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.
- 1x Minimus 32 AVR USB £6.95 (£5.00 each on qty of 5) plus £5.99 postage
- 1x MC3487NE4 £0.90
- 1x 16 pin DIL socket £0.78 for qty of 5
- 1x 5 pin DIN plug £0.65
- 1x 1K resistor
- Cable tie
- Card or other flat insulating material
- Hook up wire (I used pairs from a CAT5E cable binned for missing RJ45 lugs)
- The Minimus firmware in HEX format (in the EconetPWM\Release directory of the EconetPWM.zip attached to this post)
- Atmel Flip (can be downloaded from Atmel's website)
- 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.