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Tarbell Electronics -- 1011A Floppy Disk Controller

Tarbell Electronics was rather late to the scene with a Z80 CPU board. It is thus a rather rare board. It had a number of unique features not found on other boards of the era. Its memory management was like that of the Intersystems Z80-II board and allowed teh Z80 to address up to 1MB of RAM.

Z80 CPU Board

The Tarbell Z-80 CPU board is designed to use the full power or the Z-80 CPU chip along with 2 serial ports, a timer chip, 8080 vector interrupt or the Z-80 mode 2 interrupt, and extended memory management of 1 Megabyte with the capability of relocation on 4 Kbyte boundrys. The Tarbell Z-80 CPU board is designed around the IEEE S-100 standard and is directly compatible with all products sold by Tarbell Electronics.

The Tarbell Z-80 CPU board will run at either 2 MHz or 4 MHz by means of a jumper selection on the board.

The on-board timer, which is an 8253 timer chip, will generate timing intervals from 1 micro-sec. to 327 milli-sec. using 1 of the 3 timers available in the 8253.  Provisions are made on the CPU board to chain the additional 2 timers together for increased time intervals. The 2 additional timers may also be used independantly of the others.

The Tarbell Z-80 CPU has provisions for accepting 8080 vector restarts. The level of interrupt is also maskable with one exception in that neither RST 0 or RST 7 are maskable because of their use in CPM. The other 6 restarts are maskable by way of a register on an I/O port on the CPU board. As an option jumper, provisions have been made to allow the Mode 2 interrupt ot the Z-80 to function.

The CPU board uses two 8251 UART's for serial I/O operation. Each channel's baud rate is set by an on-board switch which controls an SMC-5016 baud rate chip. Baud rates from 50 to 19,200 are switch selectable for each channel. Each serial channel is RS-232 compatible with, hand-shaking provided for DTR,RTS, DSR, and CTS lines.

One unique feature of the Tarbell Z-80 CPU is it's Memory Management circuit. By using a ram mapping techinque, we can make the Z-80 appear to address 1 Megabyte address range, even though the Z-80 will only address 64 Kbytes directly. Besides the main advantage of being able to address 1 Megabyte of memory, the ram mapping technique also allows the programmer the ability to relocate software on 4 Kbyte boundries for dynamic memory allocation schemes.

The manual for this board can be obtained here.

Other Tarbell Electronics S-100 Boards
Cassette Interface     Single Density FDC      Double Density FDC       32K RAM      Z80 CPU Board


This page was last modified on 06/06/2015