Using & Programming the Epson HX-20 Portable Computer by Eric Balkan has a tantalising passage on p. 172*:
T.L. Ronson of the HX-20 User's Group also notes that a monochrome monitor can be hooked directly to the RS-232 port of the HX-20. Contact him for details.
As Balkan's book was published in 1985, it is hardly surprising that Ronson's way of doing this seems lost to posterity. But I wonder if anyone else has tried and succeeded at something similar, or might have enough understanding of what an RS-232 port is capable of in general, or the HX-20 in particular to be able to say how a monochrome monitor connected to an RS-232 port might be coaxed into working as a display.
It sounds like it can be done 'directly' (ie with a single cable?), though I imagine some code would be necessary to get it to work (the HX-20 allows direct machine code input via its own Monitor function).
For clarity, I am not talking about using the official serial-to-PAL adapter box (the display controller), or the one made by the company Oval, and is described on pp.171-172 immediately preceding this passage, but am interested in Ronson's "hack" to connect a monochrome monitor to RS-232.
I have experimented by connecting a DIN cable I have, which I use to connect my Soviet ZX Spectrum clone and my Elektronika BK-0010-01 to my monochrome Elektronika MC-6105-01 monitor, I know that only two of the five DIN pins of that 5-pin DIN connector are used to connect to the monitor. Once connected I tried each of the following commands on the HX-20:
SCREEN 0,1 SCREEN 0,2 SCREEN 1,0
(These are supposed to display text and graphics on the internal and external displays in different combinations).
For each command, my monochrome monitor did react, briefly turning on the screen to display greyish flickering, before switching off again, at which point the HX-20 threw:
About which the manual says:
DU: Device unavailable. A device which is not connected to the HX-20 is specified.
But the reaction of the monitor to try to display something gives me some hope that communication between the RS232 and monitor might actually be possible.
It may be of interest to note that the HX-20 RS-232 port is an 8 pin DIN, mapped to RS-232 like so.
* This passage comes right after a description of the official serial-to-PAL converter, so it does not reference that device, but is given as an alternative.