Introduction Pictures Complete Board Complete System with a Floppy Drive Specifications Hardware Documentation Schematics PCB Version 1.0 PCB Version 1.1 PCB Version 1.2 Input/Output ports Interrupts Jumpers and Connectors Jumper JP1 - CONFIG Jumper JP2 - PIN25_GND/VCC Jumper JP3 - FDC_INT/NMI Connector P1 - POWER Connector P2 - RESET Connector P3 - SERIAL Connector P4 - PARALLEL Connector P5 - FLOPPY Connector P6 - 5V Bill of Materials (BOM) BOM Notes BOM Replacement Notes Power Supply Mods PCB Versions PCB Version 1.0 PCB Version 1.1 PCB Version 1.2 (Current Version) Ideas for the Next Version  Various Ideas Builders List and Board Characteristics

 

 

Introduction

Zeta SBC is an Zilog Z80 based single board computer. It is inspired by Ampro Little Board Z80 and N8VEM project. Zeta SBC is software compatible with N8VEM SBC and Disk I/O boards.

 

Pictures

 

Complete Board

Complete System with a Floppy Drive

 

Specifications

 

Zeta SBC features following components:

• Z80 CPU
• 16550 UART - for connecting a console
• 8255 PPI - can be used for attaching a hard drive using PPIDE or controlling some external devices
• SMC FDC9266 floppy disk controller - NEC 765A / Intel 8272 compatible, with integrated data separator
• 512 KiB of battery backed SRAM
• 512 KiB of flash memory
• RTC

 

Zeta SBC is compact and easy to build:

• Footprint of an 3.5" floppy drive (100 mm x 170.18 mm) and PCB can be mounted under a 3.5" drive.
• Uses only through hole components.
• Assumes using commonly available 3.5" floppy drives (not many people have 5.25" drives and even less 8" ones). Although it should work with 5.25" drives too.
• Only 3 configuration jumpers.
• Easy to use flash memory instead of UV EPROM.
• PCB mounted connectors, no need to build cables. 
• Uses widely available components
• An easy way to get a "taste" of CP/M era computing.

 

Hardware Documentation

 

Schematics

Zeta SBC - Schematics - Color - 1.0.pdf

Zeta SBC - Schematics - BW - 1.0.pdf

 

PCB Version 1.0

Zeta SBC - Board - Color - 1.0.pdf

 

PCB Version 1.1

Zeta SBC - Board - Color - 1.1.pdf

 

PCB Version 1.2

Zeta SBC - Board - Color - 1.2.pdf

 

Input/Output ports

• Compatible with N8VEM SBC and N8VEM Disk I/O boards (FDC part only, no XT-IDE)
• 30h (aliases 32h, 34h, 36h) - FDC Main Status Register (N8VEM Disk I/O uses 36h)
• 31h (aliases 33h, 35h, 37h) - FDC Data Register (N8VEM Disk I/O uses 37h)
• 38h (aliases 39h - 3Fh)
• Write - FDC Diginal Output Register (DOR), also known as latch.
• Bit 0 - TC
• Bit 1 - MOTOR (0 = Motor off, 1 = Motor on)
• Bit 2 - MINI
• Bit 3 - P2
• Bit 4 - P1
• Bit 5 - P0
• Bit 6 - DENSEL (0 = High density, 1 = Low density)
• Bit 7 - ~FDC_RST (0 = FDC RESET active, 1 = normal operation)
• Read - FDC Digital Input Register (DIR)
• Bit 0 - ~DC
• Bits 1-7 - unused  
• N8VEM FDC compatibility notes:
• Bit 1 is inverted, so upon system reset floppy drive motor(s) will be turned off
• Bit 6 is inverted, but it is unlikely that it will cause any compatibility problems, as modern 3.5" drives don't use this signal
• Bit 7 of the latch is used differently (and incorrectly) on N8VEM Disk I/O as an input to ~DC (disk change) line. In Zeta SBC this output is connected to FDC RESET input, and allows software controlled FDC reset.
• ~DC (disk change) output from floppy drives can be read from DIR register, bit 0. It could be used by the system to detect floppy disk change. In such case CP/M should be warm rebooted.
• 60h-63h (alias 64h-67h) - PPI Registers
• 68h-6Fh - UART Registers
• 70h (aliases 71h-77h) - RTC Registers
• Write:
• Bits 0-3 - unused
• Bit 4 - RTC Chip Enable
• Bit 5 - RTC Write Enable
• Bit 6 - RTC Clock
• Bit 7 - RTC Input
• Read:
• Bit 0 - RTC Output
• Bits 1-5 - unused
• Bit 6 - Configuration Jumper (JP1)
• Bit 7 - unused
• 78h (aliases 79h-7Bh) - RAM Page Select Register (CFG1)
• Bits 0-3 - RAM page selection (A15-A18)
• Bits 4-7 - unused
• 7Ch (aliases 7Dh-7Fh) - ROM Page register (CFG2)
• Bits 0-3 - ROM page selection (A15-A18)
• Bits 4-6 - unused
• Bit 7 - ROM disable (~ROM_ENA signal)
• N8VEM compatibility notes:
• Zeta SBC qualifies ~WR signal when accessing configuration registers, so accidental read access to them won't corrupt their content. This makes programs like SURVEY.COM work without crashing the system.
• Zeta SBC doesn't check A1 when accessing configuration registers range (78h-7Fh), so write access to any of 78h-7Dh addresses will change CFG1 (78h-79h in N8VEM SBC) and write access to 7Ch-7Fh will change CFG2 (7Ch-7Dh in N8VEM SBC). This should not cause any compatibility problem, unless in the future N8VEM SBC or some N8VEM I/O devices will use 7Ah-7Bh or 7Eh-7Fh, and some software will try to access these ports.

 

Interrupts

 

• UART interrupt output is connected to the ~INT of the Z80
• FDC interrupt output has three connection options, selected by JP3 jumper:
1. JP3 position 1-2 - FDC interrupt output is connected to the ~INT input of the Z80. This setting is compatible with N8VEM interrupt and fast interrupt modes.
2. JP3 position 2-3 - FDC interrupt output is connected to the ~NMI input of the Z80
3. JP3 not installed - FDC interrupt line is not connected to the CPU. This setting can be used with polling I/O mode.

 

Jumpers and Connectors

 

Jumper JP1 - CONFIG

JP1 is a software configuration jumper. It can be read by software using RTC port (70h) bit 6. This jumper is not currently used by software.

 

Jumper JP2 - PIN25_GND/VCC

JP2 connects pin 25 of the parallel port P4 to either GND or Vcc (+5). Note that pin 1 is the pin that is closer to the parallel connector.

Warning: If JP2 is set incorrectly, it is possible to create a short circuit of the 5 Volt power supply.

Jumper Position	Description
1-2	P4 pin 25 is connected to the ground
2-3	P4 pin 25 is connected to Vcc. Use this position with PPIDE, make sure to set jumper K1 on PPIDE to position 2-3. Please refer to PPIDE documentation for more information.
no jumper*	P4 pin 25 is left open

* default

 

Jumper JP3 - FDC_INT/NMI

Jumper Position	Description
1-2*	FDC interrupt is connected to the ~INT input of  Z80. This is the default setting. Both interrupt and polling mode will work with it.
2-3	FDC interrupt is connected to the ~NMI input of Z80.
no jumper	FDC interrupt is not connected. FDC polling mode will work with this setting

* default

 

Connector P1 - POWER

Connect regulated +5V power supply to this connector.

Pin	Description
tip (the inner contact)	Positive terminal - +5V
barrel / sleeve	Negative terminal - ground

 

Connector P2 - RESET

P2 is the connector for an external reset button.

 

Connector P3 - SERIAL

P3 is the serial port connector. It is normally used for connecting a console or terminal. P3 uses pinout similar to that of IBM AT serial port (with some signals missing). Use a null modem cable to connect to a PC.

Pin Number	Description
1	Not used - Not connected on the SBC
2	RX
3	TX
4	DTR
5	GND
6	DSR
7 - 9	Not used - Not connected on the SBC
shield	GND

 

Connector P4 - PARALLEL

P4 is the parallel port connector. It is connected directly to the 8255 PPI and can be used for attaching an IDE hard disk using the PPIDE mini board, for controlling external devices (e.g. printer, watering system), or for extending the SBC (e.g. connecting an LCD display and a keyboard). The pinout of P4 is the same as in other N8VEM boards (SBC V1, SBC V2, SBC-188).

Pin Number	Description	Pin Number	Description	Pin Number	Description	Pin Number	Description
1	PC0	9	PC4	17	PB7	25	GND or Vcc (see JP2)
2	PA0	10	PA4	18	PB0	26	GND
3	PC1	11	PC5	19	PB6
4	PA1	12	PA5	20	PB1
5	PC2	13	PC6	21	PB5
6	PA2	14	PA6	22	PB2
7	PC3	15	PC7	23	PB4
8	PA3	16	PA7	24	PB3

 

Connector P5 - FLOPPY

P5 is the floppy interface connector. It uses PC compatible pinout. When using a PC floppy cable with a twist, Drive A (ID 0) is the drive after the twist and Drive B (ID 1) is the drive before the twist. When using only one FDD, either a twisted cable can be used, or the BIOS can be patched to use Drive B.

Pin Number	Description	Direction (relative to the SBC)	Connected to
1-33	Odd pins are GND		GND
2	High Density. This signal is ignored by most (all?) 3.5" drives.	Output	DOR (bit 6)
4, 6	Not used		No connection
8	Index Pulse	Input	Schmidt Trigger, FDC (IDX)
10	Motor On B	Output	DOR (bit 1), Buffer
12	Drive Select A	Output	FDC (US0, US1), Decoder
14	Drive Select B	Output	FDC (US0, US1), Decoder
16	Motor On A	Output	DOR (bit 1), Buffer
18	Direction	Output	FDC (LCT/DIR), Multiplexer
20	Step	Output	FDC (FR/STEP), Multiplexer
22	Write Data	Output	FDC (WDOUT), OC Buffer
24	Write Enable	Output	FDC (WE), OC Buffer
26	Track Zero	Input	Multiplexer, FDC (FLT/TR0)
28	Write Protect	Input	Multiplexer, FDC (WP/TS)
30	Read Data	Input	Schmidt Trigger, FDC (~DSKD)
32	Select Head	Output	FDC (HD), OC Buffer
34	Disk Changed	Input	DIR (bit 0)

 

Connector P6 - 5V

P6 is the 5V power output for the floppy drive. Alternatively it can be used instead of P1 for supplying power to the board. It is recommended to use a polarized header for P6 to avoid incorrect power polarity which probably will destroy FDD or components on the SBC board.

Pin Number	Description
1	5V
2	GND

 

Bill of Materials (BOM)

 

BOM Notes

Disclaimer: I did my best to make sure that components listed in this BOM will be compatible with Zeta SBC. Obviously I didn't order all of components listed here, and I was not able to actually verify that they will work. Please make sure to double check specifications on manufacturer's and seller's web site before ordering. Please let me know (or update the BOM) if you found any problems or incompatibilities.

 

Many components have multiple part numbers listed in BOM. There are some differences between various part numbers, such as:

• Different manufacturer (e.g. Texas Instruments vs. National Semiconductor for IC). Usually these ICs will be 100% compatible and won't have any differences in specifications. For some other components (e.g. connectors) different manufacturer also means different quality. It for example is likely to get a better quality connector from TE (was AMP/Tyco) than from some obscure manufacturer (e.g. components sold under Jameco ValuePro brand).
• Specification differences
• Different IC families (LS, ALS), different technologies (TTL and CMOS). They have different specs, and you might prefer to use certain IC family.
• Different IC speed/frequency, especially for CPU, PPI, and memory.
• Mechanical differences, especially for switches. Pick whatever suits best in your enclosure.
• Minor differences. For example: RoHS or Pb-free vs. regular components; thickness of gold plating on connectors; frequency stability of oscillators and crystals.
• Price differences

If unsure what to order, read specifications on seller's and manufacturer's web sites. There are some datasheets available in Zeta's Documentation folder. Also please read Replacement Notes below. Finally, consult people on N8VEM news group.

 

BOM

Component type	Reference	Description	Quantity	Possible sources and notes
PCB		Zeta SBC PCB Version 1.0	1	Order from Sergey
Battery Holder	BT1	CR2032 batter holder, 20 mm lead spacing	1	Jameco 355434, Mouser 122-2620-GR, 122-2520-GR, 122-2420-GR, Radio Shack 270-009
Capacitor	C1 - C28	0.1 uF ceramic, 5.08 mm lead spacing	28	Mouser 80-C323C104K5R Jameco 25523
Capacitor	C29	47 uF, 6.3 V electrolytic	1	Jameco 31114, Mouser  667-ECA-1HHG470
Capacitor	C30	10 uF, 6.3 V electrolytic	1	Jameco 94221, Mouser  667-ECA-1HHG100B
Diode	D1	1N4148	1	Jameco 36038, 179215; Mouser 512-1N4148, 771-1N4148-T/R
Diode	D2, D3	Bi-level LED indicator	1	Jameco 2006676; Mouser 696-SSF-LXH240GYD
Standoff	HOLE1, HOLE2, HOLE3, HOLE4	Standoff, M3 male / female, 20 mm	4	Mouser 534-24317, 534-25505, 855-R30-3012002 Note: The length of male end of these standoffs is 8 mm. It is too long for some floppy disk drives (you'll notice that standoff doesn't go completely inside the floppy drive mount hole). In this case cut a few millimeters using a file or a fine saw.
Screw	HOLE1, HOLE2, HOLE3, HOLE4	Screw, M3, 6 mm	4	Use regular floppy or CD-ROM drive mounting screws. Mouser 534-29311
Connector	JP1, P2	2x1 pin header	2	Jameco 108338; Mouser 649-78229-102HLF
Connector	JP2, JP3	3x1 pin header	2	Jameco 109576; Mouser 649-69190-103HLF, 649-78229-103HLF
Connector	P1	DC jack	1	Jameco 101178; Mouser 806-KLDX-0202-A
Connector	P3	DE9M, right angle PCB mount	1	Jameco 104943, 614441, 614459, 614432; Mouser 806-K22X-E9P-N, 806-K22X-E9P-N-99 (teal color), 571-1734351-1, 571-7478404
Connector	P4	13x2 pin header	1	Jameco 53495; Mouser 649-68602-126HLF, 649-77313-824-26LF Note: don't use shrouded connector, as it will interfere with the standoff and other components.
Connector	P5	17x2 pin header shrouded	1	Jameco 68583, 753547; Mouser 737-BHR-34-VUA
Connector	P6	2 pin header with friction lock	1	Jameco 232266, 613931; Mouser 571-6404562, 571-3-641126-2, 571-3-641215-2 Corresponding female connector: Jameco 234798; Mouser 571-770602-2; Contacts (2 contacts needed): Jameco 234923, 736501; Mouser 571-770666-1 It is recommended to use a polarized header to avoid incorrect power polarity.
Resistor	R1	10 Ohm, 1/4 W	1	Jameco 690380 Mouser 291-10-RC
Resistor	R2	10 kOhm, 1/4 W	1	Jameco 691104 Mouser 291-10K-RC
Resistor	R3, R4	470 Ohm, 1/4 W	2	Jameco 690785 Mouser 291-470-RC
Resistor Array	RR1	4.7 kOhm, 6 pin, bussed resistor array	1	Mouser 264-4.7K-RC, 652-4306R-1LF-4.7K
Resistor Array	RR2	1 kOhm, 6 pin, bussed resistor array	1	Mouser 264-1.0K-RC, 652-4306R-1LF-1K
Switch	SW1	Tactile switch, right angle	1	Mouser 611-PTS645VL39LFS, 611-PTS645VL58LFS, 611-PTS645VL83LFS, 611-PTS645VL15LFS (Note: these switches have different actuator length, the number at the end of the part number denotes the length measured from ground terminal - from 3.9 mm to 15 mm. You might want to select it according your own preferences, for example the enclosure type you want to use, and whatever you want the reset button to stick out of the enclosure, or to stay hidden inside) Jameco 1953575, 202956
IC	U1	Z80 CPU, CMOS, 40 pin DIP - Z84C00xxPEC	1	Mouser 692-Z84C0010PEG, 692-Z84C0008PEG Jameco 35781, 35705 (It appears that Jameco doesn't have 8MHz+ Z80 any more) Frequency of U21 (CPU clock) oscillator should be less or equal to CPU frequency. 8MHz CPU recommended for 1.44 MB floppy disks support.
IC	U2	512 KiB flash, 32 pin DIP - 29F040, 29C040, 39SF040	1	Mouser 804-39SF0407CPHE; Jameco 242667, 242659
IC	U3	512 KiB SRAM, 32 pin DIP - AS6C4008-55PCN	1	Mouser 913-AS6C4008-55PCN; Jameco 1927617 Note: Jameco 242448 and 157358 (BS62LV4006P) should work too, but they are way too expensive
IC	U4	16550 UART	1	Mouser 701-ST16C550CP40-F; Jameco 27596, 288809
IC	U5	8255 PPI	1	Jameco 52417 Unicorn Electronics (order 82C55); Utsource; eBay
IC	U6	FDC9266 FDC	1	Order from Sergey; Utsource; eBay (beware, some sellers have way too high price on this IC). Can be replaced with FDC9268, in this case U23 frequency should be 16 MHz.
IC	U7	74LS273	1	Mouser 595-SN74ALS273N, 512-DM74ALS273N, 595-SN74LS273N, 595-SN74AHCT273N; Unicorn Electronics; Jameco 47386, 308398, 45049
IC	U8	74LS240	1	Mouser 595-SN74ALS240AN, 595-SN74LS240N, 595-SN74AHCT240N; Unicorn Electronics; Jameco 47141, 308291, 45014
IC	U9, U10, U11	74LS174	3	Mouser 595-SN74ALS174N, 595-SN74LS174N, 595-SN74AHCT174N; Unicorn Electronics; Jameco 46931, 301760
IC	U12	74F139	1	Mouser 595-SN74ALS139N, 595-SN74LS139AN, 595-CD74ACT139E, 512-74ACT139PC, 595-SN74AHCT139N; Unicorn Electronics; Jameco 46623, 301268, 63773, 239011
IC	U13	74LS138	1	Mouser 595-SN74ALS138AN, 512-DM74ALS138N, 595-SN74LS138N, 595-SN74AHCT138N; Unicorn Electronics; Jameco 46607, 46608, 301233, 44927
IC	U14	74LS125	1	Mouser 595-SN74LS125AN, 595-SN74AHCT125N; Unicorn Electronics; Jameco 46501
IC	U15, U16, U17	74LS32	3	Mouser 595-SN74ALS32N, 512-DM74ALS32N, 595-SN74LS32N, 595-SN74AHCT32N; Unicorn Electronics; Jameco 44134, 47466, 47467, 295515
IC	U18,U19	74LS14	2	Mouser 595-SN74LS14N, 595-SN74AHCT14N; Unicorn Electronics; Jameco 46640, 295460, 44935
IC	U20	74LS06	1	Mouser 595-SN74LS06N; Unicorn Electronics; Jameco 46359
IC	U21	CPU clock oscillator, full can	1	See frequency selection note for U1. 4 MHz: Jameco 27967, 354889; Mouser 815-ACO-4-EK, 520-TCF400-X 6 MHz: Mouser 520-TCF600-X 8 MHz: Jameco 27991; Mouser 815-ACO-8-EK, 520-TCF800-X (see Mods section below) 10 MHz: Jameco 27887; Mouser 815-ACO-10-EK, 520-TCF1000-X 20 MHz: Jameco 27932; Mouser 815-ACO-20-EK, 520-TCF2000-X
IC	U22	1.8432 MHz oscillator, full can	1	Mouser 520-TCF184-X, 73-XO54B184 Jameco 27879
IC	U23	8 MHz oscillator, full can	1	Mouser 815-ACO-8-EK, 520-TCF800-X Jameco 27991
IC	U24	MAX232A	1	Mouser 595-TRS202ECN, 701-SP202ECP-L, 700-MAX202CPE, 700-MAX232ACP Jameco 875384, 1127599, 1800552,
IC	U25	DS1302	1	Mouser 700-DS1302, 700-DS1302N Jameco 176778, 780481, 1194644
IC	U26	DS1210	1	Mouser 700-DS1210, 700-DS1210N Jameco 114198, 2052040, 861880
IC Socket	U1, U4 - U6	40 pin 600 mil DIP socket	4	Jameco 41111 Mouser 649-DILB40P223TLF, 517-4840-6000-CP
IC Socket	U2, U3	32 pin 600 mil DIP socket	2	Jameco 112301 Mouser 649-DILB32P223TL, 517-4832-6000-CP
IC Socket	U7, U8	20 pin 300 mil DIP socket	2	Jameco 112248 Mouser 649-DILB20P-223TLF, 517-4820-3000-CP
IC Socket	U9 - U13, U24	16 pin 300 mil DIP socket	6	Jameco 37373 Mouser 649-DILB16P-223TLF, 517-4816-3000-CP
IC Socket	U14 - U20	14 pin 300 mil DIP socket	7	Jameco 37162 Mouser 649-DILB14P-223TLF, 517-4814-3000-CP
IC Socket	U25, U26	8 pin 300 mil DIP socket	2	Jameco 51571 Mouser 649-DILB8P223TLF, 517-4808-3000-CP
Oscillator Socket	U21 - U23	4 pin 300 mil DIP full can oscillator socket	3	Jameco 133006 Mouser 535-1107741
Crystal	X1	32768 Hz crystal	1	Mouser 732-C002RX32.76K-EPB, 695-CFS206-327KB-U Jameco 14584

 

 

 

 

Replacement Notes

• 74xx logic
• TTL logic families: 74LS, 74ALS, 74F, or TTL-compatible CMOS: 74HCT and 74AHCT could be used.
• Plain 74LS should work with CPU frequency up to 8 MHz. But it is recommended to use higher speed and lower power 74ALS ICs.
• It is recommended to use 74F139 or 74ACT139 for U12, especially if using older 5.25" FDDs. I tested it with 74LS139, 74ALS139, and 74AHCT139 and it worked for me with modern 3.5" FDDs.
• Tested with 74LS, 74ALS and 74HCT / 74AHCT  (except of 74LS06) logic families. Works on frequency up to 20MHz.
• Z80-CPU
• Either NMOS or CMOS Z80 CPU works. It is recommended to use at least 4 MHz CPU. 6 MHz or faster CPU is required for 1.44MB disk support. 8 MHz is recommended.
• CMOS versions - Z84C00xxPEC or Z84C00xxPEG are recommended. Note: PEC or PEG suffixes mean 40 pin plastic DIP package, PEG is RoHS compliant, PEC is not.
• Tested with CMOS Z84C00 (6MHz, 8MHz, 20MHz) and NMOS Z8400 (4MHz)
• 16550 UART
• 8250, 16450, or 16550.
• 16550 is recommended
• Tested with: Texas Instruments TL16C550, Exar ST16C550, National Semiconductor NS16550AFN, California Micro Devices CM16C550, Goldstar GM16C450, and UMC 8250B
• 8255 PPI
• It is recommended to use higher speed CMOS versions. For example 8 MHz Intersil / Harris CP82C55A part (note CP82C55A-5 is 5 MHz), 10 MHz Toshiba TMP82C55AP-10 or NEC D71055C-10 parts. 
• Tested with: Harris/Intersil CP82C55, TMP82C55AP-10, NEC D71055C-10, Intel 8255-5
• MAX232A
• Can be replaced by pin compatible devices like Intersil HIN232 and Analog Devices ADM202. Make sure to use part that works with 0.1uF capacitors. See mods section below if using part that works only with 1uF capacitors.
• Bi-Level LED Indicator
• Can be replaced by two 3 mm LEDs with pins bended at 90 degrees.

 

Power Supply

Zeta SBC requires a regulated 5V power supply. Make sure that the tip of the power supply is the positive lead. System's power consumption varies depending on components used on SBC (CMOS CPU/PPI/UART vs. n-MOS components, CMOS logic vs. TTL/LS vs. TTL/ALS) and CPU clock frequency. Also floppy disk drives are relatively power hungry (about 1 A max). Jameco carries quite a few regulated switching 5V wall adapters with different amperage. See these catalog pages:

http://www.jameco.com/Jameco/catalogs/c113/P105.pdf

http://www.jameco.com/Jameco/catalogs/c113/P106.pdf

Alternatively it is possible to use a linear power supply with 7805 or similar voltage regulator (make sure to use a heatsink, especially if powering a floppy drive).

 

Mods

 

Mods described here were intentionally not implemented on PCB to keep it simple.

• It is possible to use MAX232 for U24 instead of MAX232A, in this case capacitors C25 - C28 need to be replaced with 1uF electrolytic capacitors. Please refer to MAX232 datasheet for proper polarity.
• It is possible to use half can oscillators instead of full can ones. In this case install oscillator at pins 4 - 11 and connect oscillator's pin 14 (VCC) to pin 11 using a piece of wire.
• If using 8 MHz CPU it is possible to save one oscillator, by using single 8 MHz oscillator for both CPU and FDC. In this case connect pins 8 of U21 and U23 using a piece of wire. (Douglas says: I am doing this and it works great)
• It can be possible to use 1 MiB 27C080 EPROM. It has a bit different pinout from 29F040, so doing this will require cutting a couple of traces and adding some wires. Please compare datasheets for proper connection. Connect pin 12 of U10 to A19 of 27C080.  
• It is possible to use 128 KiB 29F010 or 256 KiB 29F020 flash, and 128 KiB SRAM. Lower size flash devices should work without any changes (note, some AMD AM29F002 parts use pin 1 as ~RESET pin, apparently in this case pin 1 should be connected to Vcc). 128 KiB SRAM parts have two chip select inputs, in this case the second chip select CE2, pin 30, should be connected to Vcc.
• If you don't plan to use 8255 PPI, it can be omitted. In this case it is not needed to install U4, C4, P4, and JP2.
• It is possible to build Zeta SBC without floppy disk controller (for example for the test purposes, or if you don't need floppy). In this case following components can be omitted: U6, U7, U8, U19, U20, U23, C6, C7, C8, C19, C20, C23, JP3, RR2, P5, P6 (Note U20 also used for UART interrupt, so don't take it out if using interrupt-driven UART I/O)
• It is possible to build system without RTC, in this case following components can be omitted: U11, C11,  U25, X1
• If you don't want battery backup for SRAM, it is possible to omit U26. In this case it is needed to connect U26 pin 5 to pin 6 (~RAM_CS) and pin 1 to pin 8 (VCC).

 

 

PCB Versions

 

PCB Version 1.0

• First PCB run, no major issues reported.
• Reverse power supply polarity protection would be nice.

 

PCB Version 1.1

• No changes to copper layers, a few minor updates to the silkscreen:
• Added a drawing indicating power jack polarity.
• Added outline for shrouded floppy interface connector, and moved floppy connector labels outside of that outline.
• Added outline for keyed floppy power connected, and moved (+), (-) signs outside of the outline.
• Added a logo.

 

PCB Version 1.2 (Current Version)

• No schematics changes
• Minor updates to copper layers and to the silkscreen:
• 20% wider power traces - 24 mils instead of 20 mils.
• Some minor traces optimization.
• Updated version and copyright information on the silkscreen.
• Blue solder mask

 

Ideas for the Next Version 

• Rotate RTC crystal by 90 degrees, so it can be installed horizontally (lay flat against the PCB)

 

Various Ideas

• Image enhancements
• NVRAM support (BIOS configuration using NVRAM), NVRAM configuration utility 
• Enhance utility for programming the BIOS, CP/M, and ROM disk separately. (It already supports the full image programming)
• Add incorrect power supply polarity protection
• Option 1: Schottky diode in series
• Option 2: Diode connected in parallel to power supply, in reverse (could be dangerous if power supply is not short circuit protected)
• Option 3: Use FET

 

Builders List and Board Characteristics

 

Builder	CPU Clock Speed	Floppies	PPIDE	Firmware	Notes
Don	8 Mhz	3.5"	Planned	RomWBW 1.4	Board not yet operational
Nik	8 Mhz	3.5" 720K	Yes	RomWBW 1.5.2	9600 baud
Wayne #1	8 MHz	1 SONY 3.5"	Yes	RomWBW 1.4
Wayne #2	20 MHz	None	Yes	RomWBW 1.4
Jim
Bob Devries	20 MHz	2 TEAC FD235HF 3.5"(1.4MB/720K)	waiting on rom update
Doug Jackson
Ian Board 1
Ian Board 2
Jason
Leon Board 1	Works on 8MHz,  16MHz & 20MHz. Random lockups at 24MHz.	1 TEAC FD-235HF 3.5" HD	Yes - tested with several CF cards: (Kingstone 4GB, SanDisk 64MB, SanDisk 512MB, Sandisk Ultra II 512MB, SanDisk Ultra 2GB) and Seagate ST380215A Hard Drve. Not working with Imation 256MB CF.	RomWBW 1.4 ZDOS	Using 74LS logic (except U12 74F139 as recommended)   Zilog 20MHz Z80 CPU Z84C0020PEC.
Leon Board 2					Not built yet
Dylan
Martin
Sergey	20 MHz, also tested with 4 MHz, 6 MHz, 8 MHz and 10 MHz.	1 TEAC FD-235HF 3.5" (1.44MB/720KB), also tested with two drives	Yes - Tested with: - Toshiba MK2729FC 1.2GB laptop hard drive - works properly - Transcend 128 MB flash IDE module - works properly with additional ground - 32 MB and 4 GB CF cards using IDE to CF adapter - work with additional ground wire between PPIDE and SBC	RomWBW 1.4
Douglas Goodall	20MHz	2 Mitumi D359M3D 3.5" HD	Working well with Compact Flash chip (256MB)	RomWBW 1.4.0.0
Bill Lewis	8 MHz osc with 6 MHz CPU chip.	1 TEAC FD-256HG 3.5"		ZetaRomWW (2011-OCT-13) Rebuilt with FDTST 2.3
David Giles	8 MHz	3.5" 1.44M/720k	2GB CF card 2GB SD card	RomWBW 1.5.1.0	Mounted in case TTLS chips AMD 29F040B FLASH
Sergio Gimenez	6.144 Mhz.	3.5" 1.44Mb	Yes with Samsung SV0322A HDD	ZetaRomWW cbios v 1.5.1	HDD with Titan cooler