MSX Info pages

Slot extender

Simple slot extender for MSX

Hans Otten, 2002/2005

The two slots of a normal MSX are quickly filled when you need a hard disk controller, a GFX9000, a Moonsound, a modem, too much to fit. And a slot expander is rare and expensive. In that case: build a slot extender! Cheap and nearly as good!

Circuit diagram of the slotextender

Slot extender? What is the difference with a slot expander?

Slot expanders are convenient extensions for the MSX. With an external box a cartridge port is trransformed into four cartridge ports, and each slot accepts nealry all types of cartridges. A well designed slot expander does not allow to insert a cartridge the wrong way, preventing a premature death of both MSX and cartridge.

Many types of slot expanders have been produced. To name a few: MK Pblic Domain sold the first version on the market. The engineer who made it work right and kept improving it is Kees Folst a.k.a. Digital KC. The last variant with four slots and two I/O slots can be bought on order by Hans Oranje. Sunrise alos mentions slotexpanders for sale on a waitinglist.

What is an I/O slot

The slot expanders made by Sunrise and Hans Oranje have some extra I/O slots. An I/O slot can be used for MSX cartridges only requiring I/O ports. Some examples are the GFX9000, Moonsound and modems like Telcom, NMS1250. Cartrdiges with builtin ROM (games) or RAM or certainly no I/O cartridges. But also hardware working with memory mapped I/O are not, like many floppy disk controllers.
Examples of cartridges not working in an I/O slot are:

- ROM cartridges
- SCC cartridges (even with the switch to turn off the game)
- memory mappers
- Harddisk controllers
- FM-PAC
- floppy disk controllers

The slot extender

A slot extender makes it possible to add one or more I/O slots to the MSX in a cheap and simple way while allowing the full use of the original slot. With a switch (SLOT SELECT) one can switch the cartridge in the original slot off or on. That can be used to dump a ROM cartridge: start the MSX with SLOT SELECT in the OFF position and, after boot, switch to ON.

Build a slot extender

The slot extender is quite a simple device,. In fact it is not more than parallel wired connectors. SLOT SELECT is only connected to the first connector. The job requires a lot of wires to be soldered and needs to be done carefully: a short circuit between wires means always damage to the internals of the MSX with fatal results. Check all connections before testing the device!

Picture 2. The three extender, originally made by Ries Vriend and extended with the BUSDIR circuit

Several of these slot extenders have been built:
- the Zemina 2 slot extender, a device that does not work with all alle MSX hardware werkt (see the discussion onBUSDIR en INT why) and therefore adpted to the slot extender recepy.
- A three slot extender build by Ries Vriend (see picture 2) . Also here the BUSDIR problems with for example the NMS8250/55/80 and therefore enhanced with the 74LS21 BUSDIR circuit.
- a quick to built version with crim-pon conenctors on flatcable, see poicture 3. A bit difficult to find a suitable case, the 74LS21 circuit is underneath.
- a four slot version with a backplane, a good case and a cartridge connector..

All these slot extenders work as expected. The 2 and 3 slots variants work fine on 7 MHz. The four slot version has a long cable and since no buffering of the signals is done it may have problems at standard speed or 7 MHz and with the drain on the power supply if all slots are used by powerhungry cartridges.

How to build

This howto is for a four slots slot extender.

The required parts are:

Number	Part	Description
50 cm	50 wire flatcable	To connect the extender with the slot, dont make it too long (30 cm is fine)
1	Cartridge case	A standard MSX cartridge case is the best, but a audio cassette case has the right dimensions
1	50 pins edge connector	The cartridge connector of the MSX has a 2.54 mm spacing and is double sided, HArd to find, an old PC ISA card has a suitbale connector connector with gold plated connectors. See picture 4 how with some work it can be used.
1	Case	Not to hard to find. An old modem case is a good example. Make sure the cartridges cannot be inserted the wrong way!
2 of 3	50 pins connectors	The slot connector. In the Netherlands: Voor print montage, printkonnektor Display Elektronica nr:05.33.2.258 2x25 polig print straight Conrad nr 74 26 35-60 2x25 connector page. 772 For a flatcable: Conrad nr 74 19 06-60 snijklemuitvoering voor platte band
1	74LS21	TTL IC for BUSDIR en INT
1	Switch	two pole switch
	Wire	Thin isolated wire, the thinner the better!
2 of 3	Resistor 2k2	To mix the audio, experment for the best balance between audio devices
12	Resistor 10k
1	Circuit Tester	Check the connections, also for short circuits, never with a MSX switched on!
3	Capacitor 0.1 uF	To decouple the powerlines
1	Capacitor 47 uF 16V	Elco, for the +5V decoupling

 

Step 1: Connector for the MSX slot

The first step is to built a cartridge to feed all signals via a flatcable to the actual slot extender. In picture 1 and example is shown of a conenctor found in the junkbox with goldplated print edge connector from an old ISA card, adapted to the right size of 2x25 pins.

Picture 4 A connector made from an old PC ISA card

Step 2 Construction of the slot extender

Take a perf board/experimenters board, for example of eurocard format and solder the edge connectors in place, see picture 1). Then connect all pins via the following tables, a lot of work!.

See the pictures for the location of pin 1, 25, 2 and 50!

Alternative way of using a flatcable and crimp-on connectors

Circuit diagram of the slotexpander

Step 3 Active components

Solder the 74LS21 and resistors in plave according the circuit diagram. Also connect the SLOT SELECT switch. The capacitors are to be placed between the powerlines and ground, mind the polarity!

Step 4. Case

Place the extender print in the case. Finish the case to your wishes, but make sure it is done in such a way that inserting cartridges the wrong way around is impoosible!

Step 5 Test

Use an ohnmmeter or circuit tester for all conections on all connections. Not only the expected connectiosn should be checked, also adjacent lines for short circuits.

Step 6 Test, test , test!

Connect the slot extender to the MSX without a cartridge present. Then switch the MSX on and see if all is fine. Bad connections or short circuits are the danger here!

Step 7 The first cartridge

Insert a good cartridge and check the SLOT SELECT switch

Slot pins 1-49 connections

Name	MSX Slot connector	Slot 1	I/O slot 2	I/O slot 3
CS1	1	1	1	1
CS12	3	3	3	3
Reserve	5	-	-	-
WAIT	7	7	7	7
M1	9	9	9	9
IORQ	11	11	11	11
WR	13	13	13	13
RESET	15	15	15	15
A9	17	17	17	17
A11	19	19	19	19
A7	21	21	21	21
A12	23	23	23	23
A14	25	25	25	25
A1	27	27	27	27
A3	29	29	29	29
A5	31	31	31	31
D1	33	33	33	33
D3	35	35	35	35
D5	37	37	37	37
D7	39	39	39	39
GND	41	41	41	41
GND	43	43	43	43
+5V	45	45	45	45
+5V	47	47	47	47
SOUND	49	*	*	*

* see text and circuit diagram

Slot connector pins 2-50

Name	MSX Slot connector	Slot 1	I/O slot 2	I/O slot
CS2	2	2	2	2
SLTSL	4	4	*	*
RFSH	6	6	6	6
INT	8	**	**	**
BUSDIR	10	***	***	***
MERQ	12	12	12	12
RD	14	14	14	14
Reserve	16	option	option	option
A15	18	18	18	18
A10	20	20	20	20
A6	22	22	22	22
A8	24	24	24	24
A13	26	26	26	26
A0	28	28	28	28
A2	30	30	30	30
A4	32	32	32	32
D0	34	34	34	34
D2	36	36	36	36
D4	38	38	38	38
D6	40	40	40	40
CLOCK	42	42	42	42
SW1	44,46
SW2	46, 44
+12V	48	48	48	48
-12V	50	50	50	50

* SLTSL only connect to one slot

** INT see text and circuit diagram

***BUSDIR see text and circuit diagram

Technical background

I/O in two ways

The way the Z80 cpu addresses I/O devices is important for the concept of I/O slot and the slot extender/expander.

I/O is performed in a computer by special ICs for the communication with the outside world. These ICs show to to the cpu socalled registers that allow via read/write operations I?O to take place. A good example is the PSG sound generator.

I/O based

The Z80 uses port mapped I/O to address I/O devices in the form of special ICs. The Z80 has special instructions to do I/O read and writes to the ports (the registers of the I/O device like IN and OUT and some more specialised ones. The RD (read) and WR (Write) signals of the CPU indicate, together with the address, which opertion on what port to perform. MThe MSX machine is filled with these ports (see the Ports map in the MSX Info pages) like PSG, VDP etc. DO not mix the I/O port mechanism and the slot concept!

Memory mapped I/O

Port mapped I/O is one way of doing I/O and standard in the Intel/Zilog world. But an I/O device register can also be accessed as normal memory location. Advantages are that no special I/O instructions are required and the limit of 256 I/O ports is gone. It does take away part of the 64K memory space available in the MSX, but this already is solved with the slot mechanism and memory mapper concept.This is done in complex MSX machines, for example disk controllers are mostly memory based, the device gets its own page.

Cartridges in a MSX slot and SLOT SELECT

An external MSX slot (one is required by the standard) is a socalled primay slot in the memory map, slot 1 (and 2) usually. This means the SLOT SELECT on the slot connector is activated if the slot is selected by the program. SLOT SELECT is therefore essential for merory based activities. When SLOT SELCT becomes low it means memory (or memory mapped I/O) access is possible for the cartridge present in that slot. When a primary slot is expanded the most important feature of the expander is to generate SLOT SELCT signals for each expanded slot.

Cartridges in a MSX slot and memory mapped I/O

On the cartridge connector no signal is present to indicate the equivalent of SLOT SELECT for memory access. This means every cartridge and all MSX internal devices will see an I/O request from the processor. Only the port address determines if a device feels addressed.
This means:
- only 256 I/O ports in total are available on the standard MSX
- an I/O based slot does not need SLOT SELECT and is unaware of primary or secunday slot

BUSDIR and INT

Some MSX computers like the VG8235 or NMS8245 and nearly all MSX 1 computers do not require the BUSDIR signal, because the databus is not buffered inside the MSX. BUSDIR is a signal that indicates during read operations that the (bidirectional) data buffer needs to send data to to cpu. External cartridges need to assert BUSDIR when they are want to send data to the CPU. In the circuit diagram therefore an OR circuit is used to combine the BUSDIR's from all slots.

INT is a signaal activated by a cartridge when it has a hardware interrupt to report. In the circuit diagram therefore an OR circuit is used to combine also the INTs from all slots in the extender.

Future developments

The slot extender presented here is a convenient addition. Not as convenient as a slot expander but it can be made more close with still a limited amount of hardware:
- better sound. The resistor circuit is quite simple to mix sound from the slots. A better solution, also for slot expanders, is to feed the audio acpins to an external mixing panel. Or built a simple mixing circuit with a opamp.
- add a real complete slot (see also the Third slot article). For example the MSX machines with the S3527 or S1985 MSX engines have at least one expanded slot signal available. On the S3527 slot 301 is available, see the IC description and the service manual.
- use a rotary swicth to enable any of the slots as a full slot with SLOT SELECT.

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