The Thurnal Modular System. I/O Port: $34; Motherboard: $30; 8-Way Indicator Unit: $24; 8-way Switch Unit: $24; 8-way A/D Converter: $45.75; Transistor Driver: $18. Thurnal Electronics, c/o Sinclair Place, PO Box 2288, Redmond, WA 98052.
The Bus
As the general public becomes progressively more “computer literate,” the topic of conversation at many cocktail parties drifts to personal computers. These conversations are usually liberally modified and amplified with “buzz” words and phrases (one only need mention these; it is not necessary to know what they mean at cocktail parties.)
One such phrase is, “Is it expandable?” As a ZX/TS owner you may validly answer, “Yes, my computer is expandable.” Though it does not have a Versabus, Multibus, Unibus, or S-100 bus (some of the more famous micro/minicomputer buses), it does have the expansion port in the rear. This may be called the “Sinclair bus.” You usually attach the RAM pack at this point.
A “bus” in computer terminology should not bring to mind a sleek racing dog (Greyhound), but rather the collective lines that connect the various inner parts of the computer. The bus is subdivided into an Address, Data, and Control bus. In our 8-bit, Zilog Z80 based ZX/TS, there are 8 Data lines, 16 Address lines, 13 Control lines, and another 3 lines for +5V, ground, and the clock signal. This gives a total of 40 pins which go into the Z80 computer (CPU) chip. These lines, as well as +9V (unregulated from the power supply) and a memory line, can be seen labeled in the chapter on machine code in your manual (p. 124 of the Timex edition).
The Thurnal Electronics (TE) devices “expand” from this edge connector.
The TE I/O Port
The TE I/O Port is the first part to connect to the computer, and it is literally the portal through which everything passes.
There are two main ways that data can be passed in and out of a computer. The first is the way a program is loaded from the tape recorder, i.e., over one wire with the bits queued up one behind the other, parading single file. This is known as serial transmission. A typical serial interface is the RS232 standard. The other way of moving data in and out of the computer is on the bus, with the data marching in, shoulder to shoulder, 8 bits wide. This is known as parallel transmission. A parallel standard is the IEEE 488 (sometimes called the Hewlett Packard bus).
The TE I/O Port (I/O = Input/Out put) is a parallel interface. The interface is accomplished via an integrated circuit developed by Zilog (designer of the Z80 CPU) to be a parallel interface for this microprocessor. It is called a Z80 PIO (Parallel I/O). From the outside the chip looks identical to the Z80 CPU. The inside is quite different though it has the same lines. The PIO is designed for receiving parallel data from the CPU and outputting it to a peripheral.
There are two separate ports, termed A and B, which have 8 data lines plus “handshaking” (control lines). The TE box has a connector which is identical to the RAM connector which connects directly to the computer. A memory pack is not necessary to operate the I/O port, but, if you will be using one, the computer edge connector for the RAM pack, continues out the rear of the TE box.
The PIO is fully “decoded.” What this means to you is that the use of the TE expansions will not interfere with any other devices you intend to attach to the computer such as 64K RAM packs, printers, serial interfaces, modems, etc.
However, because the port is decoded you will need a very short machine code program to access the ports. The documentation from TE offers you an option here, a stepwise entry of the necessary codes into a Basic REM statement which is clearly explained with absolutely no mention of machine code. This is guaranteed to operate the port. They also have a section which completely explains the machine code routine if you are interested.
The connector to the ports has all the lines to port A on the top of the circuit board and the lines to port B on the bottom. Any of the TE devices can be connected singly to this edge connector (one of the first things I connected directly here was the TE joystick.
If more than one of the TE devices is to be used, a motherboard is available. The motherboard will connect up to 4 of the TE devices at the same time (since the ZX/TS is not a multitasking computer, you will be able to address them only one at a time. The motherboard contains no electronic devices; it is just an extension of the bus or wiring, namely, the data and control lines—2 to port A (A1 and A2) and 2 to port B (B1 and B2). The proper connecting sequence of the devices and the lines is diagrammed on the cover of the motherboard.
The TE Joystick
I set up the TE joystick and entered a rather extensive (16K RAM required) Basic program supplied by TE which constructed a maze that used the joystick to manipulate through the course without crashing into the wall. There is very good documentation for the operation of the joystick, so those of you interested in games could quickly adapt one or more of these to your game programs. The Timeblaster video game from Caliope Software would be particularly interesting to play with a joystick. In this case, since the software is mostly Z80 code, you would have to prevail upon the author to provide you with a version that used the codes you would need for the joystick, instead of for the movement keys which are used.
Other TE Devices
Let’s take a quick look at five other TE devices. The TE Indicator Unit, with 8 LEDs, lights up according to the binary equivalent of what is sent to the port (a good way to learn to count binary). The TE Switch Unit, just like the original micros—the MIT’s Altair 80 and the IMSAI 8080—allows you to switch your programs in through the port by setting the binary equivalent of the code on the eight switches (on=1; off=0) and then reading the port. (This would be purely academic as far as I am concerned).
Another TE device can do Digital to Analog (D/A) control of devices: The TE Transistor Driver controls low voltage electronic devices while the TE Relay Box switches on low current household equipment using line voltages.
In all, the TE devices are a sound investment: They are well built and at a reasonable price for Sinclair expansion. If you do not have any electronic experience or applications along those lines, you will probably be interested only in the parallel port and the joysticks.