@retrochallenge 2012 – Acoustic Coupling
Since contracting this retro computing bug, after prolonged exposure to Urbancamo, I’ve had a yearning to try to recreate one of my earliest and fondest computing memories. That memory, when recalled, still has the ability to send a little shiver of excitement down my spine. Picture the scene if you will, a musty old office in the maths block of my local upper school. Two spotty teenage boys, myself and the aforementioned Urbancamo. In one dingy corner of the room an ASR 33 Teletype, on the window sill an acoustic coupler and telephone and on a piece of paper, a phone number.
Thinking we were Matthew Broderick from WarGames we eagerly dialled the number inserted the handset into the coupler and waited excitedly for the Teletype to burst into life. Sure enough the ASR 33 started clanking away and we were in! Not NASA or anything like that you understand, but a nearby college upon which we were able to play a game called Shark Attack! A momentous moment in my computing history and the first and last time that I ever used an acoustic coupler.
I have for some time therefore been on the lookout for a working acoustic coupler that I could use to try to recreate some of that experience. After failing to bring back to life a very old Anderson Jacobson ADC 212 model, I was recently lucky enough to sport an Epson CX-21 on Ebay.
It’s in lovely condition, and the original NiCad battery even appears to hold a reasonable charge. It came with an Epson HX-20 in a custom-made case which no doubt accounts for its excellent condition. There was a little blooming on the rubber cups but I cleaned this off with a mild detergent solution and the whole unit looks almost new.
I will of course also need an old telephone with suitable handset and fortunately we still have a couple of 1970′s models at work that thankfully were never thrown out. These old BT models were built to last and the one I chose cleaned up really nicely, in fact it looks so cool I may use it in place of our current home set.
The CX-21 only has two options to select, half or full-duplex and answer or originate mode. It has a standard DB25 port and I have the appropriate cable to hook it up to my PX-8, an Epson 724 cable. This is a modem cable with DB25 at one end and an 8 pin Mini DIN connector at the other. I have two options for terminal software on the PX-8, both previously downloaded, Kermit and Mex. The PX-8 does have a rudimentary terminal application on the built-in ROM but I’ve never had much success with it.
After charging the CX-21 and PX-8 I hooked up the TF-20 FDD and copied over all the software I’d need to the PX-8′s RAM disk so I had the minimal amount of stuff to take home from where I would be calling back to the box running my BBS.
I then spent rather too long trying to get my head around whether I was originating or answering the call, clearly I was originating it and the BBS box was answering but for some unknown reason I’d convinced myself that the coupler would have to ‘answer’ the handshaking attempts of the remote modem.
Nevertheless once sanity had been restored I set the coupler to ‘originate’ mode at full-duplex and dialled the BBS with the telephone. After several attempts the ready light finally illuminated on the coupler and it seemed a connection had been established. However once I’d connected via the terminal software all I received was a screen full of garbage.
I checked all my settings, made sure background noise was minimal and continued to make many more attempts to create a good connection. Eventually I discovered that contrary to my instinct to push the handset more deeply and securely into the coupler, lifting the mouthpiece end slightly out of the cup resulted in a much better connection! Whether the proximity of the transducers was causing some sort of distortion I don’t know but whatever the reason I now had a working connection and was reliably able to re-create it. There are still a few erroneous characters coming through but I can live with that.
I shot a quick video showing the process below, all I need now are a couple of ASR 33′s.