A1200 Clockport serial scheme

Current Status: Concept only, untested
Date:                  19th July 2002
License:             Creative Commons Attribution 3.0 Unported License
Design summary:
Probably a first, a complete but untested, design for the A1200 clock port.
Schematics, design notes and some timing diagrams included.

Why do this?

I have authored a technical note  on Aminet on the clock port and received a few emails asking how to design a card for the clock port. The best way to answer the emails would have been to send a design, which I did not have. So I set about designing a serial port card to  learn more. This is the result.

What about software?

This is the biggest problem. A driver would need to be written. It may work with other clock port serial cards as they are of similar design but I have not tried this.

What are the specifications?

  • Uses a PC16550D UART, the original from National Semiconductor that is used in most PCs, capable of 1.5MBaud, limited to 250KBaud in this design.
  • The UART has a 16 byte FIFO to reduce CPU overhead.
  • 250 KBPS max data rate.
  • Only requires 3 ICs.

Design files


You can download everything here.

 By Ian Stedman, 19th July 2002

1200 Clock port Serial Card design Notes

Preliminary release, 4th July 2001 by Ian Stedman


A public domain design for a high speed serial port for the Amiga A1200.

September last year, I wrote a short note on the A1200 clock port which I
posted to Aminet. I received a few emails asking for more info and how to
design something for this port. Whilst I had a few ideas, I needed to design
something myself to help others, hence this project was born.

I know there are commercially available, high speed, Serial ports for the A1200
so I decided to try and design one to show you, the reader, how they are built.

The design is untested for 2 reasons:

1) I can’t make surface mount PCBs yet. Using through hole parts would make the
   pcb way too big.
2) I do not own an A1200. I own an A600 and CD32/SX32.

If I get the chance I intend to build this design to make sure it does work 😉

The Design process

This falls into 3 stages:

1) Research on the A1200
2) Research of high speed serial parts (UARTS)
3) the actual hardware design

Stage 1, A1200 Design notes

I advise you to have a quick read of my previous note on the clock port
contained in this archive and available on aminet before continuing.
Some info is expanded upon here.

Relevant documents

Obtaining detailed design information on the A1200 is tricky.
Using Google I found some useful bit of information, namely the A1200
schematics and the A1200 functional specification.

A1200 Schematics http://www.amiga-hardware.com/a1200.html
Goto the bottom of the page and download the LZX file

A1200 Functional specification
Any aminet mirror,
A1200FuncSpec.lha  hard/misc   35K  96 A1200 functional Specifications

Oh and don’t forget the following

A1200Hardware.lha  hard/misc   73K 195 All you need to know about the A1200 Hardwardware
a1200INFO6.lha     hard/misc    5K 241 A1200 HARDWARE info, vol.6 (11/96)- RTC

By studying these I managed to piece together the following on the clock port, most
of which comes from my previous A1200 clock port notes.

Pins Available and their function

What follows is the relevant signals available for add on expansions.

All are located on the bottom half of connecter P9B

Unless stated overwise, all signals are TTL compatible and have the following characteristics

Logic 0<=0.8V for an input, <= 0.4V for an output, 3.2 mA drive.
Logic 1>=2.0V for an input, >= 2.4V for an output, 400 microamp (uA) drive. 


Chip select for the real time clock that was never fitted.
It decodes/selects a 64K memory space from $DC0000 to DCFFFF.
There are 3 Wait states on a read and 4 on a write to ports using
this address strobe.
Active low.


An Intel/PC style I/O Read signal.
Active low.
Used for Read strobes on a device connected to the clock port.
The IORD signal is also used by the IDE interface.

Signal sourced from Gayle.


An Intel/PC style I/O Write signal.
Active low.
Used for write strobes to devices connected to the clock port.
Also used by the IDE interface.

Signal sourced from Gayle.


The RESET line appears to come from the RESET line by the keyboard micro?

PWR_BAD is a power up reset signal, as far as I can tell.

The KB_RESET signal is used to Warm Reboot the Amiga and this signal comes from the
keyboard array by using the 3 finger salute!


A level 6 interrupt which goes to Paula. Paula feeds this to the 68020’s IPL
(Interrupt Priority Level) lines. Can be used by add on peripherals


One of 2 chip selects available on the clock port.

Was originally intended for the optional clock!

It is active low for any address access in the range of $DC0000 to $DCFFFF


The second of the chip selects.

It is active low for any address access in the range of $D80000 to $D8FFFF


8 bits of the 68020 Data bus.

These are the only data bus lines available for add on peripherals.

These lines connect directly to the 68020 so bear in mind their limited drive
capability, 400 uA for a logic 1, and 3.2 mA for a logic 0.

Timing in relation to IORD and IOWR is detailed later.


4 bits of the 68020 Address bus.

DC drive capabilities as above for the Data bus.

Timing info to be added later.


A +5V supply pin.

No idea of current limits, depends on PCB tracking.
Limit to 200 mA to play safe.


2 0VL (0V logic) pins.

The Next Step + some analysis

Looking at these signals and the schematics made the following observations.

– Only 8 data bits available, limiting but fine for a serial or parallel port or
  any other communications part that needs an 8 bit interface (maybe ISDN).

– 2 Chip selects that each decode a 64K address space. This eliminates the
  need for any address decode logic to provide chip selects

– There are only 4 address lines available!
  This is a real pain as you can only access 16 memory locations.

– There are separate Read (IORD) and Write (IOWR) lines which mimic 80X86 style
  Read write lines. Timing information difficult to obtain though.

So in summary, everything you need for a small peripheral is there but some careful
design is needed.

The only thing I was lacking was some timing information on IORD, IOWR and the two
chip selects, more on this later.

PC Peripheral, high speed UART

Virtually all PCs use a 16550 compatible UART, be it an IC from National
Semiconductor (the original designers) on an expansion card or embedded
within a southbridge chip, as on most modern PCs.

This is a popular chip as it contains a 16 byte FIFO for transmit and
receive functions, helping to lower CPU interrupt overheads. It also allows
data rates upto 1.5 Mbit/second. MS windows limits serial ports to 115,200 bits
per second, Amiga’s are only limited by CPU speed.

This is also the part used on all high speed serial port cards I have seen
for the Amiga. I have also designed using this part before so I knew there
was a good chance it would work with the Amiga.

The datasheet is available from National Semiconductor’s web site at;


Search for PC16550 or look for CPU accessories/support.

Interfacing the UART to the Clock Port

The first page of the datasheet shows a basic configuration which with a little
modification, would work with the A1200. Section 9, contained a clearer typical
application for an 8088 CPU, see included example1.gif and example2.gif

With a little amount of interface circuitry we can interface this to the clock port.

Taking the example circuit as a reference we can connect as follows


*    Tie CS0 & CS1 to +5V so that CS2 is used to select the device.
**   Tie the RD and WR pins to +5V.
***  The UART has an active high reset, we need to invert it.
**** INT is active high on the UART, we must invert it.

All we need is a 7404 Hex inverter chip for the 2 signals that need
to be inverted.

RS232 Transceiver chip

I chose the MAX208 from MAXIM as I had data to hand on this part.

It is limited to a maximum data rate of 120KBPS. There are faster parts available.
If you used his part, you could lower the clock crystal to 18.436 MHZ to achieve
a 115,200 BPS data rate.

Connection of the transceiver to the UART is easy enough, see the schematics.

Make sure you use the correct value capacitors for the charge pumps (C4-C7).


Each IC has a 100nF capacitor for power supply decoupling.
A 10 uF capacitor whould be placed across the power rails, as close as
possible to the clock port header to reduce power supply transient effects.

Timing Analysis

Obtaining full timing information for the clock port has been extremely difficult.

I have timing information on the address and data lines, which came from the 68020
user manual.

What I really needed was the accurate timing for the IORD, IOWR and various chip

What I have managed to gather is as follows.

A1200 clock speed is around 14.3 MHz, which is approximately 70 ns.

This is important when you consider the number of wait states associated with
the chip selects.

_SPARE_CS has 3 wait states (210ns) for a read and 4 wait states (280ns) for a

From an email by Jens Schoenfeld

The IOR/IOW timing is very slow (several waitstates!), and you can always
rely on data being valid over the whole active-time or IOW, and should
make data valid before the rise of IOR. CS is always asserted way before
IOR or IOW, since it is generated without AS, so you may encounter
glitches on this signal without accelerator boards.

Sorry, no further info. These signals are generated by the Gayle chip, and
that seems to be totally undocumented. I have heard that it has registers
where you can configure this timing, but nobody has found out yet.

This should fit in with the UART timing but the Address may not be present at
the right time! The fact the signals are slow could help. The 68020 asserts the
address at S1, and data at S2 (70 ns time).

This could potentially make the design malfunction. Without either a logic analyser
capture or some probing with an oscilloscope I can not say any more.

Hardware Design Conclusion

I have two concerns with the design at the moment, mainly due to not having
built a prototype yet. They are:

1) Some of the control signals, _IORD, _IOWR and SPARE_CS may need AC
   termination. Probably an 68 Ohm resistor and a 100 pF capacitor
   should do the trick.

2) The timing info is sketchy. Hopefully by releasing this design I will
attract some attention and maybe someone will provide the info I need.

The trickiest part of the design was finding out about the A1200 clock port,
once done it took about an hour to design the circuit.


Well we need to access the part.

The 16550 UART needs 8 address locations as follows

SPARE_CS Base address $D800000

You would probably need to access the port using 16 bit accesses as the
part is on bits 16-23 of the address bus. So all addresses start from $D800001.

We only have access to A2-A5 so all address increments are offset by 4 locations.

The upper 8 bits (D24-D31) of the data bus will be ignored.

The function of the internal register in part depend upon the Divisor latch
address bit (DLAB) which is bit 7 of the Line control register.

Register NameFunctionAddress
RX BufferHolds a byte received$D80001 (DLAB=0)
TX BufferHolds byte to transmit$D80001 (DLAB=0)
Interrupt EnableInterrupt control register$D80005(DLAB=0)
Divisor Latch lowerUsed to set the serial speed$D80001 (DLAB=1)
Divisor Latch higherUsed to set the serial speed$D80005 (DLAB=1)
Interrupt Ident Details of raised interrupt$D80009 (Read Only)
FIFO Control RegisterControls internal FIFOs$D80009 (Write Only)
Line Control RegisterSet serial data control$D8000D
MODEM Control RegisterSetup of some modem bits$D80011
Line Status RegisterVarious status bits$D80015
MODEM Status RegisterModem status bits$D80019
Scratch RegisterFor scratch data?$D8001D

For more details on the internal registers, see the 16550 user manual.

To read and write data to the serial port you access address $D80001.

NOTE: If you used the _RTC_CS chip select the address offsets would be the same but
they would start from, $DC0001.

There are 8 unused addresses from $D80021 to $D8003D. If you used a Dual UART, like the
16C552, you could map the second UART in this address range.

I am unsure if this design will work with available serial port drivers.
Any clock port device will have to use addresses similar to those listed above
so it could work.

Future Enhancements to the design

As I was completing this write up I came across the 16C552 Dual UART with
enhanced parallel port from Texas Instruments (Document number SLLS102B).

This includes 2 independent 16550 compatible UARTS and an enhanced parallel
port. All that would need to be done to use this part is to modify the design to use
the other available chip select, _RTC_CS so that the Parallel port can be used and
to modify the chip select configuration.

Final Conclusion

I designed this system as an educational exercise for myself and hopefully, for
others who wish to design peripherals using the A1200 clock port.

There may be some errors in this design, use it at your own risk.


This design and associated document is copyright Ian Stedman, 4th July 2001.

You may distribute freely and use the design in a product on one condition,
if you manufacture a commercial card, you must send one to me.

There may be errors and omissions in this document, I can not be held responsible
for them, you use this information at your own risk.

By Ian Stedman, 19th July 2002