RS232c/V24 INTERFACE BOARD

The RS232c/V24 board grants communication with the 'outside world'. This board allows connection of printers, modems, terminals and other devices which use serial communication with the Aster CT-80. 
To maximize the number of serial channels, this board is relocatable over the entire I/O port range using DIP-switches (SW1). From software point of view, the maximum number of cards is 63.
Physically this runs into problems, as the BUS should have room for this much boards, but for special purpose situations systems with a large number of serial interface ports can be created.
The RS232c/V24 input IC's are provided with a condensor on the compensation input to reduce noice suppression.

Port addressing (SW1: 1-6)
Decoding is realized with IC's Z13, Z14, Z16, Z18 and Z19, switches SW1-1 to SW1-6, and jumper block J1, J2, and J3.

Internal data bus
The board features an internal, isolated data bus with the following IC's connected:
- Z5; extra available signals to the RS232c connector, depending on the jumper block and IC Z13,
- Z6; outgoing signals to the RS232c connector,
- Z7; incoming signals from the RS232c connector, and a bit to inspect the flip-flop of IC Z13,
- Z8; the programmable BAUD rate generator. This IC allows for software selection of 16 receive BAUD rates and 16 send BAUD rates,
- Z9; the UART IC, converting parralel data to serial and vice versa.,
- Z15; the hardware configuration data for the UART for TRS-80 compatible software.

Outgoing signals
The board has two connectors:
- A 9-pin connector for a Viditel Modem and
- A 25-pin connector for Modems, printers, terminals, etc.

Configuration of DIP-switches and jumpers.
DIP-switch SW1-8 is for test purposes. It shorts the Tx signal with the Rx signal. This allows testing of UART and hardware. The send and receive BAUD rate have to match.
DIP-switch SW1-7 always has to be open for TRS-80 applications and Jumper J2 is closed. This disables Z5.

If DIP-switch SW1-7 is closed, Z13 is toggled at every USART-reset, resulting in either loading of the UART control register or the handshake latch. The handshake-latch has three parts:
- D0 to D2 (Z6),
- D3 to D7 (Z5),
- Modem-Status register (bit 1)

If the jumper J1 is closed, Z6 is linked to the UART Control register. In this case, switching Z13 selects either the UART Control register with Z6, or just Z5.

If the jumper J3 is closed, Z5 and Z6 are linked and either these or the UART Control register is selected.

The BAUD rate genaertor is a BR1941