Category Archives: RML380Z

RML380Z โ€“ and you’re back in the room.

I replaced the 74s32 with a 74ls32 (the closest replacement I could get) and the machine didn’t leap beck into life.

Disappointed doesn’t come close.

I walked away for a few days and came back to it and it came up.151108-IMG_20151108_141830344

I powered it of and on again and it didn’t. That’s a bit odd.

Out came the boards and chips were pressed into sockets. Power up again and Bingo! Off and on and still OK.

So here’s the question… Was it the keyboard adapter that caused it to fail or was it just coincidence? I’m not sure if I dare find out yet.

RML380Z – One step forward…

In my previous post, things were looking up. I had replaced a couple of faulty chips, re-seated all of the chips on the CPU and Video cards and swapped out the RAM from the CPU card with the expansion cards and now my 380Z was getting to the COS prompt.

All I needed was a key press. Specifically, I needed a ‘B’. Sadly, I don’t have a keyboard.

Screenshot from 2015-10-25 20:08:23Looking at one of the technical documents shows that the keyboard interface is a simple 7 bit parallel interface with a strobe. How hard can it be?

Arduino to the rescue?

151025-IMG_20151025_182536405_HDRI had a supply of Arduino pro mini compatible boards and so I set to building an interface and a sketch to drive the circuit. The idea is to catch characters from the ‘duino’s serial port and squirt them out of the parallel port the way the 380Z likes them.

Not so Fast.

I plugged in the board and nothing happened. I restarted the machine and nothing happened again. Just a blank screen, no COS prompt. Doh!!.

I went over the circuit for the interface again and a again and couldn’t find anything wrong with it. It does appear to have toasted the 380Z though.

A clue?

By coincidence I have just been to the Centre for Computing History in Cambridge and they have a 380Z on a table. I quick word with Jason who runs the place and I have a look inside. Their machine has a different keyboard cable to mine. Mine has an additional connector in-line with the cable. Theirs doesn’t. Sadly, I didn’t take a picture of theirs but it definitely only had one connector at each end.

151025-IMG_20151025_182559575You can see from the photo here that mine has the additional connector. After that connector, wires go off in the direction of the PSU. I’m wondering if mine has been modified for some reason and that has made it incompatible with my relatively simple interface.

I’ll have to trace where all of the wire go and see if thatโ€™s significant. I could also do with confirmation that the keyboard ribbon should be straight through.

 

RML380Z – Signs of life.

151014-IMG_20151014_213240378OK. Getting excited now ๐Ÿ™‚

I swapped the 4116 DRAMS from the CPU board with those on the RAM expansion card. And now… Ta Daa. I havenโ€™t’ tried the RAM expansion card yet and so I don’t know if the chips are faulty or just dirty and re-seating them has done the trick but so far so bloody brilliant. I’ve waited 35 years for this.

๐Ÿ™‚

RML380Z – Arrival of the new chips.

A small envelope arrived in the post Today. A tube of 74LS00 and 74LS393 to replace the ones identified as broken by my chip tester. The chip tester says that the new 393s are also faulty so I suspect the tester is a little flaky. Anyway, on with the show.

151014-IMG_20151014_180418795There has been some improvement but not a fix. I also reseated the chips that were not tested (ROMs, DRAMs etc.). I moved the 4116s around too to try and flush out the problem.

Back to square one?

It looks a bit like it. I think this might be a clue.

151014-IMG_20151014_181530814_HDR

 

380Z Repair – Send for supplies.

151009-IMG_20151009_162919625_HDRToday I received an IC tester through the post. Yay.

this is an important bit of kit for me. All I have to do is remove a chip from its socket, dorp it in the tester and press ENTER. This little gadget identifies and tests the chip in a fraction of a second. Crikey.

I’ve worked through the VDU and CPU boards and found one faulty 74LS393 counter on the VDO board and a 74LS00 NAND gate on the CPU board. Hopefully these are the source of my problems. If not, I’ll keep looking.

Supplies are on order. I’ll keep you posted.

 

380Z Repair – Another try.

After a break of a couple of years I’ve decided its high time I had another go at repairing my RML-380Z. You can read about this machine and my previous attempts here.

A quick recap… 53e525645f5aa2c829d66d1d878e69f6

When I power the machine on I get a a fixed, pseudo random image on the screen but no other signs of life. I’ve re-seated chips in their sockets and swapped the RAM chips around the board but that’s about it.

It has been suggested that I swap matching logic chip to see if that gives any clues…

380Z entering a hiatus…

The progress has been a bit slow on the 380Z front at the moment.

As you may have read, It displays nonsense on the screen and doesn’t do much else. I wondered if this was just a fault with the display controller card or more. At startup the machine is supposed to display a boot prompt and wait for a key press. however, I don’t have a keyboard. The keyboard interface is a simple parallel port with 8 bits for the decoded key code and a strobe to say that the data is ready. To try and make some progress I made a simple one key keyboard built into a 15 way connector.

All it does is permanently tie the lines needed to send a ‘b’ and then it has a push button on the strobe line. I push the button and the 380Z thinks I’ve pressed a ‘b’.

 

This didn’t change anything and so I think the problem is more with the logic board.

 

ROMS?

It has been suggested that the ROMS (EPROMS) might have lost their memory. I think their original life expectancy was only about 10 years and we’re way past that now. I would like to read them and compare them with the image files that I have. The chips used are TMS-2716, 2kx x8-bit EPROM from Texas Instruments. Their power requirements are the same as the 2708s and different to non-TI 2716s. I have been able to borrow an EPROM programmer and I have the needed adapter on order from the USA but unfortunately the seller has had some problems with his supplier and it’s taking longer than I had hoped.

When it gets here I’ll be on the case.

 

RML380Z

 

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Like a lot of people who were at school during the 1970/80 period, the first microcomputer I used was an RML380Z. It wasn’t the first computer I ever used, that was a DEC System 20 (IIRC), but this shiny black box was the first machine that I could sit at and get its full attention.

 

I have been wanting to get a 380Z for a number of years but the silly prices on ebay have made that impossible until now. A chance meeting with a fellow computer collector led to me getting the example you see in the photo.

As you can see, it’s not in the prime of life, it’s covered in stickers and there is no keyboard. When it’s powered on I get a screen full of rubbish but it’s mine and I’m delighted.

53e525645f5aa2c829d66d1d878e69f6

 

Exterior

The case is a 19โ€ type made from pressed steel and aluminium. It has a handle on each side. The front is dominated by the two full height floppy disk drives (8โ€ drives were available IIRC), the Logo and the power switch/reset button combination.

It was possible to lock the machine off, on or locked. Off and on are self-explanitory and โ€œLockโ€ disabled the reset button. Perhaps thought useful for public installations.

At the back there is a host of connections.

Keyboard. This uses a 15 way D-type socket. The keyboard uses a parallel interface down which the ASCII encoded characters are sent. There is a micro-controller on the keyboard PCB that looks after scanning and translating the key-presses.

Cassette. This uses the DIN socket typical of the time.

TV. The usual TV antenna socket. My unit doesn’t have a modulator and so this doesn’t work.

Monitor. A composite video output. Rather unusually the 380Z uses a SO259 socket much used on amateur radio equipment. This is a high quality connector and is still an expensive item. I don’t know of any other manufacturer of micros using this.

Serial port. A standard issue RS-232 port.

Another D25. I’m not sure about this one. When time permits I’ll go in and have another look.

 

There are plenty of other cut-outs covered by blanking plates so expansion was not a problem.

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Inside the case.

 

The photo above shows my machine with the lid off. The back of the machine is at the bottom of the photo, the front at the top.

As you can see, there is plenty going on.

Power Supply.

To the left you can see the power supply. Unlike later machines, this is not a self contained, switch-mode unit. Instead it is a linear supply made from a collection of components fixed to the nearest piece of case or folded aluminium. Working on it is quite tricky as a fair amount of the machine needs to be taken apart to get the the components. It’s not something that you can take out and hold in your hand.

Card cage.

Like the PSU this term is a bit of a misnomer as the cards stand between pairs of guide rails individually screwed to the bottom of the case. There is plenty of room here. As you can see, I have five cards and there is room for four more.

Floppy disk drives.

On the far right of the case is a pair of 5.25โ€ floppy disk drives. I havent taken these out to check the model numbers but early machines had DSSD drives giving about 84kb per side. Unusually, each side was addressed independently and so A: and B: were different sides of the same drive.

The cards.

I you can see, I have five cards in my machine.

VDU80.

6529f6d02e0556c5885a5f2c392016f7Next to the power supply is the video card. This supports 80 columns and gives composite video out. I noted the high quality connector on the back of the case. There is just a pair of twisted wires running from the connector on the card to the SO-259 on the case. Not the highest quality solution.

CPU.

2813ff65f9d8f5ffb48290dbf2e581e4The CPU card contains the Z80 processor so typical of business machines of the age. It has 32kb RAM in 4116 DRAM chips (the Z80 looks after refresh) and three TMS-2716 2kb EPROMs. It’s unfortunate that they used the TMS 2716 EPROM as these are very hard to get hold of now and tricky to read as they require several power supplies. The โ€œordinaryโ€ 2716s only need 5v and so became far more popular.

32kb RAM card.

 

1c04cc10496f433a822af21e5ad8d5d5Another collection of 4116s gives another 32kb RAM, maxing out the Z80’s address space.

Floppy disk controller.

391d2f4079ff9d7e668fadedabdbd119This really needs a better photo ๐Ÿ™‚

Bus terminator card.

fd828ca323c3dfd912897b754c38e4c4Just a collection of resistors and test points really.