ZX Interface 1

The ZX Interface 1 is a communications and storage interface for the ZX Spectrum released by Sinclair Research.

It provided an RS-232 serial interface, as well as storage using Microdrives and networking capabilities through the Sinclair Network, a proprietary interface of Sinclair's. The Sinclair Network would later be supported by MGT's DISCiPLE interface.

Port		Bit
		7	6	5	4	3	2	1	0
0xf7	Read	TX Data							Net Input
	Write								Net Output / RX Data
0xef	Read				Busy	DTR	Gap	Sync	Write Protect
	Write			Wait	CTS	Erase	R/W	Comms Clock	Comms Data
0xe7	Read	Microdrive data read
	Write	Microdrive data write

• TX Data is the value received by the serial port.
• Net Input is the value read from the Sinclair Network port.
• Bit 0 of port 0x7f is output to the serial port if Comms Data is set, or to the network port if Comms Data is reset.
• DTR signals that the remote station wishes to send data, CTS signals that the Spectrum wishes to send data
• Wait is used for network synchronisation.

• When Erase is reset the tape is erased and data is written, when set data is read.
• R/W instructs the microdrive to read or write. When R/W is set the microdrive runs, when reset it stops
• Comms Data is the value to be shifted into the microdrive select flip-flop chain.
• Comms Clock shifts the microdrive select bit along the chain.
• Write Protect is raised when a writeable cartridge is present in the selected microdrive.
• Gap and Sync are used to locate the correct block of data on the microdrive.

Interface 1 Pin Assignments

The Interface 1 pin assignments contain 7 data pins, one of which is a high-voltage open-collector data pin, a 9v supply and several ground pins as well as a 2-pin keyway to avoid incorrect insertion. This is all configured as a 16 pin edge connector to connect to an IDC ( Insulation Displacement Connector ) plug.

Microdrive Edge Connector

PIN		Edge Connector
		15	13	11	09	07	05	03	01
Function	TOP	GND	GND	GND	ERASE	Comms Data	KEY	Comms Clk	D1
	BOTTOM	GND	GND	GND	R/W	9v	KEY	WR-PROT	D0
PIN		16	14	12	10	08	06	04	02
		Viewed from side.

Signals:

• D0/D1 - Interleaved data, a byte at a time, from each of the read/write heads in the microdrive. Data rate approximately 80 Kbps per track.
• Comms Clock - Drive select clock, identifies which drive to turn on. TTL level.
• WR-PROT - A "low" on this indicates a cartridge is write protected. 9 volt = Not protected.
• Comms Data - Drive select is serially clocked through drives. Logic 1 = selected. Line is inverted from register.
• 9v - Power supply for drive,
• ERASE - Open Collector via 270 Ohm resistor, to ERASE head on selected microdrive.
• R/W - Write is active low. Select to read or write data to drive.

Microdrive Performance Characteristics

In many ways, the Microdrive, despite it's simple construction, performed similarly to disk systems of the era, and was roughly as fast at loading, however the lack of file allocation tables on the disk meant that the file allocation tables had to be build prior to every write operation, slowing down write access.

• "Infinite loop" - Repeating magnetic tape.
• Two tracks, operating as a single interleaved track.
• Byte by byte interleave ( Even bytes Track 0, Odd bytes Track 1, Interleave every 4 bits. )
• Directoryless Data Structure.
• Typical Capacity 90 Kb
• 160 Kbps Data Transfer Rate ( 20 Kbytes/second raw transfer rate )
• Data encoded using Biphase Frequency Modulation ( same coding as credit card )
• Does not support a cartridge change line - All sectors must be eximined before writing data.
• Typical full cycle time of around 8 to 10 seconds.
• Can handle media stretching and data rate change without losing data.

External links

• Wikipedia's article on the ZX Interface 1