Looking for info: Intel MDS-225

[Grumble]

Hello, first post here!

My name is Ed Droog, and since a few months I am working as a volonteer technical advisor at the Historic Study Collection EWI at the University of Twente.
https://studieverzameling.utwente.nl/
We have a working Intel MDS-225 system in our collection which we got running with the aid of https://homecomputermuseum.nl running Isis II and (ofcourse) I have a question:

At the back of this computer there is a number of connectors (see attached file) and I would very much like to use a printer with this machine.
I made a cable for it, and connected an old VT220 (amber ) to connector SERIAL CH1/TTY and it works! Albeid at the very low baudrate of 110bps
All I did was connecting the VT220 with my cable to J2 and typed:
COPY CREDIT.HLP TO :LP: (credit.hlp being a text file) and the text file was written on the schreen of the VT220.

I know there hides a connection for a line-printer at SERIAL CH 2 (J3) but I wasn't able to get it to work the same way I used J2 and the VT220.

My questions:
Is it even possible to address the printer from J3 using the command line in Isis II like we did with J2?
Is there a member here who can tell me what to type to have this computer sending data over J3 like I did with J2?

Thank you for your time!

[Mark1960]

Did you try any other device names in place of :LP: ?

Its been a very long time since I used ISISII, but I did find a user guide on bitsavers that might help. Device names are shown on page 2-4.
http://bitsavers.trailing-edge.com/p...uide_May81.pdf

[Grumble]

Thank you, but that didn’t help.
I know this guide, we even have the original printed version from the late 70-ties or early 80-ties.
I have to admit that English is not my native language, so maybe it is somewhere in that guide how to address the printer on J3, but buried somewhere in the intel jumbo jumbo…
Anyway, I keep searching and reading….

[JohnBHanson]

try typing
help devices

That might help

In my case I get the wrong infomation out of this - which is
HELP DEVICES

There are both logical and physical devices in ISIS. Physical devices are
mapped to logical devices with the ASSIGN command (see HELP ASSIGN).
<logical device> :<nn>: where <nn> can be:

:F0: thru :F5: Logical Drives
:CI: Console Input
:CO: Console Output

<physical devices> any of the following:

0 thru 3 Disk Drives
4 and 5 Bubble Drives
:SO: Serial output
:SI: Serial input
:LP: Line printer
:VI: Video input (keyboard)
:VO: Video output (CRT screen display)
:BB: Byte bucket
:I1: User defined input device
:O1: User defined output device
:L1: User defined list device


Devices I would try are :SO: :CO: :TO: :VO:

From what you say it appears that the connector you are using is for the teletype - hence
the 110 baud. The crt would be the other serial port which is either SO or CO.

My first guess at baud rate for the crt would be 2400. You may need to assert RTS/DTR
on the RS232 interface.

For more infomation type debug under isis which will give the . prompt. Then the Q command will give the current assignments. The A command can be used to change
these assignments (Although ISIS will prefer its own device names). There is also a consol
command under isis that may help. To return to ISIS type g


Does your MDS have a screen/keyboard in which case it maybe driven from the IOC (Input / Output Controller)?

[Grumble]

Thank you JohnBHanson, I will sit down next Wednesday and try your suggestions.
The system we have is shown in the attached picture.

[Grumble]

Attaching a picture was unsuccessfull, I'll try again..

[JohnBHanson]

Some info given here.

https://www.ricomputermuseum.org/col...lopment-system

[woodchips]

Have you actually managed to use the MDS?

I see from the listing that about 40 years ago I wrote a comms program to upload and download files to the PDP11. ISIS is not a very nice operating system compared to the equivalent MDOS for the Motorola Exorciser, I found any help was useful!

If this listing, paper, is of any use then i can post it.

[Grumble]

In my working days (I’m retired some years ago) I spent most of the time on designing and building hardware, starting with the PDP8 where I made a student safe input rack for the A/D converter, I made some interfaces for the apple II and for the XT-pc.
Later I switched to uprocessors where I also made test programs for, so not realy user friendly programs….
Though this MDS is originally from our university,I have never worked with the MDS-225, that computer belonged to a different work-group.
The desire was to use this system for demonstration purposes, so I started working on it.
First thing I did was opening the dual 8” drive, and the first thing I noticed was the difference in smoothness of the capstan rotation, which is driven by a 220 volt motor.
I took the motor apart, interchanged both the ball bearings with new ones (they are still sold as standard types) and that was a succes, because now we have a working system with a built-in and a dual (working!) external 8” floppy disks.
One of my colleagues of the Study Collection has actually did some assignments during his study (or phd, I’m not sure…) so he still knows a little about this machine.
So, maybe if you scan the listing and mail it, perhaps it can be of some help?
Thank you!

[Grumble]

I stopped trying to connect a printer to SERIAL CH 2 (J3), instead I used SERIAL CH 1/TTY (J2)
I come to think that this SERIAL CH 2 can only be used program wise.
In one of the manuals I found a small program to modify the Baud-rate and used that with the ASM80 assembler.
The used program is attached to this post.

[Grumble]

https://youtube.com/shorts/e6BSAg6vZ...ip49NogMUgcr66

I think Serial Ch2 (J3) is defective, since I was able to use Kermit to transport data to and fro the MDS2235 and a Windows XP computer using Serial Ch 1/TTY (J2) while this was not possible using Serial Ch2 (J3)

[SiriusHardware]

Most likely there is an RS232-TTL level converter or, in this period, two separate level converter chips, one for TX and one for RX - if it is a single IC doing the level conversion both ways then almost certainly a MAX232 but more likely a pair of older generation 1488 / 1489 level converters or some other manufacturer's equivalent. (SN75188 / SN75189 etc).

Normally if a serial port fails hardware-wise it will be these ICs, the ones with connections directly exposed to the outside world, which will have been damaged.

As a quick check, the RS232 TX-out output pin on the port will usually have a standing -ve voltage on it when it is idle - if you don't even see that then you should investigate why. Since you have more than one physical port you can measure and make a note of the standing voltages on all the pins of the other ports, then measure the pins of the suspect port - are any of the pins in a different state to those measured from the 'good' ports?

[ortek_service]

I doubt it would have used a MAX232 as Maxim didn't release that until around 1986: https://en.wikipedia.org/wiki/MAX232

With Maxim only being formed in around 1983, seemingly not making their own new designs until 1985: https://en.wikipedia.org/wiki/Maxim_Integrated - With the MAX232 being their breakthrough product, that made them profitably

(I first encountered Maxim, in their early second-source only devices days, when I was having difficulty obtaining an Intersil BCD to 8digit LCD driver IC - But I'd found Maxim were producing a compatible one. And recall them later producing many 'enhanced' versions of Intersil IC's like OpAmps & Voltage regulators / charge-pumps. I've still got many of their first databooks and seeming weekly new product brochures from the early 90's when they appeared to be at their peak for innovation)

[SiriusHardware]

I didn't know the launch date for the MAX232 but I had an idea, even as I wrote, that it would be too 'late' to have been used in this unit. Apart from the MC1488 / 1489 and SN75188 / SN75189 level converters mentioned earlier, were there any other 'popular' RS232 level converter ICs when this unit was manufactured?

[TonyDuell]

The Texas Insturments SN75150 (driver) and SN75154 (receiver) were sometimes used but nowhere near as common as the 1488/1489 devices

Some manufactueres used op-amps as the drivers. Discrete transistor-based driver and receiver circuits turn up too,

Of course on eary-ish serial communications (1970s) the interface might be a 20mA or 60mA current loop, not RS232 at all

[Grumble]

Serial 2 proved to be working as well!
I failed to notice that, although the serial section (pins 2 and 3) of the connectors at the outside of the system are named equally, the rxd and txd wires are twisted in the Serial 1 wiring. All it took was a null modem cable to get Kermit working on Serial 2 as well ?

Oh, btw, the system we have appears to be a MDS-226

[ortek_service]

Good to hear you've now got all of these working OK.

It seems Intel rather broke the RS-232 'Standard', allowing pins 2 & 3 to be swapped over inside a 'DTE' - When by convention Pin 2 on a DTE's DB25 should be TxD Out into pin2 of a DCE's DB25 (Which could also be labelled TxD as convention is that signal names are all referred to DTE-end).
It would seem to be better to just use a 'Null-MODEM' cable, to swap these over, to save having to open-up the Terminal.

And Intel have also broken the convention of using a Male DB25 on the DTE.
But it seems other manufacturers of Terminals did this, but usually provided a double-ended straight-wired 25w plug-to-plug lead to correct this, so effectively being a gender-changer: https://en.wikipedia.org/wiki/RS-232

Maybe the sockets were considered more-robust to have on the equipment, and cables more-easily replaced if the exposed pins in the male/plug got damaged.

Of course IBM then went on to move from a standard 25way 'D' male plug to a 9way 'D' male - but also confusing decided to swap pins 2 & 3 over relative to the original 25way 'D' in the standard whilst using the same pin numbers for these (although ground had also changed pin no. from 7 on 25way to pin5 on 9way to still be on the same row).
Maybe they moved to a different-sized connector, to not confuse it with the 25way 'D' they'd used for the Parallel Printer port (Although at least that was a female-socket, so 'standard' cables wouldn't get plugged into the wrong one)