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Cheap DIY Econet clock

Posted: Thu Mar 27, 2014 12:10 am
by dangardner
minimus-clock.png
Schematic


I've been playing with this idea on breadboard for a while using an SN75159 but I've now got it down to a nice little form factor barely bigger than most USB flash drives if you ignore the trailing DIN plug. The Minimus 32 AVR USB is a handy little ATMega32U2 breakout board with a USB connector for loading firmware and providing power. The MC3487 is an RS422 quad differential line driver; although we only need one driver, it was the cheapest I could find at CPC.

Materials:


Method:

  • Connect the Minimus to a Windows PC and use Atmel Flip to program it with the EconetPWM.hex file. Instructions for doing this can be found here. You will need to select ATMEGA32U2 as the device type and there is no need to load EEPROM data. Remove the Minimus after programming.
  • Cut the card to the size of the Minimus' main body and cut out slots to expose VCC, GND and PC6 on the back of the Minimus (the side without components, so VCC is top-left and GND is bottom-right). This is intended to insulate the Minimus from the IC. Stick the card to the back of the Minimus. I used hot glue, which seemed to work well.
  • Splay the legs of the IC socket out at right angles to make soldering to them easier. I used pressed pin sockets because I thought turned pins would probably snap.
  • Solder the hook up wire and the resistor to the IC socket and the Minimus, as shown in the schematic. I routed the resistor down the middle of the IC socket to keep it neat and avoid potential shorts.
  • Insert the MC3487 IC into the socket.
  • Tie the cable tie around the Minimus and the IC. I used this both for mechanical strength of the assembly and as strain relief for the Clock +/- lines by wrapping them around the cable tie a few times.
  • Solder the ends of the Clock +/- lines to the 5 pin DIN plug. Clock+ goes to pin 3 and Clock- goes to pin 5. See the Econet entry on Wikipedia for the Econet socket pinout (as seen when looking into an Econet socket).
  • Connect the Minimus to USB power and plug the 5 pin DIN plug into the centre socket of your Econet cabling. I used a USB wall wart with a USB extension cable to provide the best flexibility on positioning.

Of course, if you prefer to connect the clock lines to a pair of 5 pin DIN sockets in the same way as traditional clocks, that would work too. I just found the single DIN plug more convenient.

The Minimus is much cheaper when bought in quantities of 5 or more and the IC sockets only come in packs of 5. To make a single clock unit costs around £15 but that comes down to under £8 each when making 5 or more. I'm very happy for anybody who wants to to make some up and sell them at a reasonable price but I have too many uses in mind for my remaining Minimi to want to part with them. :-)

The firmware source is very simple and can be rebuilt using Atmel Studio if you want to set your mark/space to something other than the standard 1us/4us (5us period, 200KHz frequency) used in the attached firmware. Details of how to tune the clock speed can be found in the 1993 Econet Installation Guide (for which I can't find a working link at the moment) and on Rick Murray's website.

I would love to see this design enhanced to moderate the mark/space timings by monitoring the data lines for aborted handshakes, in the same way as Netmon is used in the clock tuning section of the 1993 Econet Installation Guide. I don't know if this was how SJ Research achieved the variable clock on the BEN but it seems like it would be worth exploring.

You may also want to wait until flynnjs makes his Econet to USB adapter available, which (I believe) will include a clock generator. I just needed something working sooner and hope that others will benefit from what I have done.

Re: Cheap DIY Econet clock

Posted: Thu Mar 27, 2014 3:25 am
by jgharston
dangardner wrote:The Minimus 32 AVR USB is a handy little ATMega32U2 breakout board with a USB connector for loading firmware and providing power.
Does it have to be an active device? I would have though a 555 timer would do it.

Re: Cheap DIY Econet clock

Posted: Thu Mar 27, 2014 7:22 am
by dangardner
jgharston wrote:Does it have to be an active device? I would have though a 555 timer would do it.

I think you could do it with a 555 in astable mode but my impression was that a 555 was not accurate enough for a good clock signal without incorporating a crystal. The Minimus also has the advantages of a built-in ubiquitous power connector, the facility to change clock speeds easily without having to replace components and the potential for future enhancements to the design such as the variable clock speed idea.

Also, my electronics knowledge is pretty limited and I had a couple of Minimi knocking around to play with. :-)

Re: Cheap DIY Econet clock

Posted: Thu Mar 27, 2014 8:09 am
by paulv
That looks great Dan. =D>

A nice cheap way of getting a simple Econet clock is I think the biggest stumbling block for people getting a network up and running and this goes a long way to addressing that :D

=D> =D> =D>

Paul

Re: Cheap DIY Econet clock

Posted: Thu Mar 27, 2014 8:42 am
by richardtoohey
That Minimus looks neat, but given I've not done enough with my Pi let alone my retro-kit ... :roll:

But maybe one day I'll get around to Econet! :D

Re: Cheap DIY Econet clock

Posted: Thu Mar 27, 2014 9:38 am
by 1024MAK
dangardner wrote:
jgharston wrote:Does it have to be an active device? I would have though a 555 timer would do it.

I think you could do it with a 555 in astable mode but my impression was that a 555 was not accurate enough for a good clock signal without incorporating a crystal. The Minimus also has the advantages of a built-in ubiquitous power connector, the facility to change clock speeds easily without having to replace components and the potential for future enhancements to the design such as the variable clock speed idea.

The accuracy of a 555 timer circuit (astable or indeed monostable) depends on the quality of the passive components used, especially the capacitor. Also there are a number of different configurations of the circuit for astable mode, depending on what mark-space requirements are needed.
A lot of timer/clock circuits have a bad reputation simply because they are low frequency circuits, so use electrolytic capacitors. This type of capacitor has a very wide tolerance +/-20% at best and a relatively high leakage current. The characteristics also change over time and with temperature. See the wiki entry here

But for higher frequency circuits, or where a more complex circuit design is used, better close tolerance, highly stable capacitors can be used. Combined with 1% metal film resistors, you can build a astable circuit that should be good enough for this application. I know of a 555 astable circuit being used for the reference frequency generator for a 9600 bps RS232 circuit for example.

Mark

Re: Cheap DIY Econet clock

Posted: Thu Mar 27, 2014 12:28 pm
by jms2
This looks brilliant, well done! =D>

I'm one of those people who have always regarded Econet as being too expensive / too difficult, and the price of a clock box has always been a major part of this.

I don't have time to rush out and build one, but Econet has certainly now gone up several places on my list of interesting future projects!

Re: Cheap DIY Econet clock

Posted: Tue Nov 10, 2015 9:23 pm
by JonC
Sorry for the Necro post, but it looks like;
1x 16 pin DIL socket £0.78 for qty of 5

is no longer available.

Can anyone recommend a suitable pressed pin alternative from cpc farnell?
I also went looking for 1k resistors and got bombarded with choices on the cpc farnell site, completely lost, any recommendations?

I may just get all the other bits and wait patiently for the Wakefield show next year, hopefully some friendly electronics expert will take pity on me [-o< :mrgreen:

Re: Cheap DIY Econet clock

Posted: Tue Nov 10, 2015 9:54 pm
by danielj

Re: Cheap DIY Econet clock

Posted: Tue Nov 10, 2015 11:05 pm
by JonC
Cheers Daniel 8)