Research Machines RM380z

Research Machines RM 380Z

Preliminary Information
Release Year	Introduced 1977 Discontinued 1985
Mounting Technology	Sockets, DIP, through hole and screw mount
Capacitor Types	Aluminum Electrolytic and Tantalum
Destructive Entry	No
Hardware Specifications
CPU	Zilog Z80A 4 Mhz
Memory Capacity	Up to 64K with 56K accessible.
Memory Type	8116 which is the Fujitsu labelled version of the 4116.
Storage	Early systems were single density single-sided 5¼-inch disks with a capacity of up to 72 KB later systems were up to 360K.
Graphics	40x24 or 80x24 text mode with a High-resolution graphics adapter available.
Display Interface	SO239 composite
Power Requirements
Input Voltage	240V AC 50Hz
Power Connector	IEC 13

Known Issues

The power supply was blowing fuses and was outputting low voltage.

The bridge rectifiers were also getting very hot after a few seconds.

This unit also exhibited no signal on the video output and no life on the bus, this was due to all the corrosion this specimen was subjected to. This specimen is also missing a floppy drive as that one was very corroded and the bearings were seized that is why the photo has a large hole in the front left.

Causes

• The large filter capacitors were bad.
• The tantalums on the cards were bad.
• Sockets and ICs were corroded

Solutions

• Tried reforming the large filters however this was unsuccessful so these were replaced.
• Replace the tantalums which lowered the current draw to what was expected cooling the rectifies down to normal operating conditions..
• Replaced the sockets and replaced the ICs on the VDU which cured the video output however the machine still doesn't run..

Disassembly Notes

Simple disassembly is easy with two screws on the back of the lid to gain access to the cards however the power supply requires disordering cables and removing the whole case off the base.

Removal of the floppy drives in the vertical configuration is also complicated as they require the removal of the front panel and all the cards. This can be quite difficult especially if the nuts and bolts are corroded like mine, the tight space doesn't help as it can be difficult to get pliers and/ or spanners into the right place.

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When the lid is removed the cards and backplane are easily seen and the power supply is on the right underneath the CPU serial monitor board. the floppy drive mounts are on the left for vertical mount BASF 6106s. The serial number tag should be visible on the case above the PSU which this one is hand written SN 691

Original Parts

Disk drives

The early RM 380s use BASF 6106 floppy drives which are 2/3 height SSSD 5 1/4. The drives are Shugart interface compatible. Later on IBM PC full Hight drives were chosen and the mounts were changed from a vertical to horizontal. These are easy to replace as they are more common. The early BASF drives can badly corrode depending on storage conditions so be wary of this as 2/3 height floppy drives are not as common.

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  This is the top view of the floppy drive which is a BASF 6106 single sided 2/3 Hight floppy drive. These drives use the original shugart interface.

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  This is the bottom view of the floppy drive which shows how bad the corrosion is.

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  The connector position of the BASF 6106 head should be on the top row of pins. This is possibly because the PCB is compatible between the 6106 and 6108 which is double sided though this still needs conformation.

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  The PCB top view is pretty complicated though this may be because it supports the double sided 6108 drive too.

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  Bottom view of the drive PCB

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  The back view of this drive shows that it is serial number 8251 and looks to have been made in the late 70s

Keyboard

The keyboard is just standard parallel ASCII foam and foil unit external to the unit via a 15 pin cable. The foam pads in mine are still in good shape but are the same size as most replacement pads making replacement easy. My unit was made by alphametric in the UK.

Keylock

The keylock is a old off the shelf part from RS however the keys are unique to each machine so check to see if you have the keys. This is because the switch controls the power input and also lock function of the front. This one was also seized so was soaked in WD40 to unstick it and allow it to rotate freely.

Cards

The cards are pretty easy to service however the early RM380s use Texas Instruments chips which are know for getting corroded legs. The early ones also use cheap sockets which can corrode. For mine to be a reliable machine large scale replacements of chips and sockets was the only way as the IC legs were breaking in the sockets. Below are the first party Research machines cards in this case as originally configured via the original label inside.

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  RM380z 80 and 40 column VDU. This card is text only and produces a composite video output

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  RM380z CPU and 32K ram card

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  Rm380z 32K add on ram card which is a CPU card minus most of the parts.

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  RM380z floppy disk controller and serial controller. This card is the early FDC which is for SSSD disks.

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  RM380z serial controller which is a FDC minus the FDC parts.

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  RM380z bus terminator card

The following are more first part Research machines cards which aren't in this machine at the moment for one reason or another.

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  RM380Z IEEE-488 controller this is missing the cable for the back panel.

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  RM380z prototyping card with bus interface circuitry already included and wire wrap sockets on the back.

Power supply

Early supplies use the types below. Later revisions use different types.

Capacitor Specifications and Footprint Information

RefDes	Qty	Capacitance	Voltage	Mount	Diameter/Size	Height	Lead Spacing	Temp	Type	Brand	Series
C1	3	470uf	63v	Axial	13mm	31mm		+85c	Aluminium electrolytic	Solar
C8	1	22000uf	25v	Screw mount radial	50mm	90mm	25mm	+85c	Aluminium electrolytic	Mallory	CGS223U025V3C
C12	1	15000uf	40v	Screw mount radial	50mm	115mm	25mm	+85c	Aluminium electrolytic	Mallory	CGS153U040V4C

Replacement Parts

Power supply

Capacitors

RefDes	Qty	Compatible Part Number	Order Links
C1	3	598-477TTA063M	DigiKey Mouser
C8	1	661-36DA223F025BA2A	36DA223F025BA2A Chemi-Con | Mouser United Kingdom
C12	1	647-LNT1H153MSE	LNT1H153MSE Nichicon | Mouser United Kingdom

Notes

C12 replacement is smaller in diameter to the original however is the same specifications.

It is recommended to add a crowbar to the PSU as the stock supply holds no protection on the early machines.

If parts are not available or different selection is preferred, you can use the values in the Original Parts section to perform a parametric search.

History

The Research machines 380Z is a British Z80 based microcomputer made in oxford England from 1977 to 1985. This machine found a popular use case in schools before the BBC micro was released and could be used in conjunction with Research Machines 480Zs to form a network with a 380Z running as a file sever over the proprietary RM 800kbps network. A kit version of the 380Z is believed to have been made called the 280Z however this cannot be confirmed. The RM380 can use multiple operating systems with COS (cassette operating system) been the default OS in ROM, the versions of COS changed over the lifespan adding and removing functionality. The main revisions known are.

• COS 2.2 –
• COS 2.3 – cassette-only and 40×24 video card support
• .COS 3.0 – floppy disk support for external 8 inch drives.
• COS 3.4 – main COS version supporting single density 5.25 disks and the 40×24 video card.
• COS 4.0 – 80×24 VDU text display with no cassette support due to the VDU80/02 not having the cassette interface.
• COS 4.2 – double density disk drives (380Z-D). This version is very rare due to the limited run of it.

The RM 380 could also run CP/M in both the 32K and 56K configurations. The two known versions of CP/M used on the 380 were CP/M 1.4 and later on it was CP/M 2.2. MP/M version 2 was used on 380s which formed the file server on a network of 480s using CP/NET

Related Links

• https://www.computinghistory.org.uk/det/2226/RM-380Z-(Black-Case)/
• https://www.theregister.com/2013/11/13/archaeologic_the_research_machines_380z_story/
• https://vt100.net/rm/380z_review
• https://archive.org/details/rm380zservicemanualpn13821a (This covers the later 380z but most of the schematics and information is still relevant.)
• Index of /pdf/researchMachinesLtd (bitsavers.org)
• Index of /bits/ResearchMachinesLtd (bitsavers.org)