First things first - does the TX-2, in fact, ship with Datalink enabled? Early answer: yes, it appears so. But more testing is required to verify the entire instruction set. I've been postulating on this for the past month, trying to find a definitive "it does" or "yes, but there's a catch..." post, but I figured I'd have to do my own testing anyways.  So before changing out the RCA output cable, I thought I'd run a quickee test of the Datalink channel on the main board, to see what signals were being sent, if any.  I attached a signal analyzer to the board output (going from CPU pin 19 through some circuitry to pin 1 of the white connector) and measured the results.  Presto: Datalink. I received both a "No Media" signal and a "Status Playing" signal, depending on whether I held the record sensor down. With this experiment done, I'm quite confident I'll be able to attach a Din-7 cable to the output board, and use the Datalink features on a Beomaster 5500, at least.  For the later units, I'll still need to find or create an RIAA card.

Hello again, as already answered

I've had the opportunity to tinker around a bit with a few TX-2 / 5500 PCBs - I used to think that they are nearly identical across the various iterations of Beograms from the 3000 to the 9500, but it turns out there were additional  subtle improvements, as one might expect with the introductions of several models.  I've outlined some of the changes that I can spot - perhaps there are more changes that others know of. Here's an example of an early 5005/TX-2 board:

With the Beogram power supply, the story is much the same as the main PCB -  the supplies all have the same basic capability across models, but depending on the model and region, the way they were wired varies slightly.  With the deployment of an On-board RIAA of the 6500, it became necessary to power the IRAA via two power feeds from the PS. This necessitated the inclusion of a small 2-wire pin to the PCB, from where the RIAA could be powered Of the US-based TX-2 that I've seen, it appears that both early and late examples utilized the power supply setup as seen below, irrespective of which generation main PCB was installed.  This supply was wired for 120V, with a large fuse taking up much of the PCB #2 real estate, and no support for an RIAA hookup:

Here's a "Old vs. New" 59XX schematic connecting the CPU to the Datalink bus.  The new schematic includes a 547 transistor and 10K resistor hardwired to 5V, so it will be active as soon as the Beogram is plugged into power. It turned out to be a fairly straightforward process adding a transistor to PCB1. I took a 547 equivalent transistor, and soldered a 10K resistor to the B pin, and added a bit of heat shrink to prevent a short.  Next, I repositioned R42 by removing the Datalink bus leg from the board, and soldering it to the C pin of TR11.  Last, I placed the TR11 E pin into the now-empty hole which previously connected R42 to the Datalink bus, completing the circuit.  I contemplated drilling holes in the board and routing the wire underneath the board, but in the end my eventual solution was much simpler to execute, and I think it turned out looking OK - the parts are basically in the same spot they would appear in a later board, so it will be simple enough to understand what the retrofit's purpose is. This is a picture from a 5005 board I had retrofitted earlier:

Next step is adding a power plug for the RIAA amps I purchased from adyan.  His design mimics the original RIAA boards used in the 6500-7000 models, and therefore also needs a 22V and 12V supply, which it gets from the transformer on a plug from PCB2.  None of my US-Spec TX2 transformers - early or late models - have this 2-pin plug, so I had to tap into the PCB at the correct spots.  Here again I contemplated drilling PCB2 to add a surface-mount plug. And once again, for simplicity I opted to add a pre-wired 2-pin connector instead.  I attached the red wire lead to the existing 12V wire hookup (post IC), and the black wire lead to the 25V output from the rectifier. This way, Red would consistently be the 12V supply.   The plugs are male/female, so there's no way anyone could accidentally feed a 25V input to PCB1 ground by mistakingly hooking up the incorrect plug.

I decided I'd upgrade the 5500 first, on the thought that the wiring would be a bit easier.  This proved to not be the case - B&O supplied the original output cable with an extra unattached blue wire, which was not connected to pin 7.  I made the decision to disassemble the Din connector to attach it.  This is strictly not necessary for my planned use, but I wanted to observe the convention that "Beograms with RIAA run Datalink on Pin 7". Upon disassembly, I found that not only was the blue wire not attached, it was truncated to the point where I'd have to un-solder all the pins and re-cut them to the new length.  This is fiddly work which I'm personally not terribly fond of.  I just happen to appreciate the coolness of a removable pin 6 and 7, and want to preserve that feature. I wired up both pin 6 and 7 with the blue and yellow wire.  Without thinking about it first, I moved the yellow wire to pin 7, and soldered the blue wire to pin 6.  Inside the turntable I'll have the blue wire available on pin 6 for a future new project I'm dreaming about. After a bit of squinting, cursing, and double checking my work for shorts, I was ready to reassemble the connector. Then came the "one-way" step of snipping the cable at a point I thought would provide enough wire on either side of the RIAA card to connect everything together using the original wires.

On reading, re-reading, and re-re-reading the latest version of the service manual RIAA schematic, I came to the conclusion that the "Old Version" PCB 11 would need a modification in order to electrically emulate the "New Version" mute circuit.  The original PCB11 is wired such that LG and RG from the stylus are not connected to chassis ground or shield ground until the wiring reaches the Beomaster.  Further, the mute circuit is implemented by grounding the L and R to LG. New boards designed for dual use as RIAA or non-RIAA implement a sightly different protocol, which necessitates the cutting of a wire bridge between some of the solder pads to achieve the desired circuit.  Also, the LM and RM wires work a bit in "reverse", where the mute circuit acts to ground the amplified signal on mute activation, before it reaches the output cable.  The only way I could see to mimic this implementation was to grind a cut on the PCB itself, isolating L and R from the original mute circuit. [EDIT - I've made a small change to the schematic, and included both versions for comparison] Original Version combines L+ R during 'unmute' - potentially undesirable:

On to the final steps - finish routing the wiring, and connecting the RIAA.  The original wires ended up being just a bit short to wire around the underside of the board as I would have liked, but at this point I didn't care - I wanted to see if my upgrade had worked!