have a LOT of omnibus cards, I made up a small converter PCB, see the pic, this board will also take a 7432 to replace a DEC384, another IC that I have found has a high failure rate.

With regard to the 8861 driver, in my singe back plane systems I have "cheated" and used a 7401 sometimes as the 7439 seems to be rather hard to get, however the 7438 seems to be more available and similar in spec , and looking at the pin out it looks as if this could be used if inserted inverted and the Vcc and Gnd pins suitable adjusted, I hope so as Iv just bought 50 on Ebay .

Dave H

 

[Marty]

Hi All;
Dave, the 7438 is backward to the 7439.. So, for instance in the first nand gate pins 1 and 2 are inputs and pin 3, on the 7438.. And on the 7439 pin 1 is the output and pins 2 and 3 are the inputs.. The only pin that stays the same is pin 2, the middle pin..
The 8640 I think is Open collector and the 7402 is not, but, I could be WRONG on this.. I will need to look this up.. I have Not tried to sub an 02 for the 8640..

THANK YOU Marty

 

[DaveH]

Hi All;
Dave, the 7438 is backward to the 7439.. So, for instance in the first nand gate pins 1 and 2 are inputs and pin 3, on the 7438.. And on the 7439 pin 1 is the output and pins 2 and 3 are the inputs.. The only pin that stays the same is pin 2, the middle pin..
The 8640 I think is Open collector and the 7402 is not, but, I could be WRONG on this.. I will need to look this up.. I have Not tried to sub an 02 for the 8640..

THANK YOU Marty

Hi Marty,
I understand that the pinout of the 7438 is backwards from the 7439, what I am suggesting is that one clips off the pin part of pin 7 and 14, then insert the 7438 UPSIDE DOWN, then cross wire the Vcc and Gnd pins, ok a bit gash but needs must if you dont want , or cant use a 7401 and cant find a 7439.

Re the 8640, yes if its an OC part then 7402 is not going to work, the 6380 , the omnibus bus reciever, is a normal TTL op so the 7402 is ok
Dave H

 

[pbirkel@gmail.com]

Dave: Do you have a close-up of the two sides of your converter PCB? It looks to me like the substitute IC is exactly aligned with the original footprint; I would have expected it to be offset in some manner. THX!

 

[DaveH]

Dave: Do you have a close-up of the two sides of your converter PCB? It looks to me like the substitute IC is exactly aligned with the original footprint; I would have expected it to be offset in some manner. THX!

Hi, no there is an offset, 0.15 "

dave H

 

[pbirkel@gmail.com]

Ahh. I see now :->.