*************************************** * * * Reproduction or translation of * * any part of this documentation * * without the written permission of * * HoneySoft Ltd is unlawful. * * * * An Advanced User Guide covering * * all the information supplied here * * and more is available from * * HoneySoft Limited. * * * * Copyright (c) 1985 HoneySoft Ltd. * * * *************************************** CIRKIT MODEM 1200 TO AMSTRAD CPC464/664/128 INTERFACE HARDWARE SPECIFICATION _______________________________________ Introduction This document presents the hardware specification for a minimal parts serial interface to allow the Cirkit Modem 1200 to communicate with the Amstrad range of CPC computers. _______________________________________ Features o Asychronous operation o 5, 6, 7 or 8 bit characters o 1, 1.5 or 2 stop bits o Odd or even or no Parity, generation and checking o Framing, overrun and parity error detection o Carrier Detect input (/DSR) o MUTE output (Intended for Half / Full duplex switching) o 1200/1200, 1200/75, 300/300 bps operation o RS232 compatible inputs, TTL compatible outputs _______________________________________ The Hardware An 8251 UART is used as the basis for the design, associated with this are four support ICs. _______________________________________ CPU Interface The 4MHz clock output from the expansion bus is used to derive the CLK input for the 8251. The active high 8251 RESET input is derived from the bus /RESET line. Two OR gates provide the necessary address decoding. _______________________________________ I/O Address Map Two addresses are used from the I/O address map. The addresses used are those recommended by Amsoft for communications peripherals, and are therefore compatible with all Amstrad peripherals, with the exception of other communications devices. ADDRESS READ WRITE ================================== 0FADFH DATA IN DATA OUT 0FBDFH STATUS CONTROL 0FADFH Data input/output register 0FBDFH Status / Control register When read, returns the UART status. /DSR is used as a Carrier Detect input. When written after a soft or hard reset, accesses the Mode Instruction register are used to define operating parameters. After writing the Mode Instruction word, this address then accesses the Command instruction register. /DTR is used in conjunction with the selected clock ratio to set the desired baud rate. /RTS is used as a MUTE line, to switch the Modem into half or full duplex. D7 should be sent as a 0. Refer to a data sheet on the 8251 for detailed description of register assignments. _______________________________________ Modem Interface Four signals control data transfer via the Modem and 8251, these are: Transmit Data TTL level Output. 0=Mark, 1=Space (+5V). Data to the Modem. Receive Data TTL level Input. +5V=Space, 0V=Mark Data from the Modem. Diode clamped to allow RS232 compatability. Mute TTL level Output. +5V=Mute This signal is driven directly by the 8251 /RTS output. The Modem's transmitter should be disabled when MUTE is TTL logic 1. Carrier detect TTL level Input. 0V=Carrier Detected Carrier detect input from modem. Diode clamped for RS232 compatibility. Should be connected to 0V if not in use. _______________________________________ Cable assignment The modem interface lines are brought out along a 6 core cable to a 5 pin DIN plug to allow a Modem or other device to be connected. Pin connections as follows: Black - Ground 0V Brown - Transmit data (to Modem) Red - Receive data (from Modem) White - +5V from computer Yellow - MUTE (to Modem) Green - Carrier Detect (from Modem) Shield - Ground 0V _______________________________________ 5 Pin DIN Connections The pin connections to the 5 pin DIN are: 1 - Ground 0V 2 - Transmit data (to Modem) 3 - Receive data (from Modem) 4 - Not connected 5 - MUTE (to Modem) Carrier Detect connected to ground (Green). _______________________________________ Baud Rate Generation Three fundamental baud rates are supported, these are derived from a 614KHz oscillator. Two frequencies of 1200Hz and 19.2KHz are used to support (RX/TX) 1200/1200, 300/300 and 1200/75 baud operation. The 8251 receive clock is always 19.2KHz. The transmit clock can be selected from any of the two, by use of the 8251 /DTR output. The table below details how the baud rates are derived. BAUD RATE RX TX /DTR CLOCK RATIO 1200 1200 0 16 300 300 0 64 1200 75 1 16 All three available baud rates may be used in either half or full duplex mode. It is the responsiblility of support software to control the MUTE output, for half duplex operation. _______________________________________