Introduction

The SPCOM is a multi-function peripheral board which plugs into the bus of an STEbus computer system. Based around two 8250A/16450 type serial communications controller ICs and a custom LSI chip, it provides a PC-compatible communications environment. This consists of one Centronics parallel printer port and two RS232 serial communication channels, each capable of simultaneously transmitting and receiving at rates of up to 20 kilobaud (20000 bits per second) asynchronously, each with 5 associated control bits.

The pin-out on the printer connector is arranged so that a ribbon-cable can connect directly to a 25-way D-type IDC connector. Note that this differs slightly from the Centronics standard pinout in order to maintain PC-compatibility. The pinout of the RS232 connector (20-way Bergstick) is such that a 20-way ribbon cable can be directly connected to two PC-compatible 9-way D-type IDC connectors. An Arcom standard 10-way connector allows the two RS232 channels to be taken to a SERT1. The SERT1 board carries two 25-way D-type connectors as used by most RS232 equipment. Note that only TxD, RxD, CTS and RTS are used on the SERT1. Signals RI, DCD, DTR, DSR are not used on the SERT1 and the SPCOM should be programmed to ignore them.

Typical Applications

Circuit Description

The SPCOM board handles two types of I/O address decoding:

1. Standard STEbus decoding

If link LK1A is made, then the board is accessed in the range (BASE) to (BASE+7) inclusive, where BASE is defined by the links to the address comparator IC6. IC6 compares A11 - 5 with the links in LK5, and asserts pin 13 (!SEL) on IC4 (a custom PAL chip) if they match and ADRSTB* is low. IC8 is clocked by the 16 MHz SYSCLK, and controls I/O and DATACK* signal timing, and 8250A chip selects.

2. PC compatible decoding

If link LK1A is unmade, the printer and serial ports can reside at a choice of PC compatible addresses (278-27F, 378-37F or 3BC - 3BF) can be selected by LK1 B-E.

The PC uses non-vectored interrupts, with the controlling CPU either polling the internal interrupt flags of the 8250A or performing a non-vectored interrupt when the ATNRO* line set by LK1 goes low, and clearing the interrupt so that the 8250As can operate normally again.

IC11 and IC13 are 8250A ACE (Asynchronous Communications Element) chips which control one serial channel each. IC10, 12,14,15 and 16 buffer the TTL serial signals to RS232 levels and vice versa. IC9 is a custom chip which drives a parallel printer port. It should be noted that although the signals are Centronics compatible, the PC printer port connector is a 25-way D-type. The pinout of PL2 is such that a ribbon cable can directly connect it to an IDC 25-way D-type connector.

Links and Options

The board is described as seen from the component side with the 64-way bus connector to the right.

Note:
+ = the standard jumper connection
* = the signal is active-low

Link Positions.

Link area 1. I/O Address Selection

o o o o o
E D C B A
o o o o o

The SPCOM is made PC-compatible by default, enabling the two serial ports, and the printer port is the primary printer port.

Link areas 2-4. Interrupt levels

  o
  B
oAoCo

  o
  B
oAoCo

  o
  B
oAoCo

The SPCOM interrupt levels are made PC-compatible by default, by setting links LK2A in position B.

Link area 5. STEbus address decoding

If you wish to have the SPCOM peripherals in a block of 32 contiguous I/O locations residing at any I/O base address, then make LK1A and define the appropriate address on LK5.

Making a link causes the board to respond when that address line is low. Leaving these links unmade makes the board respond when the corresponding address lines are high. Leaving all the links open therefore gives the board a base address of FF8 (Hex), i.e. A11 to A5 all high (A3 high, A2 to A0 always low, as fixed by the PPC1 chip).

Table 1. Links and address weights

I/O Map

The I/O map of the SPCOM board is given below, for jumpering as standard; i.e. with PC-compatible address decoding.

Table 2. PC- compatible STEbus I/O Addresses

The serial ports are identical to each other (apart from their address), as are the printer ports.

Table 3. Use re-definable STEbus I/O Addresses

When LK2A is made the SPCOM is accessed as a contiguous block of 24 I/O locations, whose base address is defined (in multiples of 32) on LK5. The bit functions of the registers are identical to the description given in the previous table. The SPCOM board will not respond to I/O locations base +18H to base +1FH, and so these locations are free for use by another board.

Using the SPCOM

This manual does not cover the low-level control of the 8250A Asynchronous Communications Elements (ACE), as this will usually be done by the routines in the BIOS (Basic Input / Output System) ROM. If you wish to drive the SPCOM from a processor other than the SCPC88, then the 8250A technical manual from Intel or National Semiconductor contains the relevant details.

Before sending or receiving data the relevant handshake lines must be at the appropriate level. To enable transmission of data from the SPCOM, CTS needs to be low at the 8250A, and to enable reception DCD needs to be low. This means that these signals must be high when measured at the connector PL2, because inverting buffers are used.

To test the printer and RS232 channels, first boot up your operating system which will contain the drivers. Type control-P, some text, then a carriage-return. The line of text should appear on the printer. The printer should be set up for 1200 baud, by default.

The baud rates of the RS232 channels 1 and 2 can be set up by the PC-DOS command:

SET COM <rate>

where is the channel and <rate> is the baud rate. Hence the commands:

SET COM 1 2400
SET COM 2 9600

will set RS232 channel 1 for 2400 baud, and channel 2 for 9600 baud. You can also redirect the line printer output to an RS232 channel using the command:

MODE LPT#:= COM1

after which, pressing the print-screen key will cause the present screen contents to be printed out on a serial printer on RS232 channel 1.

Example Programs

A great many example programs can be obtained from the large range of PC software. There is a public-domain program called KERMIT which is used to transfer data across a variety of serial communication links. Other programs allow a PC system run as a terminal emulator. The parallel printer can be tested from an operating system by typing control P followed by a stream of characters terminated by a carriage return. The characters should then appear on the printer.

Component List

Connections

Specification

Circuit Diagram