Topics

Questions for finalizing my Elf 2k #ELF2K

Rob Carnegie
 

I am getting to the end of the assembly of my Elf 2K mainboard.  One issue that I can't find any information on is if there is any rule on how the clock Xtal should be installed.  I have never encountered a square one with 4 pins like this before so I'm not sure what to look for in orienting it on the PCB.  The photos of the Elf 2K that I can find online seem to all show it with the inscription oriented to be right side up when looking at the TIL311 displays.  I don't know if that is just a coincidence or how to tell if mine would be the same.

The other issue is that I still don't have a DS275 for the RS232 port.  I managed to find an SN75155P which seems to have similar characteristics.  Does anyone know if this would be compatible?

Rob

cmdrcosmac
 

Rob,
 For the RS232 interface, either one should work, though the pinouts are different.
I used the MAX232 myself. They are easy to get, cheap, and known to work.

 For the can oscillators, the 4 pins are +5V, GND, OSC OUT, and ENABLE.
These things are made with sine or square wave output,3V or 5V output,
and may have HI or LO or no enable input The pinouts are fairly standard,
but study the data sheet. In the Elf2000 user manual, see Photo 6.
Notice the black dot in the corner of the oscillator. Assuming you are
using the original PC layout, that's the way the part mounts.
A CMOS- or TTL-compatible output part will work.
 See Digi-Key 535-9170-5-ND. It's 14.31818 MHz and I used a 74HC4040
to divide it down. I am now running a 9.8304 MHz oscillator divided by 2
and the IDIOT monitor works at 7200 Baud.
 If you run ElfOS with the CompactFlash disk, please let us know how
that works. I'm thinking of adapting it to a Super Elf expansion.
-Chuck

taf123
 

Hi Rob.

RS232 is nominally +/-12V but often is operated at +/-5V.

The DS275, which you can't find for love nor money, steaks the negative voltage from the remote end so you don't need a negative supply.

The 75155 is just a driver chip, it still requires a negative supply.

The newer MAX232 uses external capacitors to generate the negative voltage so it can be operated with just a positive supply.

But is certainly wouldn't fit where the DS275 is intended.

One option which I use is to use an external max232 adaptor such as https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.com%2Fulk%2Fitm%2F292602822407

This does the conversion for you and presents the Rx and Tx signals.

Hope that helps
Todd

 

  I wouldn't use a 14MHz oscillator in the Elf2K - that's not going to work!  As for the original question, this part isn't really a crystal; it's an active IC just like all the others on the board.  It has a pin 1, marked by a dot and usually a square (not rounded) corner, on the part.  Pin 1 on the board is marked with a square pad. 

  As for the SN75155P, it's similar to the DS275 but I don't think that's going to work.  The pinouts aren't the same, for one thing, and the SN75155P requires an external -ve source where as the DS275 derives one for you.

  FWIW, I don't understand all the fuss over the DS275.  If I was going to build an Elf2K now, I'd just bypass the DS275 completely and use TTL level I/Os.  There are tons of USB to "RS232" cables on the market today with 5V I/Os...

Bob

codedoctor@...
 

Hey all.  Actually I tested up to a 12 mhz oscillator in my homemade elf 2k with a regular old cdp1802ace CPU.  It worked fine with elfos (really speeds it up!), but scrambled the pixie video.  I don't remember if I tested all other functions.

I decided to use a cdp1802cd instead because I like gold ceramic chips, and that would only support up to an 8mhz oscillator. 

I have both the original 3.6mhz and 8mhz oscillators in there now, with a switch so you can change between them.

ajparent1/kb1gmx
 

in my EELF....

Crystal is 4.9152mhz seems to be happy there though trying faster
(6.144mhz) failed. IT does really speed matters up greatly.  Since pixie
is not in use that's not an issue as that must have a specific clock to
work or the timing going to the TV/Monitor is wrong.

I used external TTL to RS232 (MAX232) adapter or directly via TTL serial to USB.

Allison

Rizal Acob
 
Edited

So why is it that no one specifies the original Elf2k crystal (3.579545Mhz) as found in the schematics. That is the one I intend to use as I gather my parts for my Elf2k? I just got them from China with right size (half-can).

As for the DS275, my plan B is to use  one of the 2 suggested solution in my last post. I have seen in one of the post here where a Max232 was used ...mounted on external pcb and wired to the main Elf2k pcb in place of the DS275. Lee Hart has also a circuit that was used in his membership card which can be fitted into the DS275 area on the Elf2k board.

Rob Carnegie
 

On Mon, Sep 30, 2019 at 10:24 PM, cmdrcosmac wrote:
Rob,
 For the RS232 interface, either one should work, though the pinouts are different.
I used the MAX232 myself. They are easy to get, cheap, and known to work.

 For the can oscillators, the 4 pins are +5V, GND, OSC OUT, and ENABLE.
These things are made with sine or square wave output,3V or 5V output,
and may have HI or LO or no enable input The pinouts are fairly standard,
but study the data sheet. In the Elf2000 user manual, see Photo 6.
Notice the black dot in the corner of the oscillator. Assuming you are
using the original PC layout, that's the way the part mounts.
A CMOS- or TTL-compatible output part will work.
 See Digi-Key 535-9170-5-ND. It's 14.31818 MHz and I used a 74HC4040
to divide it down. I am now running a 9.8304 MHz oscillator divided by 2
and the IDIOT monitor works at 7200 Baud.
 If you run ElfOS with the CompactFlash disk, please let us know how
that works. I'm thinking of adapting it to a Super Elf expansion.
-Chuck

 Ah - I see now that my oscillator has a dot in the same corner as shown on the photos of assembled Elk 2K pcbs.  Thanks!

Rob Carnegie
 

On Tue, Oct 1, 2019 at 06:15 AM, Bob Armstrong wrote:

  FWIW, I don't understand all the fuss over the DS275.  If I was going to build an Elf2K now, I'd just bypass the DS275 completely and use TTL level I/Os.  There are tons of USB to "RS232" cables on the market today with 5V I/Os...

Bob
I just wanted to complete the Elf 2K as originally intended by the designer. Even though it is a replica of an earlier classic computer, it has become a classic in it's own right.  I was looking for something that would be pin compatible to the DS275 but glad to know that using a USB TTL interface is an option.

Thanks

David Madole
 

Rob,

 

They are all over eBay, there is even one US seller so you wouldn’t have to wait a month.

 

https://www.ebay.com/itm/323914873719

 

David

 

 

From: cosmacelf@groups.io <cosmacelf@groups.io> On Behalf Of Rob Carnegie
Sent: Wednesday, October 2, 2019 4:58 PM
To: cosmacelf@groups.io
Subject: Re: [cosmacelf] Questions for finalizing my Elf 2k #ELF2K

 

On Tue, Oct 1, 2019 at 06:15 AM, Bob Armstrong wrote:


  FWIW, I don't understand all the fuss over the DS275.  If I was going to build an Elf2K now, I'd just bypass the DS275 completely and use TTL level I/Os.  There are tons of USB to "RS232" cables on the market today with 5V I/Os...

Bob

I just wanted to complete the Elf 2K as originally intended by the designer. Even though it is a replica of an earlier classic computer, it has become a classic in it's own right.  I was looking for something that would be pin compatible to the DS275 but glad to know that using a USB TTL interface is an option.

Thanks