ITCI Brasov CoBra Hardware Description

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HARDWARE AND SOFTWARE SCIENTIFIC RESEARCH

AND TECHNOLOGICAL ENGINEERING INSTITUTE

HARDWARE DEPARTMENT

COBRA

HARDWARE

DESCRIPTION

BRAŞOV

1988

EDITORIAL MANAGERS:

Dr. eng. Dan Roman

Dr. Emil Munteanu

COVER DESIGN:

designer Liviu Derveşteanu

The COBRA microcomputer was designed and built by the following team:

eng. Vasile Prodan, eng. Sorin Finichiu, eng. Bernd Hansgeorg Wagner,

math. Marcel Arefia, math. Mircea Pop, eng. Mircea Ungur,

arch. Alexandru Antal.

This material has been edited by: eng. Vasile Prodan,

eng. Mircea Ungur.

Coordinator: Dr. Eng. Gheorghe Toacşe

C O N T E N T S

1. Functional description of CoBra microcomputer . . . . . . . . 4

1.1 Block diagram . . . . . . . . . . . . . . . . 4

1.2 Central Processing Unit . . . . . . . . . . . . . . 5

1.3 Memory . . . . . . . . . . . . . . . . . . 8

1.4 Video controller . . . . . . . . . . . . . . . . 11

2. Microcomputer interfaces . . . . . . . . . . . . . . 16

3. Keyboard . . . . . . . . . . . . . . . . . . . 19

4. Power supply . . . . . . . . . . . . . . . . . . 22

5. Parts list - floppy disk interface . . . . . . . . . . . . 26

6. Parts list - CoBra microcomputer . . . . . . . . . . . . 27

7. Parts list - keyboard . . . . . . . . . . . . . . . 32

8. Parts list - power supply . . . . . . . . . . . . . . 33

Connectors - pin description . . . . . . . . . . . . . 35

9. Appendix 1 --- Schematics - CoBra microcomputer . . . . . . . 39

10. Appendix 2 --- Schematics - Floppy disk interface . . . . . . . 55

11. Appendix 3 --- Component placement on circuit boards . . . . . . 59

12. Appendix 4 --- RGB interface for the 002 color monitor . . . . . . 65

1. FUNCTIONAL DESCRIPTION OF COBRA MICROCOMPUTER

1.1 BLOCK DIAGRAM

The CoBra microcomputer is built around the 8-bit microprocessor Z80A,

on three circuit boards. The microcomputer block diagram is shown in Fig.1.

Fig. 1 – CoBra microcomputer block diagram

On the mainboard there are: — the central processing unit block with the Z80A μP and address/control

bus amplifiers/separators.

— the memory block consisting of a configuration/selection circuit, a 48

KB DRAM circuit and a 2-16 KB EPROM circuit.

— the video controller consisting of 16 KB DRAM video memory, memory

access priority controller (memory access prioritizer), generators for video synchronization signals and composite color signals, video memory control signals generator, and the system clock generator.

— interface block, built around a parallel programmable interface i8255.

— RS-232 interface for use with a printer or another computer. — external memory (magnetic tape) interface. — audio interface. — keyboard interface (6x8 key matrix). — 8 bit input port for general use or used as a Kempston-compatible

joystick interface.

The conventional keyboard is built on a separate board, as an extended

6x8 key matrix plus ZX SPECTRUM+ compatible compound keys.

The floppy disk interface is built on a third board around the i8272A

Floppy Disk Controller circuit assisted by the Z80-CTC counter/timer circuit.

1.2 CENTRAL PROCESSING UNIT

The central processing unit is built with the 8-bit microprocessor Z80A.

Z80A is a MOS-LSI integrated circuit in a 40-pin DIP package, having 3 buses:

— the data bus; — the address bus; — the control bus. The data bus D0—D7 is a three-state input/output bus, used for

exchanging information with the memory and the I/O interface circuits.

Z80A falls within the 8-bit microprocessor category, having the capability

to process 8 bits of information simultaneously on its data bus.

The 16-bit address bus is used to address the memory or the I/O devices

during the information exchanges.

Having 16 bits for the address bus, Z80A can address 64 KB of memory

and an additional 64 KB space dedicated to the I/O devices.

The control bus offers the signals required for monitoring the data

transfer to/from microprocessor.

The microprocessor can perform different tasks: — reads data from memory; — writes data to memory; — reads data from I/O devices; — writes data to I/O devices; — performs arithmetic operations on data. Z80A executes a range of 158 types of instructions. In the CoBra

microcomputer, the microprocessor clock signal has a 3.5 MHz frequency.

Pin description:

A0-A15 — the address bus;

— three-state logic outputs, active in “1” logic level; — can address up to 64 KB memory and I/O devices; — in the case of I/O, the 8 less significant address bits are used to

select up to 256 input devices or 256 output devices;

— during the dynamic memory refresh cycle, the contents of the

I and R registers shows up on the address bus, the 7 less significant bits of the self-incrementing R register being used as a refresh address.

D0-D7 — the data bus;

— three-state inputs/outputs, active in “1” logic level.

M1 — machine cycle one;

— output, active in “0” logic level; — together with MREQ, indicates that the current machine cycle is the

opcode fetch cycle of an instruction execution;

— M1 and IORQ both active indicate the execution of an interrupt

cycle.

MREQ — memory access request;

— three-state output, active in “0” logic level; — indicates that the address bus holds a valid address for a memory

read of memory write.

IORQ — I/O ports access request;

— three-state output, active in “0” logic level; — indicates that the lower half of the address bus holds a valid I/O

address for an I/O read or write operation;

— together with M1, indicates that an interrupt response vector can

be placed on the data bus.

RD — read;

— three-state output, active in “0” logic level; — indicates that the CPU wants to read data from memory or an I/O

device.

WR — write;

— three-state output, active in “0” logic level; — indicates that the CPU data bus holds valid data to be stored at the

addressed memory or I/O location.

RFSH — refresh;

— output, active in “0” logic level; — together with MREQ, indicates that the lower seven bits of the

system’s address bus can be used as a refresh address to the system’s dynamic memories.

HALT — halt state;

— output, active in “0” logic level; — indicates that the CPU has executed a HALT instruction and is

waiting for either a non-maskable or a maskable interrupt (with the mask enabled) before operation can resume. During HALT, the CPU executes NOPs to maintain memory refresh.

WAIT — wait;

— input, active in “0” logic level;

— indicates to the CPU that the addressed memory or I/O devices are

not ready for a data transfer;

— the CPU continues to enter a WAIT state as long as this signal is

active without refreshing the dynamic memory. Extended WAIT periods can prevent the CPU from properly refreshing dynamic memory.

INT — interrupt request;

— input, active in “0” logic level; — interrupt request is generated by I/O devices. The CPU honors a

request at the end of the current instruction if the internal software-controlled interrupt enable flip-flop (IFF) is enabled and BUSRQ is not active. INT is normally wired-OR and requires an external pull-up for these applications.

NMI — non-maskable interrupt;

— input, negative edge-triggered; — NMI has a higher priority than INT. NMI is always recognized at the

end of the current instruction, independent of the status of the interrupt enable flip-flop, and automatically forces the CPU to restart at location 0066H.

RESET — CPU reset;

— input, active in “0” logic level; — initializes the CPU as follows: it resets the interrupt enable flip-flop,

clears the PC and registers I and R, and sets the interrupt status to Mode 0. During reset time, the address and data bus go to a high-impedance state, and all control output signals go to the inactive state; dynamic memory refresh signals are not generated. Note that RESET must be active for a minimum of three full clock cycles before the reset operation is complete.

BUSRQ — bus request;

— input, active in “0” logic level; — bus request has a higher priority than NMI and is always recognized

at the end of the current machine cycle. BUSRQ forces the CPU address bus, data bus, and control signals MREQ, IORQ, RD, and WR to go to a high-impedance state so that other devices can control these lines. BUSRQ is normally wired-OR and requires an external pull-up for these applications. Extended BUSREQ periods due to extensive DMA operations can prevent the CPU from properly refreshing dynamic RAMs.

BUSACK — bus acknowledge;

— output, active in “0” logic level; — indicates to the requesting device that the CPU address bus, data

bus, and control signals MREQ, IORQ, RD, and WR have entered their high-impedance states. The external circuitry can now control these lines;

— as long as it is active, dynamic memory refresh signals are not

generated.

1.3 MEMORY

The Z80 microprocessor can directly access any location in a 64 KB

memory block.

In order to have a maximum flexibility for applications, the CoBra microcomputer is bundled with the maximum RAM memory that can be accessed by the microprocessor, i.e. 64 KB. At power-up, the RAM memory has a random contents. Therefore the existence of a non-volatile memory is needed, one that wouldn't lose its contents at power-off (EPROM). The CoBra computer can be equipped with 2 to 16 KB of such a memory, the standard version having 2 KB.

In this memory there are recorded power-up specific programs, such as hardware tests, a test image used for adjusting the black & white or color monitor, computer configuration user selection indicating the source of the operating system to be loaded:

— BASIC interpreter – Sinclair ZX-Spectrum compatible, with a monitor program for work in assembler and with printing routines specific for RS-232 serial interface printers; — Sinclair ZX-Spectrum BASIC interpreter, unmodified, for applications that verify the genuineness of the existing ROM; — operating system specialized in assembler work, with editor, assembler, disassembler, copier (ex. OPUS); — any other user-designed interpreter. (ex. FORTH); — CP/M compatible floppy disk operating system.

Spectrum-type operating system can be loaded from 16 KB EPROM

memories, from magnetic tape or disk.

The CP/M operating system can only be loaded from disk, its operation

involving the existence of the floppy disk.

On one hand, the Sinclair-Spectrum compatibility requires that the RAM

memory between addresses 4000H and 5AFFH contain the information used by the video controller to display a 256x192 pixel image on the monitor, on the other hand running the CP/M operating system becomes impossible if the video memory shows up in the middle of the transient program area (TPA).

In order to solve this problem created by the dual nature of the computer, a memory configuration and selection circuit was created that satisfies the conditions imposed by these three configurations. This circuit is made of two D-type flip-flops (U36), a BCD-to-decimal decoder (U56) and gates, being presented in the block diagram in Fig.2.

The main advantage of this circuit is that it allows switching the memory configuration with a jump anywhere in the 64 KB space of the new configuration, even if the memory area containing the switching routine dissapears as a result of the switching.

Fig. 2 - Memory configuration and selection circuit - block diagram

Its operation makes use of the fact that the R register is 7-bit incremented after each instruction fetch cycle and appears entirely (8-bit) on the address bus lines A0—A7, the moment of its appearance being signaled by the RFSH signal.

A change of bit 7 of the R register by the instruction LD R,A is shown on the address bus (BA7) only after the next instruction code was already fetched.

This instruction can be a single-byte jump instruction, such as RST n or JP (HL), which can achieve the jump of the program counter to any address within the 64 KB memory space. The memory configuration switching sequence:

LD R,A ; JP (HL)

is detailed in Fig.3.

The two flip-flops (U36) are forced to “1” at power-up, by a circuit made of C15, D02, R09, D03.

Fig. 3 - Memory configuration switching sequence

The diode D02 makes sure the microprocessor exits the reset state a few miliseconds before the NPOR signal turns off. During this time the microprocessor executes the first instructions where bit 7 of the R register is set to “1”, which makes sure the flip-flop U36/5 is maintained at “1” after NPOR turns off. This is the temporary startup configuration, where the memory map allows the microprocessor to access the BOOT EPROM U89, the 16k BASIC EPROM, the video memory and the 16k DRAM (BANK #0). In this startup configuration, the RESET button is not to be used, because the contents of the R register would be erased, which leads to a random memory configuration. The user can choose the configuration wanted by pressing one of the following keys:

— B for BASIC interpreter resident in EPROM;

— C for BASIC or other operating system resident on magnetic tape;

— W to check the contact between the EPROM memories and their

sockets. The contents of EPROMs are verified byte by byte against the recording on the magnetic tape. In case of error, the defective EPROM chip is indicated by flashing the corresponding color in the test image (0-7);

— D to load the operating system from floppy disk.

After selecting the source of the operating system to be loaded from the keyboard, the RAM area between 8000H and FFFFH is set up accordingly, bit 6 of the output port FEH is set to 0 or 1 according to the configuration chosen (BASIC or CP/M respectively) and bit 7 of the R register is brought back to “0”. The memory configuration will change after the microprocessor fetched the code for the JP (HL) instruction from the old configuration.

The map of the memory in these 3 configurations is shown in Fig. 4.

Multiplexers U41 and U58 take care of changing the row and column address for the 48k DRAM memory (U62-69, U43-U50, U24-U31). In order to increase speed of operation in CP/M, half of the video DRAM (BANK #1) is replaced by half of DRAM BANK #0, otherwise the execution of BDOS calls would be slowed down by wait states from the memory access prioritizer. This memory replacement is achieved by the gates at U52/12, U52/8, U35/3.

Under the BASIC configuration, U54/11 synthesizes the Read Only signal for DRAM BANK #0, the gate U17/11 allows access for SPECTRUM-specific 20ms interrupts, and the asyncronous input U36/1 locks the state of memory configuration flip-flops regardless of any change in bit 7 of the R register.

1.4 THE VIDEO CONTROLLER

The video image is organized in memory as follows:

— a 6kB area, called “serial video information area”, which specifies the

type of every pixel of the screen, as follows:

if the corresponding bit is 0 then the pixel will have the color of “paper”, and if the bit is 1 then the pixel will have the color of “ink” for that particular character. The address of this 6kB area is 4000H for BASIC configuration and C000H for the other two configurations;

— a 768 bytes area, called “color attribute area”, which specifies the

color of “ink” and the color of “paper”, for each character, specifies if that character should be flashing and if it should have increased brightness. The address of this area is 5800H for BASIC configuration and D800H for the other two configurations.

The block diagram of the video controller is shown in Fig.5.

Fig. 4 - Memory map for the

three configurations

Fig. 5 - Video controller - block diagram

Starting from the 14 MHz clock built with a quartz oscillator with gates (U57), through a division by 2 the dot clock frequency SCLK is obtained - used for serialization - with a frequency of 7 MHz, at U61. This signal is further divided by 8 through the Johnson counter made with U60 (7495), in order to generate the character frequency. Gate U40/6 along with U57/8 ensure the counter is self-triggered. The main advantage of this 4-bit divider by 8 is that the outputs' transitions occur one at a time, so at any given moment only one output changes its state. Through a simple decoding using gates, more signals are generated - control signals for DRAM 16k BANK #1, control signals for the serialization registers, the color attributes and the strobe for the data separation and storage register.

The timing of these signals and also the dual access to the video memory are both shown in Fig.6.

On one hand the video controller is accessing the video memory at fixed time intervals in order to read the serial video information and color attributes, on the other hand the CPU is accessing the video memory in order to change the image, the color attributes, the system variables or to store code or data.

Fig. 6 - The timing of command signals

In order to easier calculate the address of any pixel as well as the address of the associated color attribute, we divide the 256x192 px image (or 32x24 characters) as follows:

— horizontally — 8 adjoined pixels, 8-bit encoded (B7—B0) make up

one character;

— the image contains 32 characters, 5-bit encoded:

C4, C3, C2, C1, C0;

— vertically — 8 sequential TV lines, 3-bit encoded L2, L1, L0,

make up one character;

— 8 character rows, 3-bit encoded R2, R1, R0 make

up one third of the image;

— the image contains 3 thirds, 2-bit encoded T1, T0,

the “11” combination not being used.

The address of the byte containing bit B of character C, line L, row R and third T can be found using formula (1), and the address of the associated color attribute can be found using formula (2) where:

X=0 —> BASIC configuration

X=1 —> startup or CP/M configuration

A15 A14|A13 A12 A11 A10 A9 A8 A7| A6 A5 A4 A3 A2 A1 A0

X 1 | 0 T1 T0 L2 L1 L0 R2| R1 R0 C4 C3 C2 C1 C0 (1) | +--------+ +--------+ +--------+ +----------------+ | T L | R C | | X 1 | 0 1 1 0 T1 T0 R2| R1 R0 C4 C3 C2 C1 C0 (2) +----+ +--------+ +----------------+ T R C

In these formulas we notice the sameness of the A0—A6 address bytes corresponding to the dynamic memory line address, which allows the video controller to access the memory for the two separate readings in page mode, RAS CAS CAS.

The multiplexing of the video addresses is done in two steps: circuits U03, U20, U22, U39 do the multiplexing for RAS-CAS microprocessor and RAS-CAS video controller, and circuit U51 along with gate U19/8 do the address change for the two CAS CAS page-mode accesses of the video controller.

The access of the CPU to DRAM BANK #1 (video) is less prioritized than the access of the video controller, and is controlled by the memory access prioritizer made with U02/5 and U02/9. The access request is signaled by activating the NCS1 line which in turn activates NWAIT through the asyncronous input U02/1. Possible moments of activation for the access request are marked on the CLK diagram of the CPU clock signal in Fig.6 having beside them the number of wait states inserted by the memory access prioritizer in each case. The activation of the video memory access signal VMA leads to activating the RAS, CAS, WE signals; in case of reading, the byte requested is sampled and stored in the separation and storage register i8212 (U76) — the moment marked as (1) in Fig.6. On the positive edge of the QC signal, NWAIT is deactivated, the microprocessor still maintaining the data read from memory on the data bus, by means of the DS selection signals of the i8212 circuit.

The access of the video controller to DRAM BANK #1 video is done once every 1.1µs, this way also ensuring the memory refresh.

At moment (2) in Fig.6, the data byte is loaded in the video & serialization register (U78, U82, U83), the role of U83 being that of delaying the serial data by two SCLK periods before it reaches the select input of the U80 multiplexer.

At moment (3), the color attribute byte is loaded into the color attribute register (U77 and U81).

At moment (4) the video information (serialized) reaches the QB output (U83/12). The time difference between moments (3) and (4) can be compensated by the gate U88/3 along with C36.

The color of ink or paper selected by U80 is once again multiplexed with the border color, by U85.

Gate U87/8 turns off the electron beam in the monitor during the horizontal-retrace. The outputs of U85 are normalized in order to yield the intensity signal, and separated through U86 to yield the R, G, B, I and sync signals for the color monitor. U88/11 along with T1 generates the B/W composite video signal.

The oscillator made of U88/6, U88/8 is controlled by the FD7 signal, together with gate U88/3 achieving the FLASH function. Resistor R43 ensures the blinking oscillator is in sync with the TV vertical sync signal. Its value must be chosen so that the oscillator flips during any TV line except the 192 ones that are visible.

The diode gates that synthesize the BD6N signal, along with R73, achieve the BRIGHT function, at the same time suppressing it for color black.

Circuits U12 and U13 and gates U16, U14 make up a divider by 56. The outputs of this divider are being used as character address for the video controller and are the basis for horizontal sync and retrace signals. The associated signal timings are shown in Fig.7.

Fig. 7 - The timings for TV line signals

Circuits U32, U33, U15/9 and gates U37, U34 make up a divider by 312, their outputs being used as a TV line address, character row address and screen third address on one hand, and on the other hand they are the basis for generating the NBRD and NVS vertical sync signals.

The associated timings are shown in Fig.8.

Fig. 8 - Timings for TV frame signals

2. MICROCOMPUTER INTERFACES

Gates U54/6, U54/8 synthesize reading and writing signals from/to I/O ports. These signals are required for INTEL family interface circuits. The mainboard uses a programmable peripheral interface (i8255) which has three input/output ports and a control port as follows:

port A — input port, address 254 (FEH);

port B — input port, address 31 (1FH);

port C — output port, address 254 (FEH);

control port, address 223 (DFH), value 146 (92H).

∘ Bytes 0—5 of port A are used for reading the columns from the keyboard

matrix.

∘ Bit A6 is used for reading data from the external memory on magnetic

tape. The audio signal from tape is limited by R98, D10, D9 and formatted by U92 (op. amp.).

∘ Bit A7 is used as serial input, protected by R94 and D05. It can be used as

RS-232 serial input when driven by a serial data reception routine (driver).

Input bits of port B can have a general use as an 8-bit parallel port at

address 223 (DFH), with protocol signals PA5 for input and PC5 for output.

∘ Bits 0—4 of port B can be used as Kempston-compatible Joystick interface.

Resistors R99:106 ensure a “0” reading in stand-by, resistor R107 ensure a

“1” level through one of the joystick contacts.

∘ Bits 0—2 of port C are used for storing the BORDER color. ∘ Bit 3 serves as output for tape recording. Resistors R97, R98 and diodes

D06, D07 ensure an optimum level for the majority of tape recorders.

∘ Bit 4 is used as audio output. The i8255 circuit can directly drive a

telephone speaker through C45, speaker mounted in the microcomputer's case.

Bit 5 is a general use output bit. Can be used as a protocol line for port ∘ B. Bit 6 is an output line. It indicates the configuration selected at startup. In∘

a particular configuration it can be used as a general use bit.

Bit 7 is a serial output. It is separated and inverted by ∘ U87/11, the level

being adapted by T2 to be RS-232C compatible. It is used as a serial data transmission bit to a printer or another computer, using an emission routine.

The floppy disk interface is built on a separate board using the dedicated floppy disk controller circuit i8272.

To be able to use the features of Mode 2 Interrupts of the microprocessor, the interrupts generated by i8272 are passed through the counter/timer channels circuit Z80-CTC (U01). This circuit has four counters of which counters 0 to 2 are cascaded. By programming, channel 0 CTC generates one interrupt for each byte to be transferred between i8272 and microprocessor; channels 1 and 2 cascaded generate an interrupt at the end of the sector also generating the Terminate Count (TC) signal for i8272. In this hardware configuration, i8272 can be programmed to work without direct memory access, simplifying the interface very much.

The floppy disk interface block diagram is shown in Fig.9. The clock signal generated by the 16 MHz quartz oscillator is divided by 2 or by

4 depending on the position of the switches SD0-3 and is used as a clock for i8272. Remarkable is this original configuration where i8272 - by means of the selection signals US0, US1 - selects its own 8 or 4 MHz clock frequency for work with 8” or 5¼” floppy drives. This configuration allows the use of 2 floppy drives simultaneously regardless of their sizes, without having to intervene by means of software to switch the clock frequency to the floppy disk controller.

The circuit made with U04/9 and U03/4 performs the multiplexing for READY signals coming from floppy drives.

The double decoder circuit U05 generates SELECT signals for floppy drives, as well as HEAD LOAD signals for the reading/writing heads of the drives.

Signals HL0-3 (HEAD LOAD) can also be used as MOTOR ON signals for 5¼” floppy drives which have this input line.

Fig. 9 - Floppy disk interface block diagram

In order to reduce the symbol crossover interference at write operations, a circuit made with U18, U12 is used, that ensures precompensation for data to be written.

Gates U16, U03, U06, U10 take care of multiplexing and level translation for command signals of the floppy drives.

Gates U16, U15, U14/6, U14/8 and counter U08 generate the write clock signal WCK and clock signal CK depending on the selected simple or double (MFM) density.

Circuits U13, U07, U14/12, U09 make up a phase locked loop circuit (digital PLL) that is used to synthesize the data window RDW signal (Read Data Window) starting from the transitions of data received from the selected floppy drive. If input U07/13 is in “1” logic level, U13 along with U07 make up a regular divider-by-16 circuit. The positive edge of the USD signal from the floppy drive

triggers a sampling of the current count of this divider. If the sample is 0, the frequency generated by the 16-divider is considered to be synchronous with data arriving from the floppy drive and the counter keeps counting. If the sample is not zero, a positive or negative jump by 1 or 2 is made in the counting sequence, according to the shift that was detected, so that the number in the 16-divider is brought closer to the correct synchronousness value. The 71488 PROM is programmed as follows:

Address Contents Shift Address Contents

00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F

01 01 02 03 03 04 05 06 0B 0C 0D 0E 0F 0F 00 01

-1

-2

-2 +2 +2 +2 +2 +2 +1 +1 +1

10 11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E 1F

01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F 00

Circuit U09/6 performs a division by 2 on the output signal coming from the 16-divider, so that the RDW signal (data window) is generated from the clock signal CK through a division by 32 with a compensation of the shift occuring between RDW and the data read from the floppy disk.

3. KEYBOARD

The keyboard consists of 58 keys, of which 48 are organized as an 8x6 matrix, and the other 10 are used for generating some commands that on a standard ZX SPECTRUM are generated by simultaneously pressing CAPS SHIFT and another key.

The detection of a key press is done as follows: when interrogating the keyboard, the keyboard lines are connected to the higher 8 microprocessor address lines (A8—A15) separately through diodes D11—D18. During a keyboard reading cycle, these lines are sequentially brought to a “0” logical level, one by one, one at a time while the other 7 are in a “1” logical level. If a key is pressed, by the electrical contact made in that particular matrix node, the “0” level is propagated through the column (k0—k6) corresponding to the key that was pressed, all the way to the PA input port of the i8255 circuit.

The keyboard schematics are shown in Fig.10 and Fig.11.

Fig. 10 - Keyboard schematic

Fig. 11 - Keyboard schematic

The generation of a command with one of the 10 special keys mentioned above can be studied on the example in Fig.12; command DEL (deletion) which on ZX SPECTRUM is obtained by simultaneously pressing the CAPS SHIFT and 0 keys, on CoBra can be generated by pressing the DEL key.

Fig. 12 - Special keys

functional example

The TAB key in the group of 10 special keys is different from the others by the fact that it validates three different keys simultaneously: CAPS SHIFT,

SYMBOL SHIFT and P. When pressing the TAB key, the contacts representing the CAPS SHIFT and SYMBOL SHIFT keys are activated, the command being

propagated through the differential circuit RD, CD. The contact representing the P key does not yet get activated due to the presence of the integrating circuit R411, CI. Shortly after the deactivation of the contacts representing the CAPS SHIFT and SYMBOL SHIFT keys, the contact representing the P key is activated

too.

4. POWER SOURCE

The power source of the CoBra computer can generate 3 different voltages:

+5V — for a load of max. 3A;

– 5V — for a load of max. 50mA;

+12V — for a load of max. 0.3A

The power source schematic is shown in Fig.13. The +5V voltage stabilizer is built as a switched-mode circuit. The main elements are:

— the group of T5, T4 (representing the switching transistor), IC3 (second generation stabilizer ROB 317);

— the group of L1, C15, which is the energy storage element;

— diode D5, which closes the load circuit during the off-time of the switching transistor;

— the group of R10, C14, R9.

Fig. 13 - Power source schematic

At startup, the current through the load is supplied by the stabilizer circuit IC3. As the input current increases, the voltage on S1 opens up the switching element made of T5, T4 and the current through the coil starts rising in a linear fashion. The load current being constant, the current supplied by the linear stabilizer IC3 decreases, as the current through L1 increases. The process described eventually leads to the blockage of transistor T5 and thus of the switching element T5, T4. At this time the voltage on coil L1 changes polarity, diode D5 opens up and the energy stored in coil L1 supplies the load current during the on-time of T5, T4. After a while, the output voltage tends to decrease below the preset value, this being sensed by the IC3 stabilizer which initiates a new conduction process in the T5, T4 transistors. The positive feedback loop R10, C14, R9 introduces a hysteresis in the voltage applied on the linear stabilizer input, helping its turning on or off.

The +5V voltage is adjusted with the trimmer resistor S2.

The optimal efficiency is adjusted with the trimmer resistor S1.

The –5V voltage stabilizer is a DC–DC type. It is made up of an oscillator built with IC1 (ßE 555), R6, R7 and C8, a voltage doubler rectifier circuit built with C10, D3, D4 and C11, followed by a shunt regulator built with R8, Z5 and the filter capacitor C12.

The oscillator is fed on pin 11 with a voltage of about 14V obtained from the regulator built with R2, Z3, Z4 and the filter capacitor C7. On output pin 6 of IC1 a rectangular signal is generated with a frequency f=20kHz and a duty cycle close to 1/2.

The +12V voltage stabilizer is an emitter follower transistor regulator. It is built with R1, B, T1, Z1 and the filter capacitor C6.

Restrictions and protections:

To avoid damaging the RAM memory it is necessary that the first voltage applied to the memory be the –5V voltage. The power source of the CoBra microcomputer achieves this condition with the help of the R5, Z2 group and the T2, T3 transistors. At source power-up, the output of the –5V stabilizer is at 0V. Diode Z2 is blocked and T2, T3 are saturated due to the current being injected in their bases through R5. The negative potential from the cathode of diode D2 appears on the collectors of transistors T2, T3 and thus also on the base of T1 and pin 2 (reference) of IC3. The two voltage stabilizers (+12V and +5V) will be as such kept at a positive value close to 0V. When the –5V power source output gets close to the preset value, diode Z2 opens up and pulling the voltage potential on the bases of transistors T2, T3 down blocks them, allowing the +12V and +5V sources to be turned on. The +5V source is designed with a protection against exceeding a 3A output current. This protection works as follows:

Increasing the output current over 3A causes a voltage on R12 equal to the voltage required to open the base-emitter junction of transistor T6. The positive potential which appears on the collector of T6, divided by R14, R15 and applied to pins 5, 9 of IC2, is bigger than the upper trigger limit of IC2, which works as a flip-flop circuit.

IC2, which has the output pin 6 in a high state (0V) at startup (poweron), flips to a low state (–5V). The Zener diode Z6 opens up holding the potential of pin 2 of IC3 relative to ground at approx. –1.2V. The output voltage will be equal to the sum of the voltage on S2, R9 and the voltage on R11 (1.2V — datasheet value), that is approx. 0V. Rearming the +5V voltage can be done by turning the main switch off and then on. Due to the tight admissible tolerances for the +5V voltage (±0.25V), in order to avoid accidental increases of the +5V voltage, a crowbar protection was also designed, built with TH1, Z7, R17, which upon exceeding a voltage of approx. 6V triggers the overcurrent protection by opening TH1.

The protection on the +12V source works as follows: when a shortcircuit on +12V occurs, on the base of transistor T1 a positive potential less than 1V relative to ground appears, which divided by R3, R4 is applied to pin 7 (ALO) of IC1, blocking the oscillator. Thus the –5V voltage dissapears, Z2 is blocked, transistors T2, T3 are opened and through T2 saturated the oscillator built with IC1 is held in a blocked state. The light bulb B was included in the +12V stabilizer schematic due to the nonlinear character of its resistance, this way achieving a limitation of the shortcircuit current by limiting the base current for transistor T1. Through T3 saturated, the +5V voltage will be close to 0V too, so in conclusion a shortcircuit on the +12V output will cut all three voltages off. rearming is done by turning the main switch off and then on.

The protection on the –5V source works as follows: a shortcircuit on the –5V output causes Z2 to block, T2, T3 get saturated and through T2, R3, R4 the

potential on pin 7 of IC1 is close to 0V, blocking the oscillator built with IC1. It is obvious that this state is maintained until the source is rearmed. All the three stabilizers are blocked until turning the main switch off and then on.

Powering up:

After verifying the connections, one pin of diode Z7 is disconnected to avoid triggering the crowbar protection, S1 is positioned in the middle and S2 set to its minimal resistance, and then the circuit is plugged in. The presence of the +5V voltage is verified. If it is not present, the oscillator built with IC1 should be checked. After this, the presence of the +12V voltage is verified after which the +5V voltage is adjusted without load. The oscilloscope is connected between the cathode of D5 and ground. The system should be oscillating.

After this, a load of approx. 1.3 Ohms and at least 12W is connected to +5V, a load of 39 Ohms/5W connected to +12V and a load of 100 Ohms/0.5W connected to –5V.

All voltages are again checked, adjusting the +5V voltage with S2. The voltage on D5 is visualized on the oscilloscope checking that the frequence is around 33 kHz (+2 kHz, – 4 kHz) and the amplitude around 18V.

A fine adjusting of the frequence can also be done by modifying the resistance of R10.

In order to get to the maximum efficiency, the fuse is pulled out of the socket on the PCB, inserting instead an AC ammeter (MAVO-35 set to the 5A~ position). The main switch is turned on and S1 is carefully adjusted until a low

is found. Then the +5V voltage is checked again and adjusted.

The shortcircuit protections on +12V and –5V are tested, respectively the triggering of the +5V protection against a load current in excess of 3A. Z7 is then connected and the +5V voltage is increased by adjusting S2 until TH1 is activated. After this last check the +5V voltage is readjusted. After this, the power source is ready to be used.

WARNING!

Do not insert and do not pull out the power connector (j9) while the power source is on; the memory chips with three voltages (4116) might get destroyed.

5. COMPONENT LIST - FLOPPY DISK INTERFACE

CODE TYPE

------------------------------------------------------

u01 Z80A-CTC u02 i8272A u03 74(LS)14/7404 u04 74(LS)153 u05 74156/74155 u06 7438/7403 u07 74S174 u09 74(LS)74 u10 74(LS)08 sw dip-switch u12 74(LS)153 u13 74(S)188 u14 74(LS)10 u15 74(LS)51 u16 74(LS)04 u17 74(LS)74 u18 7495

xtal 16 MHz

R01 150/0.5W R02 150/0.5W R03 150/0.5W R04 150/0.5W R05 1K/0.5W R06 150/0.5W R07 150/0.5W R08 150/0.5W R09 150/0.5W

R10 470/0.5W R11 470/0.5W R12 470/0.5W R13 470/0.5W R14 680/0.5W R15 680/0.5W R16 4K7/0.5W

c01 1µ/35V c02 100n/50V c03 1µ/35V c04 100n/50V c05 220p/30V c06 220p/30V

jex 201.607 RC jfdd 201.243 RC

pcb CX-FDC PCB rel 2.0

921442432

6. COMPONENT LIST - COBRA MICROCOMPUTER

CODE TYPE PCS.

------------------------------------------------------

u01, 2716 (2732) 8(4) u18, u23, u42, u59, u75, u84, u91

u02, 74(LS)74 4 u15, u36, u61

u03, 74(LS)153 4 u20, u22, u39

u[04:11], 4116 (4516) 32 u[24:31], u[43:50], u[62:69]

u12, 74(LS)193 4 u13, u32, u33

u14, 74(LS)10 3 u40, u52

u16, 74(LS)00 6 u17, u37, u54, u87, u55

u19, 74(LS)08 2 u38

u21 74(LS)51 1

u34 74(LS)20 1

u35, 74(LS)86 2 u88

u41, 74157 2 u58

u51, 74(LS)157 3 u80, u85

u53, 74(LS)04 2 u57

u56, 74(LS)42 2 u70

u60, 74(LS)95 6 u[77:78], u[81:83]

u[71:74], 74(LS)07 5 u86

u76 i8212 1

u79 i8255 1

u89 2716 1

(2732) (2764) (27128)

u90 Z80A-CPU 1

u92 741 1

xtal 14 MHz 1

t1 2N2222 1

t2 BC251 1

p[01:04], 1N4148 16 p[06:07], p[09:18]

p05, DZ4V7 2 p08

1s phone speaker 1

j1 300.066 1

j2 300.064 1

300.060

j3, 303.608A 3 j4, j9

j5 201.577 1

j6 300.062 1

j7 201.561 1

j8 201.161 RC+ 1

201.146 RC 1

jex 201.619 RC 1

k1 EA-5993 1

sb SWITCH 1

ss DIP-SWITCH 1

R[01:05], 33 22 R[07:08], R[18:19], R21, R[24:25], R[28:30], R[32:33], R[36:37], R[39:41]

R06, 1K 12 R20, R22, R23, R26, R27, R31, R34, R42, R73, R94, R96

R[99:106] 8K2 8

R09, 2K2 4 R84, R87, R107

R[10:17], 4K7 16 R69, R85, R[108:113]

R35, 680 2 R38

R43 1–4K7* 1

R44 330–390 1

R[45:47], 620 21 R[49:66]

R48, 150/0.5W 2 R92

R70 330/0.5W 1

R[67:68], 10K 7 R[89:90], R95, R[97:98]

R[71:72], 100/0.5W 5 R[75:77], R91

R72, 470 7 R[78:83]

R73 1K 1 R74,RN 240 1 R86 220/0.5W 1 R93 75/0.5W 1 Rw 4K7 1

c[01:05], 100n/30V 34 c[07:13], c[16:23], c[25:29], c[31:32], c[37:39], c41, c[46:48]

c14, 150pF 4 ccS, cAT, cHB

c15, 10µ/10V 2 c45

c24 220pF 1 c30 15pF 1

c[33:35], 1–10µ/35V 4 c40

c36 270pF 1

c42 100–220µ/6V 1

c43 100–330µ/6V 1 c44 150–330µ/10V 1

cXR, 390pF* 2 ccc

pcb 921442431 1

7. COMPONENT LIST - KEYBOARD

CODE TYPE PCS.

------------------------------------------------------

t201, BC171 11 t210, t411, t[502:503], t[506:511]

d[100:101], 1N4148 9 d[103:104], d209, d[301:302], d501, d512

cd, 10µ/35V 2 ci

r201, 4K7/0.125W 3 r411, ri

rd 10K/0.125W 1

rpu 1K5/0.125W 1

jkb 201.606 1

369.108.151 116

369.108.152 58

369.108.154 58

369.108.155 58

369.108.169 2

369.108.170 1

k[100:101], 369.108.153y 11 k[103:104], k200, k209, k211, k[300:302], k313

k102 369.108.159x 1

k201, 369.108.158x 2 k210

k[202:208], 369.108.153x 16 k[303:311]

k312 369.108.163x 1

k400, 369.108.156y 2 k412

k401 369.108.162y 1

k[402:411] 369.108.156x 10

k500, 369.108.157y 2 k513

k501, 369.108.164y 2 k512

k[502:511] 369.108.157x 10

8. COMPONENT LIST - POWER SOURCE

CODE TYPE

------------------------------------------------------

CI1 ßE 555 CI2 ßE 555 CI3 ROB 317 (TO-39) T1 BD 237 T2,T3 BC 171 T4 BUR 608 T5 BD 138 T6 BD 251 PR1 3PM05 D1 1N4001 D2 6DRR1P D3,D4 1N4148 D5 6DRR1P D6 LED ROL 02 Z1 PL 13Z Z2 PL 4V7Z Z3,Z4 PL 6V8Z Z5 PL 5V1Z Z6 DZ 3V3 Z7 PL 5V6Z C1,C2,C3,C4 3n3 (CLX 12.15) C5 4700µF/40Vcc (EG 76.91) C6 1000µF/16Vcc (EG 61.44) C7 10µF/25Vcc (EG 52.53) C8 1nF (CLY 12.06) C9 10nF (CLX 12.15) C10,C11,C12 10µF/25Vcc (EG 52.44)

C13 100nF (CLY 32.15) C14 220pF (CLZ 12.06) C15 100µF/35Vcc (CTS-M) C16 100µF/16Vcc (EG 61.21) C17 10µF/16Vcc (EG 52.43) R1 1.2/3W (RBC-1002) R2 2x47/3W (RBC-1002) R3 8K2 (RCG 10.50) R4 1K (RCG 10.50) R5,R6 4K7 (RCG 10.50) R7 27K (RCG 10.50) R8 18K (RCG 10.50) R9 100 (RCG 10.50) R10 27K (RCG 10.50) R11 120 (RCG 10.50) R12 0.15 (RBC-1002) R13 1K (RCG 10.50) R14,R15 2K2 (RCG 10.50) R16 220 (RCG 10.50) R17 270 (RCG 10.50) S1 250 (P32824) S2 2K5 (P32824)

Other materials:

— TR 220V/19V S=8 cm

N2=96 turns 0.95mm enamelled copper (≈19AWG)

N1=1067 turns 0.2mm enamelled copper (≈32AWG) — DIN 5 pin connector (303608) — B — auto light bulb 24V/2W — Fuse socket for PCB mount — L1=600µH (1.2mm enamelled copper (≈17AWG) on 34x28mm ferrite pot core)

Ex: AL=570nH/turn : n=33 turns.

— L2=25µH (1.2mm enamelled copper (≈17AWG) on 23x17mm ferrite pot core)

Ex: AL=280nH/turn : n=10 turns.

CONNECTORS – pin-signal listing

j1 — video connector

1 — VVM — modulator power 2 — GND — ground 3 — VR — color red 4 — VG — color green 5 — VB — color blue 6 — VI — brightness 7 — VC — monochrome composite video signal 8 — VS — positive synchronization signal 9 — VNS — negative synchronization signal

j2 — RS 232 connector

2 — TxD 3 — RxD 7 — GND

j3 — auxiliary connector

1,4 — AA — speaker out 2 — GND — ground 3,5 — ATR — formatted audio signal

j4 — tape recorder connector

1,4 — ATO — output to tape recorder 2 — GND — ground 3,5 — ATI — input from tape recorder

j5 — 8 bit input port connector address 0DFH

1a — 8a — GND — ground 9a — 10a — Vcc — +5V 1b — PB0 2b — PB1 3b — PB2 4b — PB3 5b — PB4 6b — PB5 7b — PB6 8b — PB7 9b — K5 — bit 5 port 0FEH — input 10b — 05 — bit 5 port 0FEH — output

j6 — Joystick connector

1 — PB0 — right 2 — PB1 — left 3 — PB2 — down 4 — PB3 — up 5 — PB4 — button

6 — NC

7 — NC 8 — JSC — common

9 — NC

10 — NC

— 1) The original PCB layout had PB4 connected to j6/pin5 but the original manual had PB4 listed at j6/pin6. I kept the original PCB layout and I changed this manual to match the PCB. — 2) Same as above. — 3) The original PCB layout had JSC connected to j6/pin10 but the original manual had JSC listed at j6/pin8. I changed the PCB layout to match the original manual (maintained above). — 4) The original manual only had 9 pins listed for the Joystick connector. The original PCB layout though had 10 holes drilled for this connector and pin 10 was even connected to JSC as mentioned above at 3). I changed the original manual adding pin 10 to the list, even though it is now not connected.

j7 — keyboard connector

1a — NC 2a — Vcc — +5V 3a — DIF — phone speaker out 4a — KA8 — A8 — keyboard 5a — KA10 — A10 6a — KA12 — A12 7a — KA14 — A14 8a — K4 — bit 4 port 0FEH — input 9a — K2 — bit 2 10a — K0 — bit 0 1b — NC 2b — VCC — +5V 3b — GND — ground 4b — KA9 — A9 keyboard 5b — KA11 — A11 keyboard 6b — KA13 — A13 keyboard 7b — KA15 — A15 keyboard 8b — K3 — bit 3 9b — K1 — bit 1 10b — K5 — bit 5

j8 — expansion connector

a b c 1 VDD VDD VDD 2 GND GND GND 3 VBB VBB VBB 4 VCC VCC VCC 5 BA15 — BA14 6 BA7 — BA8 7 BA6 — BA9 8 BA5 — BA11 9 BA4 — — 10 BA3 — BA10 11 BA2 — NPO 12 BA1 — D7 13 BA0 — D6 14 D0 — D5 15 D1 — D4 16 D2 — D3 17 BA12 — BA13 18 — — — 19 — — — 20 06 — 05 21 IEI — — 22 IEO — — 23 NIOWR — NIORD 24 BNRD — BNIORQ 25 BNWR — BNMREQ 26 BNRFSH — BNM1 27 NHALT — NWAIT 28 NBUSACK — NNMI 29 NRST — NBUSRQ 30 POR — NINT 31 GND — GND 32 BCLK BCLK BCLK

j9 — power connector

1 VDD +12V 2 GND ground 3 VCC +5V 4 VBB -5V 5 GND ground

jEX — floppy disk interface connector

a b 1 VCC VCC 2 D3 D4 3 D2 D5 4 D1 D6 5 D0 D7 6 — — 7 POR BNRD 8 BA1 NIORD 9 SI/TRG3 NIOWR 10 BA4 BNIORQ 11 BA3 IEO 12 NRST NINT 13 BA2 IEI 14 BCLK BNM1 15 GND GND

jFDD — floppy disk drive connector

1a NSS 2:20a GND

1b NUSD 2b NTR00 3b NWP 4b NIX 5b NRDY0 6b NRDY1 7b NRDY2 8b NRDY3 9b NS0 10b NS1 11b NS2 12b NS3 13b NHL3 14b NHL2 15b NHL1 16b NHL0 17b NWD 18b NST 19b NDIR 20b NWG