@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.

ADC 212 Internals

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.

Epson CX-21

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.

CX-21 Battery

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.

Epson PX-8 With The CX-21

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.

BBS’ing With The Epson PX-8 Geneva

I’d hoped to acquire an acoustic coupler to use with the Epson PX-8 however it wasn’t to be. Plan B involved connecting up an external modem and I therefore needed some software that would allow me to communicate with the US Robotics unit that I have, Mex from NightOwl Software was the obvious choice.

Using this guide I was able to determine which files I needed specifically for the PX-8 which were as follows:-

  • MEX114.COM

    Assembling MEX PX8 Overlay

  • MXO-SM14.ASM

The MXO-PX8 overlay is I believe written specifically for the Epson Multi Function Unit which has a built in modem and sits below the PX-8 connecting via the system bus.  It is therefore also necessary to include a generic Hayes compatible overlay if you need to communicate via RS232 with an external modem.

Generating MEX For The PX-8

I used Filink to move all the files that I needed across to the PX-8 and saved them on disk with the TF-20 and used ASM.COM to assemble the overlays.  Using MLOAD you can then generate a MEX executable tailored specifically to your PX-8/Modem set up.

Epson Cable 724

In order to connect a modem to the RS232 interface on the PX-8 you need the Epson 724 cable, which is mini din to DB25.  Fortunately one of my PX-8’s came with this cable and it’s not to be confused with the similar looking 725 cable which is a null modem version.

Px-8 & U.S.R. Modem

So then the moment of truth, what chance this would all work? I decided I should try and find a dial up BBS other than mine own to try.  Wgoodf’s recent blog entry pointed me in the direction of the Plasma Sphere BBS which I didn’t have any luck with but this did lead me to the Arcade BBS.

A nice feature of MEX is you can use either ‘Call’ or ‘Dial’ to initiate the call, the latter will allow you to drop back into the command line and fire up other software if necessary, Kermit for instance.  I stuck with ‘Call’ initially and it worked! Sort Of.  Clearly there are some issues, I’m guessing buffer overruns but I’m sure these can be ironed out.

So here’s a video of me connecting to the Arcade BBS with my Epson PX-8 using Mex:-

(Update, I restricted the RS-232 port to 300 Baud and tried again with better results and have therefore updated the video, ahhh 300 baud, those were the days.)

Epson PX-8 PF-10 & TF-20

Having declared in an earlier post that I’d managed to get the PF-10 working again I thought I should really post some proof.

I apologise for the shaky, and frankly poor quality of this video, I forgot my tripod and camera and had to fall back on the iPhone.

I just show the PX-8 going through the available drives:-

A: Ram Disk

B: Basic ROM

C: System ROM

D: TF-20 5.25″ FDD 1

E: TF-20 5.25″ FDD 2

F: PF-10 3.5″ FDD

H: Mini Cassette (Realised I forgot this, bless it)