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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:

DU Error

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.

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    My guess is that someone was confused, and somehow "you can connect a monitor using na H00DC-IA display controller connected to the serial port" got misinterpreted as "you can connect a monitor directly to the RS-232 port". – Ross Ridge Dec 30 '19 at 20:49
  • I very much doubt it, as the above quote comes right after a description of the pal-serial box and is given as an alternative. – harlandski Dec 31 '19 at 2:46
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    That only shows person you quoted from believed there was an alternative, not that the author or someone else wasn't confused. I'm sorry, but it still seems likely someone misunderstood when someone else was describing an H00DC-IA. If it was actually possible to connect a monochrome monitor directly to the RS-232 port like how you're doing then that would mean that the HX-20 has a video display chip hidden inside somewhere that no one knows about. A video chip that would make the video chip in the external display controller unnecessary. – Ross Ridge Dec 31 '19 at 3:26
  • Ok that's reasonable, thanks for explaining. – harlandski Dec 31 '19 at 6:01
  • Since it takes pains to call out "monochrome," there are some TTL monochrome CRTs out there. I just can't imagine how you would be able to bit-bang one fast enough from an HX20. – ravuya Dec 31 '19 at 23:49
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It sounds like it can be done 'directly'

No, it can not. The interface hardware is not able to generate a valid TV signal.

  • The RS232C port is a true RS232C operating as positive and negative voltage. The HX20 uses +8/-8 Volt level (*1). Voltage is generated by a TL479 step-up/down converter and used to feed the 75188 line drivers. This Voltage is way outside the allowed range of 0..1V for a composite signal (0V for sync; 0.3V for black; 1V for white).

  • The SERIAL bus port does as well use levels of +8/-8 Volt. In addition is this bus not only used for external devices, but also for communication between master and slave CPU (in this case TTL level is used). Even without the voltage issue, using it to generate a different signal would render everything handled by the slave CPU inoperational.

Beside all the technical reason, what reason should Epson have go the length of developing a display controller (and Epson UK offering a third party controller), if everything needed is already build in?


As Ross Ridge already mentioned, this most likely references to the H00DC-IA Display Controller Epson offered:

enter image description here

The Display Controller is a box about 5x15x25 cm in size with connectors for power, Serial Bus, Composite Out and TV out. It is a genuine Epson product, not to be mixed up with the Oval HO-20 sold in the UK (see below). It outputs a 60 Hz NTSC compatible signal. The hardware is based on a (nother) 6301 CPU with a 6847 VDG, 2 KiB of Video RAM and a 4 KiB character ROM.

The controller is connected via the 5-pin SERIAL bus, which it could share with a floppy drive named TF-20 (5.25"; 320 KiB; CP/M format). See page 65 of the manual. I have no idea if there were other devices for the HX-20, but the bus is compatible with the PX-4/8 SERIAL (both can use the TF-20) which had printers supporting this protocol (*2).

The serial bus is looped thru the devices and operates at 38,400 bps. The connector is a 5/180° DIN like used for PC keyboards or MIDI.

Pinout:

1 - GND
2 - TX
3 - RX
4 - POUT
5 - PIN

(Caution, on this connector pins are not numbered in sequence but as 1, 4, 2, 5, 3, when looking at the pins, notch at the top)

PIN/POUT are used as handshaking to define who's talking. AFAIK it's not used - in fact, the Display Controller does not forward PIN from a subsequent device.

Beside original controller, there were as well compatible third party products, like the Oval HO-80 which was even sold by EPSON UK. Unlike the Epson product can it produce 50 Hz frames as well as PAL modulation.

enter image description here

Software wise they are compatible, but the HO-20 seams to offer additional modes for higher resolution as well, as I found a a note in one of the marketing brochures, but no further data.


The well known Norbert Kehrer did develop a quite useful emulation of setup of 4 drives and a display controller under Windows called FLASHX20. One rather great feature is that all access is done on logic level. Files are not managed in any special format, but as regular files on your disk drive. THere is a directory for each of the four emulated drives. Copying to a HX-20 'disk' (or back) is done by simply moving it into the directory. A RasPI based Linux version is announced, which would allow creating a handy storage and display box.

Another PC-based emulation was created by Martin Hepperle about a year ago.


Similar interesting, albeit older is the (DOS) program called PX8VFS which emulates a set of compatible disk drives which should as well work with the HX20.


*1 - RS232C asks for 3 Volt minimum and 25 Volt maximum.

*2 - The PX-4/8 had quite a lot of peripherals like TF-15 5.25 dual floppy, PF-10 single 3.5" floppy.

| improve this answer | |
  • As I said, I am not asking about the serial-pal conversion box. – harlandski Dec 31 '19 at 2:47
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    @harlandski There is no other TV interface. The serial port is as well used for communication between the master and slave CPU. Even if generating a TV signal would be possible (which isn't), doing so would make communication between both CPU impossible, essentially bricking the HX-20. – Raffzahn Dec 31 '19 at 3:47
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    Ok it seems reasonable that it's just a mistake in the document I quoted, or in its source. Pity, as working on the tiny built in HX screen is a pain! – harlandski Dec 31 '19 at 6:04

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