Friday, January 2, 2009

Serial Interfacing.

The 8085 is equipped with two pins which provide for sending and receiving of serial data. Actually, the pins simply accept or generate a voltage level out according to program control. Whether or not the timing of the highs and lows on these pins constitute real serial-by-bit data or not is determined by the programming and other components involved and surrounding the ‘85.

Accepting single-bit serial data into the 8085 is accomplished by the use of the Serial Input Data (SID) line. A voltage from an outside source is applied directly to this pin. It may be sampled at any time by executing a RIM instruction. Upon completion of the RIM, bit 7 of the A register will contain either a 1 or 0, indicating the conditions existing on the SID pin at the time of the instruction’s execution. The arrival of a bit on SID or the change of the level applied to it between high and low in no way effects the interrupt system. No interrupts are generated, nor is any electrical indication of the activity on SID made in any way. Only the execution of a RIM, at the time desired by the program, will indicate the condition of the SID pin.

Generating single-bit serial data out of the 8085 is accomplished by the use of the Serial Output Data (SOD) line. This pin has placed upon it a voltage high or low that is determined by the SOD flip-flop. This flip flop, in turn, is controlled by bits 6 and 7 of the interrupt mask byte loaded via the SIM instruction. If bit 7 is high and a SIM is executed, the SOD flip-flop is set high, and SOD pin will be high accordingly. If bit 7 of the mask byte is low and a SIM is executed, the SOD flip-flop is reset low, and pin contains a low output. Bit 6 of the mask byte acts as a permissive toggle. The contents of bit 7 will be transferred to the flip-flop if bit 6 is high. If bit 6 is low, the SIM instruction will have no affect on the SOD flip-flop. This permits independent execution of SIMs to service either SOD or interrupts without affecting the other.

It is obvious that the SID and SOD controls are relatively dumb, and that sort of signals appear or are accepted by them are under control of the program to a great extent. This is important if a serial data transmission is in progress and interrupts must be serviced at the same time.

SUPPORT DEVICES FOR THE 8085

At the time of introduction of the 8085, several additional support devices were introduced to enable the ‘85 to be adapted to a great variety of system organizations. A few are listed here. The student is encouraged to look them up in the documentation and review their usage characteristics.

  • 8155 WORKS IN A DRAWER: (For lack of a better description.) This device provides several items of general use for a small ‘85-based control environment. It includes 256 bytes of RAM, two 8-bit wide parallel ports, one 6-bit wide parallel port (usually used as control lines for the other two ports), and an interval timer. The timer is programmable, and the ports may be used for input, output, or both. The device accepts standard 8085 electrical interface lines and operates on a single 5 volt supply.
  • 8755 EPROM WITH I/O: This device provides 2Ks worth of 8-bit bytes of UV-erasable EPROM, and two 8-bit-wide bi-directional parallel I/O ports. It uses the 8085 electrical interface, and a 5 volt supply.
  • 8259 PROGRAMMABLE INTERRUPT CONTROLLER: This is later device which interfaces with several processors, including the 8085 and the 16-bit 8086. It handles 8 vectored interrupts by itself, and is cascadable to a total of 64 interrupts by adding additional 8259’s to each of the first one’s input lines. It requires initialization bytes to be sent to it before it can perform, and these bytes can tailor its operation to a variety of conditions. It requires a single 5 volt supply.

In addition to these devices above, many Intel and non-Intel RAMs and ROMs have been developed to provide data storage to meet every conceivable need. Also, the Intel documentation lists many general purpose I/O devices, usually aimed at the Multibus configuration, which may be used with a variety of MPs. The astute technician will acquaint her/himself with these, as the use of LSI support devices is becoming very important

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