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The 3P+2S IO Board     
  Fuinal Board


S100Computers has a number of well established IO boards by now. All are fairly well debugged and seem to function correctly. Some are almost 10 years old by now. Since that time the have been a number of advances in small computer interfaces.  Boards like the Raspberry Pi or Arduino series have revolutionized computer interfacing.  Many add-on "modules" have been developed that allow quite powerful computer setups. There is no reason why these modules cannot be incorporate into S100 Bus Systems as well. We have here a new IO board  with 3 parallel ports and 2 serial ports.   The board has a large "patch" area where the user can use these ports to add various components such as sound generators, serial terminals etc. It has the following capabilities.

1.    Two generic RS232 Serial port capable of operation up to 115,200 Baud with a DB9 Connector or a 8X2 Ribbon Cable Pin Connector
2.    Alternatively one or both Serial ports can be used with Serial to USB port adaptors
3.    Three 8 bit parallel ports.
4     Only two sets of S100 bus ports are utilize. These are  four  8 or 16 bit contiguous ports.
6.    A large "patch area" is on the board to add other components/modules
7.    Important S100 bus control lines are available near the patch area.
8.    A simple program is supplied to test the board is provided.

For the serial port the default baud rate for this connection is 19,200 BAUD,1 stop bit, no parity (19,200 BAUD 8N1).  This can easily be changes in software

It is absolutely essential you use a Zilog 85C30's.  I could not get the board to work with other UARTs such as the OKI equivalent.  The reason for this is unclear to me.
   Zilog 85C30
Don't start this project without this chip.

Step By Step Building the Super IO Board

The build instructions are fairly simple for this board however building it should not be rushed.  As always, first examine the bare board carefully for scratches or damaged traces, use a magnifying glass if need be.  A broken trace is almost impossible to detect by eye on a completed board.

Solder in all the required IC sockets, resistors, resistor arrays, capacitors and jumpers.  The 4.915 Oscillator usually is inserted wit its own "Oscillator socket".  However you can also use a 14 pin DIP socket.
Note there are two sockets for the Pololu 5V regulator. While the older ones (D24V25F5) are still available and use P1, it seems Pololu is suggesting users use the newer D24V22F5's (5V, 2.5 Amp) units, it has a different pinout, use this one in P2.  Be sure you get the regulator correct. To be safe once inserted, check the voltage in your system is a 5V with no chips yet inserted on the board.

You can also use the EZ-SBC regulators. It is soldered into K3. These EZ-SBC regulators are currently cheaper and as best I can tell just as good -- though they have a larger footprint however.
As a last resort you can even use an old 7805 regulator with a heat sink in K3.

At this point insert the board into your system and check your S100 bus Z80 monitor boots fine.  If not check for solder bridges etc. Also do one last check to be sure every IC solder pin is soldered.  Seems trivial, but I don't know how many times I got caught with this error.
 Here is a picture of the board at this stage:- 
  Board with Sockets
Place the board in the S100 bus and check you are getting 5 volts on U5 pin 14.

Install all the 74LSxx IC's.  Do not install the 4 MAX232 IC's yet (U21,U19,U20 and U18). Install the 8255A  (U14)  but not yet the Z85C30 (U8).
Jumper K1 1-2, so the s100 bus reset line will reset the 8255A configuration (see below).

This board utilizes two sets of ports. One set of 4 ports for the parallel ports (the 8255A), and one set of 4 ports for the serial ports (the Z8530) .   Both sets utilize a contiguous set of 4 ports. 
This board has the ability to place the ports anywhere in the CPU's IO space of 256 bytes, (or for 16 bit CPU 64K bytes).
The tricky part is setting the dip switch jumpers for this.

To understand how this is done you should understand how the
74LS682 adders selector chip works. Please read here
There are 3 of them on this board.

IEEE-696 16 bit S100 bus port IO addressing is assumed (the upper 8 lines being zero) for this board. This can be easily bypassed within the Jumper K2 1-2.  Do so if your 8 bit CPU board does not zero the high address lines for port I/O. To be safe we will jumper K2 2-3 for now. Note 16 bit CPU's will probably require K2 1-2

Let us look  at how we set the dip switch settings.
For now we will assume an 8 bit S100 bus master CPU, so for any IO port address the address lines A15...A8 will all be low.
There eight all switches of SW1 should be closed (i.e. grounded). See the schematic below. So jumpering K2 1-2 would insure the IC1 pin 19 output (low) would get to pin 2 of IC2 and IC3. To be safe for now we will bypass this option and simply ground K2 (pin 2-3).  This in effect bypasses the use of SW1.

This switch is used to set up the 4 ports for the Zilog UART (U8).  Pick a port range that is not already used in your system.  
We will use ports
50H (Port 1, control), 51H (Port 0, control), 52H (Port 1, data) and 53H (Port 0, data) here, so Port 50H to 53H.
We must match the inputs of IC2 pins 2,4,6,8,11,13,15 and 17 to exactly match the switch settings of pins 3,5,7,9,12,14,16 and 18.  Only then will
pin 19 of IC2 go low (and select the UART).
Here is how things should match up:-
Upper Address, A7, A6,  A5, A4, A3, A2, GND  
        0      0    1   0   1    0   0   0    Port 50H to 53H
SW2     on     on  off  on off  on  on  on

This switch is used to set up the 4 ports parallel ports of the 8255A (U14).  Pick a port range that is not already used in your system.  
We will use ports 58H (Port A),  (Port B),  5AH (Port C)  and 5BH
(Port D) here.
Again we must match the inputs of IC2 pins 2,4,6,8,11,13,15 and 17 to exactly match the switch settings of pins 3,5,7,9,12,14,16 and 18.  Only then will pin 19 of IC2 go low (and select the 8255A).
Here is how things should match up:-
Upper Address, A7, A6,  A5, A4, A3, A2, GND  
        0      0    1   0   1    1   0   0    Port 58H to 5BH
SW3     on     on  off  on on  on  on  on

Here is a picture of the DIP switch settings.
  Dip Switches

It may be confusing to see that pin 19 of IC2 and IC3 are always pulsing.  This is because only the address lines are fed into the chips.  The sOUT and sINP signals go directly to the 8255A or Z85C30 and are internally AND'ed with CS*.

We us a common 8255A IC to provide 3 parallel ports on the board.  This is a old IC but one of the most widely and reliable chips used for over 30 years.  It's  a 40 pin IC with individual pins for three 8 bit IO ports.  The catch is the chip is somewhat tricky to setup because it has numerous modes in which these pins can be utilized. Either output, input or bi-directional.   if you are not familiar with the chip you must read the spec sheet here.

The fourth port (port D) of the 8255A is the important port that determines the function of ports A, B & C.  It is an internal port on the chip with no external pins.

Here we will configure the 3 parallel ports (A,B, & C) on the chip by sending 8 bits to its D port, (in our case
We will configure the chip as:-
A output, B input, and C (bits 0-3) output, (bits 4-7)input)
We do this by simply one time outputting to port 5BH 10001010b (8AH).  This configuration must be done after any power-up or reset of the chip.

If for example you wanted the configuration to be:-
A input, B output, and C (bits 0-3) output, (bits 4-7)input)
You would do so by outputting to port 5BH 10011000b (98H).

There are numerous other ways to program the 3 parallel ports on this chip. Please see here.

In order to test the board I have written a small program 3P+2S_IO.Z80.  It can be obtained at the bottom of this page. 
It runs at 100H and only uses the Propeller Console IO for input. This can be easily modified in the code if need be.

Here is the signon screen after transferring the .com file from  PC (with XMODEM):-
      loading Code
We will select menu item D.  This will initialize and configure the 8255 and cause the 8 bits on its port A (pins 1-4 and 37-40) to continuously flip 0H to FFH.
Check this is happening wit a logic probe.  Hit ESC to abort the test.

Before installing the Serial circuit it might be useful to review facts about the RS232 transmission protocol.

The RS-232 was originally introduced in 1960 for serial communication transmission of data. It formally defines signals connecting between a DTE (data terminal equipment) such as a computer terminal, and a DCE (data circuit-terminating equipment or data communication equipment), such as a modem. The standard defines the electrical characteristics and timing of signals, the meaning of signals, and the physical size and pinout of connectors.  The RS-232 standard was commonly used in computer serial ports and is still widely used in industrial communication devices. A serial port complying with the RS-232 standard was once a standard feature of many types of computers. Personal computers used them for connections not only to modems, but also to printers, computer mice, data storage, uninterruptible power supplies, and other peripheral devices.  USB has displaced RS-232 from most of its peripheral interface roles. Thanks to their simplicity and past ubiquity, however, RS-232 interfaces are still used—particularly in industrial CNC machines, networking equipment and scientific instruments where a short-range, point-to-point, low-speed wired data connection is fully adequate.

Because the standard did not foresee the requirements of devices such as computers, printers, test instruments, POS terminals, and so on, designers implementing an RS-232 compatible interface on their equipment often interpreted the standard idiosyncratically. The resulting common problems were non-standard pin assignment of circuits on connectors, and incorrect or missing control signals. The lack of adherence to the standards produced a thriving industry of "breakout boxes", patch boxes, test equipment, books, and other aids for the connection of disparate equipment. A common deviation from the standard was to drive the signals at a reduced voltage. Some manufacturers therefore built transmitters that supplied +5 V and −5 V and labeled them as "RS-232 compatible".

In RS-232, user data is sent as a time-series of bits. Both synchronous and asynchronous transmissions are supported by the standard. In addition to the data circuits, the standard defines a number of control circuits used to manage the connection between the DTE and DCE. Each data or control circuit only operates in one direction, that is, signaling from a DTE to the attached DCE or the reverse. Because transmit data and receive data are separate circuits, the interface can operate in a full duplex manner, supporting concurrent data flow in both directions. Note the standard does not define character framing within the data stream or character encoding.

The standard specifies a maximum open-circuit voltage of 25 volts: signal levels of ±5 V, ±10 V, ±12 V, and ±15 V are all commonly seen depending on the voltages available to the line driver circuit. Some RS-232 driver chips have inbuilt circuitry to produce the required voltages from a 3 or 5 volt supply. RS-232 drivers and receivers must be able to withstand indefinite short circuits to the ground or to any voltage level up to ±25 volts.

RS-232 devices may be classified as Data Terminal Equipment (DTE) or Data Circuit-terminating Equipment (DCE); this defines at each device which wires will be sending and receiving each signal. According to the standard, male connectors have DTE pin functions, and female connectors have DCE pin functions. Other devices may have any combination of connector gender and pin definitions. Many terminals were manufactured with female connectors but were sold with a cable with male connectors at each end; the terminal with its cable satisfied the recommendations in the standard.

The following table provides a list of the major RS-232 signals and pin assignment.
Direction is given with respect to the DTE - 'In' means the signal goes to the DTE, 'Out' means it comes from the DTE; the signal names also derive from this convention.

Name Abbreviation Direction DB-25 pin DE-9 pin
Transmitted Data TxD Out 2 3
Received Data RxD In 3 2
Request To Send RTS Out 4 7
Clear To Send CTS In 5 8
Data Terminal Ready DTR Out 20 4
Data Set Ready DSR In 6 6
Data Carrier Detect DCD In 8 1
Ring Indicator RI In 22 9
Force Busy Out 25 N/A
Signal Quality In 21 N/A
Signal Rate Both 23 N/A
Serial Clock Transmit In 15 N/A
Serial Clock Receive In 17 N/A
External Clock Out 24 N/A
Signal Ground G Common 7 5
Protective Ground PG Common 1 N/A


The Request To Send (sometimes 'Ready To Send') and Clear To Send pair are for hardware flow control, but use of them is fairly rare.

The signal ground provides a voltage reference for all the other signals, and also serves as a common signal return for them.

In order to interface to the outside RS-232 world the zero or +5 volt Low/High levels of the Zilog 8532 must be converted to the -12V & +12V  levels required for RS-232 transmission. In the 70's the two chips to do this voltage level conversion were 1488' and 1489's. These worked fine except you needed extra + and - 12 volt regulators.  Today we use the +5 volt (only) MAX232 chips that internally generate the required voltages. So the serial data I/O from the SCC pins goes through the MAX 232 chips to convert the signals to RS-232 levels. (BTW one word of caution, be careful with your logic probe if you are checking this circuit -- the MAX output voltages are not TTL levels).

The most common pin connections between DTE and DCT is as follows.
The P8 and P9 jumpers on he board can be used to change the pinouts if required.

RS232 pinouts
The Zilog UART Chip
This is a more recent UART in the history of UARTs.  It has the advantage  that it is totally self contained needing no other "support chips" such as a Baud rate generator for example.  It is a DUAL UART allowing the use of two completely independent serial channels to be used.  There is almost no serial protocol it cannot work with.  Finally it is easy to program. You do so, by one time, sending a series of bytes to its two control registers.  Serial data is then continuously read from its two data ports.  The cost of all this flexibility is the task of sending the correct configuration bytes to these control registers at the start. 
Please carefully read the spec sheet if you wish to reprogram the chip. It can be obtained here.
Insert the Zilog 85C32, U8.

To make things simple here is the code to program the Zilog 85C30 port 0 to 19,200 Baud, N-8-1  (19,200 BAUD 8N1).
You would use the same code for the Zilog 85C30 port 1.
    LD   A,ACTL             ;Program Channel A of UART_0
    LD   C,A
    LD   B,0EH              ;Byte count for below
    LD   A,(HL)
    OUT  (ACTL),A
    INC  HL

    DB 04H                 ;Point to WR4 >>>>>>>>>>>>>>>>>>>>> (RS232 UART)
    DB 44H                 ;X16 clock,1 Stop,NP
;   DB 04H                 ;X1 clock,1 Stop,NP
    DB 03H                 ;Point to WR3
    DB 0C1H                ;Enable receiver, Auto Enable, Receive 8 bits
;   DB 0E1H                ;Enable receiver, No Auto Enable, Receive 8 bits (for CTS bit)
    DB 05H                 ;Point to WR5
    DB 0EAH                ;Enable, Transmit 8 bits
    DB 0BH                 ;Set RTS,DTR, Enable. Point to WR11
    DB 56H                 ;Receive/transmit clock = BRG

    DB 0CH                 ;Point to WR12
;   DB 40H                 ;Low Byte 2400 Baud
;   DB 1EH                 ;Low Byte 4800 Baud
;   DB 0EH                 ;Low Byte 9600 Baud
DB 06H                 ;Low byte 19,200 Baud
;   DB 02H                 ;Low byte 38,400 Baud
;   DB 13H                 ;low byte 115,200 Baud (With 1X clk)
;   DB 00H                 ;Low byte 76,800 Baud
;   DB 06H                 ;Try for 460800
    DB 0DH                 ;Point to WR13
    DB 00H                 ;High byte for Baud
    DB 0EH                 ;Point to WR14
    DB 01H                 ;Use 4.9152 MHz Clock.
    DB 0FH                 ;Point to WR15
    DB 00H                 ;Generate Int with CTS going high
Please study the 3P+2S_IO.Z80 to understand better how to use this chip.
BTW, I have found the fast Z80 OTIR block output instruction does not work reliably with some Z8530 UARTS.

To test the chip, insert the 85C30 in U8.  Do not install the MAX232 chips yet.
First we will do a very simple test. We will send data out on Port A (TXD1, pin 15), jumper it directly to (RXD1, pin 13). 
We also need to connect RTS1 pin 17 to CTS1 pin 18.

Using the program above, first initialize the UART (Menu A).
Then select Menu B.  This will continuously send a character out through the UART pin 15 and receive it back on 13.
Check with a logic probe that pin 15 and 13 are pulsing.
This must be the case before going further.

Next we will move the signal out to the MAX232's. Insert U21, U19,U20 and U18.
Jumper P9 1-5 and 3-7.
Do the same Menu B check. Check you are getting a pulse U21, pin 14.
Here is a picture of the setup.
    MAX232 Jumpers

Finally we bring the signal out to the DB9 connector J2.
To do this all the vertical pins of P9 need to be connected.
Jumper the DB9 pin 2 to the DB9 pin 3
Jumper the DB9 pin 4 to DB9 pin 8.

Do the same Menu B check. Check you are getting a pulse U21, pin 14.
Here is a picture of the setup.
    RS232 Jumpers

At this point if you have an RS232 terminal you should be able to communicate with it at 19,200 Baud using the DB9 socket.

These days a very popular way of communicating serial data is via a USB connection.
Fortunately there are a number of simple modules these days to convert a 5V serial signal to a USB signal.
Here is a picture of three common ones.
      AdaFruit USB Adaptor             SparkFun USB Adaptor                  Pparkfun Adaptor 2

The good news is they all utilize the same pinout along the bottom row.
Please read up on these adaptors on the Sparkfun or Adafruit web sites before using them.

In order to utilize these adaptors on either or both UART ports the corresponding MAX232 chips must be removed from the board.
UART port 0 utilizes P31 or 51.  UART port 1 utilizes P50 or P20.
Both pairs of ports have the same pinouts. One can be for adaptors with pins the other for adaptors with female sockets.
Also jumper P50 and if used P53.

For testing load the 3P+3S_IO program, Initialized the UART and run menu item B.
The USB terminal (set to 19,200 Baud) should display 3's continuously
Next run menu item C.  Anything you type on the USB terminal should appear on your Z80 S100 bus console.
Here is a picture of the USB adaptor attached to the UART port A.
   USB Adaptor Connect
If these menu items all work you should have a functional Zilog 85C30 UART.
BTW, everything explained above works in an identical manner for the UART port B
It's unclear to me why P50 (or P53) should be jumpered. This pin supplies +5V from the USB PC connection.
Without it data entry on the PC terminal is not received by the USB module.
Here is what you should see on the USB terminal.
    Writing on teh USB terminal


To Order a Production S-100 Board
A number of people may want to utilize a board like this.  Together with a group of people on the  Google Groups S100Computers Forum, a "group purchases" will be setup on that forum.
If you would like a bare board please keep an eye on that forum.


Please note some of the above pictures of the boards during the build process are late stage prototypes. The position of certain items may be slightly changed from the final (V1.0) versions.



The links below will contain the most recent schematic of the above boards.
Note, it may change over time and some IC part or pin numbers may not correlate exactly with the text in the article above.

3P+2S_IO Board Schematic 
                                 .PDF File  (Note V0.3),  not yet available)
KiCAD files for the 3P+2S IO Board                      .ZIP File     (Note V0.3  11/23/2023)

3P+2S_IO.Z80 (Text File)                                      (11/23/2023)
3P+2S_IO.Z80 folder (zip file)                             (11/23/2023)

3P+2S IO Board BOM  (Text file)                     

Other pages describing my S-100 hardware and software.
Please click here  to continue...

This page was last modified on 12/10/2023