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
The Tarbell Z-80 CPU board is designed to use the full
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
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
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
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
Management circuit. By using a ram mapping techinque, we can make the
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