CompuPro - Interfacer 4
The Interfacer 4 was fully software compatibility with the Interfacer 3 as well as the ability to intermix Interfacer 3 and 4 boards at the same port addresses. Additional features included 3 fully programmable asynchronous serial channels, 2 of which were capable of high speed synchronous transmission and one capable of current loop operation,  five RS-232 handshaking lines per channel plus bi-directional clock drivers on both the synchronous channels, a pin compatible Centronics parallel interface port with the full complement of handshaking lines, a universal parallel port with 16 data and 3 handshaking lines, expandability to 32 users with eight boards using only 8 port addresses, a flexible interrupt structure with full maskability and pending status on both transmit and receive interrupts, and a conservative design for operation with most CPUs operating to beyond 10 MHz.

The Interfacer 4 was designed for efficient operation in interrupt driven/ multi-user microcomputer systems as well as polled mode single user systems. Eight distinct interrupts are generated on-board by the three USARTs and two parallel ports, and these are brought out for jumpering by the user to the eight vectored interrupt lines on the S-100 bus. Since these interrupt lines were open collector, they could be configured to interrupt on any or all of the vectored interrupt lines. In addition, a transmit and receive interrupt mask port is provided  for  inhibiting  unwanted   interrupts. The Interfacer 4 provided multi-user operation with a minimum number of I/O ports by incorporating a user select register to activate the required I/O channel. This five bit register is used to select a particular channel, which allows up to 32 users (up to eight boards) on the same 8 port addresses. When a particular user is selected, the four USART registers associated with that specific serial channel or the parallel registers are made available for exami­nation and alteration by the host processor or other temporary bus master. In addition, whenever a particular channel is selected, the interrupt registers on that particular board as well as the registers on another board in the same group of  eight  users  are  available  for  examination  and   alteration.  This was a very cleaver idea used on later S-100 boards and got around the dwindling I/O space becoming available on the bus. The board utilized INS 2651 UARTS.
 
The manual for this excellent board can be found here.

 

Other CompuPro S-100 Boards
CPU8085-88  CPU86-87  CPU-Z  Disk1  Disk1A  Disk1B  Disk2  Disk3   EconoROM2708  Interfacer 1  
RAM Boards   Interfacer 3  Interfacer 4  Interfacer II  M-Drive  MPX-Board  PC-Video  
System-Support1  System-Support2   SPIO  Spectrum  SP186   CPU-286   68000  32016  SPUZ