Rs232 on The Last Psion

USB RS232 Shenanigans (Updated 2025-05-28)

Fri, 18 Aug 2023 00:00:00 0000

I spent some time today testing out a couple of RS232 adapters with various bits of Psion software. I wanted to write a quick summary of what works and what doesn’t.

Software	CH340	PL2303TA	PL2303RA	FT232R
plptools (Linux)	Yes	Yes	Yes	Yes
PsiWin 2.3 on XP in VirtualBox (adopted USB)	Yes	Yes	No	Yes
PsiWin 2.3 on XP in VirtualBox (via virtual serial)	Not tested	Yes	No	Yes
PsiWin 1.1 on XP in VirtualBox (via virtual serial)	Not tested	Yes	Yes	?
MCLINK or SDBG in DOSBox Staging	Unreliable	Yes	Yes	No

Notes

CH340

Seems to work in most places, but DOSBox and DOSBox Staging are very unreliable with it. It’s very cheap and up until recently was what I recommended, but as I’ve tried to use SDBG for debugging on real hardware, I’ve needed something more reliable.

PL2303TA

Great all-rounder so far! I’ve been using an adapter by UGREEN with this chip in, and it’s been solid.

There’s just one problem. Prolific discontinued the chip in 2020. Getting drivers to work in newer versions of Windows is a pain. This won’t affect you if, like me, you’re not running anything newer than Windows 7. Modified versions of older drivers are out there to make the adapter work on Windows 11. Also, stocks will probably run out in the not-too-distant future. Linux, of course, works fine with it.

PL2303RA

I really wanted this to work with everything. You can get boards very cheaply from AliExpress, and my plan was to create two all-in-one Psion RS232 cables - one with a 9 pin mini-DIN for the older machines, one with a Honda connector for the newer ones. But PsiWin 2.3 won’t talk to it.

FT232R (Updated 2025-05-28)

Interesting one. This adapter works really well with PsiWin 2.3.3 and plptools, even at 115200. However, It’s unusable with DOSBox at any baud rate - it starts off being incredibly laggy and then gives up (see screenshot below). But even better than that, trying PsiWin 1.1 in an XP VM with the adapter completely locked up my laptop! I may investigate this further in the future.

Conclusion

I really want to be able to recommend a good all-rounder for Psion work, no matter what you want to do with the device. The PL2303TA might well be that, but its EOL status might put people off. The PL2303RA was so close to being perfect, but the pickiness of PsiWin 2.3 has scuppered my plans.

It’s safe to say that you can pick pretty much any of these adapters if you want to use plptools on *NIX. Reconnect, a macOS wrapper for plptools, will also be fine if you have the right macOS drivers (be careful with the PL2303 chips!).

I will update this as and when I test more chips. As of May 2025, the UGREEN PL2303TA is still my adapter of choice for all-round Psion goodness.

In fact, I like the UGREEN adapter so much, I’ve hacked one apart and plan to solder it to a Honda cable when I get the chance.

The Siena SSD Drive

Sun, 17 Nov 2019 00:00:00 0000

It’s been an interesting morning. I’ve been digging around in the internals of the Siena SSD drive and made some discoveries.

The Siena SSD drive was released so that the diminutive Siena could still read SSDs. I bought one of these on eBay a week ago because I wanted to answer a question: “How did Psion get the SIBO Serial Protocol to work over RS-232?”

At first I thought there were two ways that Psion could have done this. First, the SIBO-SP packets (12 bits in length, although only 9 are ~~useful~~ significant) are repackaged so they fit into 8 bits. Or second, the Siena SSD drive tells the SIBO machine that it’s SIBO-SP compatible and then changes from RS-232 using to SIBO-SP (maybe still using RS-232 signal levels that are downconverted for ASIC4/5?).

Turns out there was a third way.

The only documentation I have for the Honda connector is from a Series 5 service manual. In it, pins 10,13 and 14 are shown as not connected. Turns out that’s not the case with the Honda SIBO machines (3c, 3mx and Siena). On the 3c board (pictured with the stripped-down drive) they connect to a MAX3212, an RS-232 transceiver.

This means that the Honda serial cable has a second set of serial connections, separate to the main RS-232 used for things like PsiWin.

The Siena drive has a passthrough RS-232 port with pins 9 to 14 disconnected. Having a second serial channel would explain how a Honda SIBO machine can do both regular RS-232 and read an SSD at the same time.

The white square in the top-middle of the Siena drive’s board seems to be another custom ASIC! All the SSD pins connect to this. I’ve not seen any information about this chip anywhere. My guess is that it splits the single-wire SIBO-SP into two wires for transmission along the second serial link, but until I can get a logic analyser on it I can’t say that for certain. The rest of the board must be providing things like Vpp for writing to Flash SSDs and lowering the 9v power supply to 5v.

This does sadly mean that, even with drivers, there is no way to get the drive to work on an EPOC32 machine, but it does potentially mean more exciting things for SIBO machines with Honda connectors, as it does look like there’s a way to talk SIBO-SP over the Honda connector.

UPDATE: 2026-04-15

Last year a few of us in the Psion Community came across an update to the SIBO C SDK that we didn’t have before.

At a conference in the late 90s, Psion handed out Word documents for developers to update version 2.10 of the SDK to 2.15. The latest version that I have of the Hardware Guide is 2.10. This includes a table that I have reproduced here with added information on ARM/EPOC32 machines:

Pin	Function	Direction
1	Siena: Vin at +6V DC ±10%	`<--`
	Series 3c/mx: 5V power out, 30mA maximum	`-->`
	ARM: 5V power out (max current unknown)	`-->`
2	`RTS` (RS232)	`-->`
3	`DTR` (RS232)	`-->`
4	`TXD` (RS232)	`-->`
5	`DSR` (RS232)	`<--`
6	`DCD` (RS232)	`<--`
7	`CTS` (RS232)	`<--`
8	`RXD` (RS232)	`<--`
9	SIBO: `SDOE` - data direction control	`-->`
	ARM: Vpcc	`-->`
10	SIBO: `EXTSTAT` - active low peripheral detect line	`<--`
	ARM: N/C	N/C
11	SIBO: `EXON` - external turn-on	`<--`
	ARM: Vpp	`-->`
12	SIBO: `INT` - peripheral interrupt	`<--`
	ARM: N/C	N/C
13	SIBO: `SD` - serial data	`<->`
	ARM: N/C	N/C
14	SIBO: `SCK` - data clock	`-->`
	ARM: N/C	N/C
15	`GND` - Signal and power ground	`---`
16	`GND` - Shield frame ground	`---`

Key:

--> Out of machine
<-- Into machine
<-> Bidirectional
--- Ground

Notable differences here between SIBO and ARM machines are pins 9 and 11. As no one has yet blown up a Siena drive by plugging it in to a Series 5, I assume this wouldn’t cause a problem with other peripherals.

Also, the last time I tried to attach an oscilloscope to a Siena SSD Drive, I slipped and blew up a regulator - one that’s no longer manufactured. Luckily I have two other drives, but these things are rare. I’ll be much more careful in the future.

Striving for simplicity: A Psion Serial (Honda) to USB cable | Details

Sun, 30 Dec 2018 00:00:00 0000

As I’ve got older, I’ve grown to like simplicity. I want to faff around less with the minor details and give myself more time for the fun stuff. This project aimed to remove some faff, although at first it didn’t quite go to plan.

My plan was to make a Honda USB cable, meaning I could connect a 3c or 3mx to a PC without needing two cables and a load of bulk.

DISCLAIMER: This project comes with the usual “thermonuclear war” caveats. It worked for me, it might not work for you. In fact, you could damage your Psion, your USB cable, your computer’s USB port, yourself… Do this at your own risk.

I used:

1x USB to RS-232 adapter with a CH340 chip (I used this one)
1x Unterminated Honda serial cable (like this)
Soldering iron
Scalpel
Heatshrink (I think I used 1x 1cm diameter and 2x 0.5cm, but I can’t remember)

My initial idea was to buy a 5v TTL adapter from eBay, solder an unterminated Honda cable to the serial pins and, hey presto, a Honda USB cable. This, however, turned out not to be the case. Because Psion RS-232 ports use true RS-232 at +12v and -12v. So, back to the drawing board.

My work on this was on a Saturday a couple of weeks before Christmas. The day hadn’t gone too well - I had struggled with this project, then tried (and failed) to get the Microsoft Macro Assembler running in DOSBox, and finally tried (and failed) to cram as much VHDL into my brain. I’d gone to bed feeling frustrated and disheartened. I woke up the following day still feeling frustrated and disheartened. So I decided to do something about it.

I grabbed my existing USB-to-serial cable and an unterminated Honda cable and, armed with a soldering iron and a scalpel, got to work.

Entombed inside the DB9 end of the cable is a 1cm2 PCB. And I really do mean entombed - the entire board and connections had been put in a mould which had then been filled with soft translucent plastic.

It took me a good hour or so of hacking away with the scalpel to free the board from its rubbery tomb. I also had to cut off the DB9 connector as I didn’t have any desoldering tools good enough to do the job. This is what it looked like when I was done.

I reused the now unterminated USB cable, slipping some heatshrink onto the cable for later before soldering it to the board. I then soldered an unterminated Honda cable to the serial connectors on the board, again with heatshrink already on the cable. Here’s the table with the cable’s colour coding and the RS-232 pins they correspond with. (Note that the colours have changed over the years, but these colours worked with the cable I got from eBay.)

DB9	Honda cable colours	Honda pins	RS232	Description
	Shield	GND	GND	Shield
1	-	nc	DCD	Data carrier detect
2	Yellow	4 (TX)	RD	received data
3	Grey	8 (RX)	TD	transmitted data
4	Green	5 (DSR)	DTR	data terminal ready
5	Black	15 (SG)	SG	Signal ground
6	Orange	3 (DTR)	DSR	data set ready
7	Red/White	7 (CTS)	RTS	request to send (= DTE ready)
8	Pink	2 (RTS)	CTS	clear to send (= DCE ready)
9	Blue	6 (RI)	RI	Ring indicator

On my first attempt I had soldered the USB’s D- and D+ wires around the wrong way, so I quickly swapped those over. Once I’d fixed that, I crossed my fingers that I hadn’t damaged the board or my PC.

Luckily, I hadn’t. It worked perfectly.

Once that was done I tidied up my soldering, put the large piece of heatshrink in place and aimed a hairdryer at it. I then moved the two smaller pieces of heatshrink and did the same.

I now have a 2m USB to Honda cable that will work on the Series 3c, 3mx and Siena, plus the Series 5 and 5mx, all for about £11.

If I did it again, I would probably find a USB plug enclosure that the CH340 board could fit inside. There is a risk of damage to the solder joints and the PCB and fitting the board inside a small box would fix that. Also, I’d really like to find a supplier who will sell me CH340 boards that don’t require me to hack away at a load of blue plastic.

Oh, and I did work out how to get MASM working, but more on that another time.