PiDP-11 I/O Expander Software

This repository contains the latest revision of the PiDP-11 software with updates to support the PiDP-11 I/O Expander in SIMH. Releases from this repository are intended to fully replace the original PiDP-11 software. A complete list of changes can be found here.

Project documentation is hosted on Hackaday and assembled boards are available for purchase from Tindie.

Installation

Note: These instructions assume you have a working hardware installation. Be sure to follow the steps documented on Hackaday and verify that the I/O expander is responding on the I2C bus before continuing.

Installing releases from this repository follows the same steps as installing the official PiDP-11 software:

$ sudo mkdir /opt/pidp11
$ cd /opt/pidp11
$ sudo wget https://github.com/sstallion/pidp11/releases/latest/download/pidp11.tar.gz
$ sudo tar -xvf pidp11.tar.gz
$ sudo /opt/pidp11/install/install.sh

Once installed, reboot the system and the front panel should light up.

Theory of Operation

The general-purpose I/O (GP) device bridges one or more MCP23016-based I/O expanders to connect SIMH to the outside world. The programming interface matches that of the MCP23016 with the addition of a CSR to manage interrupts. Up to 8 units may be attached with the unit number corresponding to the position of the I/O expander on the I2C bus.

As this is not a simulated device, state cannot be saved and the file associated with the unit is ignored, which by convention is /dev/null.

GP11 units are mapped to a fixed set of addresses with a single auto-configured interrupt vector:

UNIT	BASE ADDRESS
GP0	17776000
GP1	17776020
GP2	17776040
GP3	17776060
GP4	17776100
GP5	17776120
GP6	17776140
GP7	17776160

GP11 registers are mapped to the following offsets:

REGISTER	OFFSET
PORT	00
OLAT	02
IPOL	04
IODIR	06
INTCAP	10
IOCON	12
CSR	14

For example, to examine the contents of the GP0 CSR register from the SIMH console, issue:

sim> examine 17776014

Note: The examine GP0 CSR command can also be used, but be aware that the values returned by this command are cached and do not represent the true state of the underlying hardware.

With the exception of the CSR register, each register is documented in the MCP23016 datasheet. The CSR register contains two bits of interest: IE (bit 6) and ERR (bit 15). The IE bit controls whether or not changes to input state will interrupt the system, which must be set by software and can be cleared by either software or hardware. The ERR bit is used to report low-level errors communicating with the MCP23016. It is set by hardware and must be cleared by software.

Usage

An I/O expander using the default I2C address can be attached from the SIMH console by issuing:

sim> attach GP0 /dev/null

Note: GP0 is attached by default when booting IDLED to simplify GPIO programming using the front panel switches.

Once attached, registers can be examined and deposited using the EXAMINE and DEPOSIT commands in SIMH or by using the front panel switches.

To verify that the I/O expander is attached and functioning properly, the IODIR and CSR registers may be examined using the front panel switches:

1. Toggle the HALT switch to stop execution.
2. Load address 17776006 using the S0-21 switches followed by the LOAD ADRS switch.
3. Press the EXAM switch to display the contents of the IODIR register in the data display; bits 0-15 should be lit indicating that all ports are configured as inputs (POR default).
4. Load address 17776014 using the S0-21 switches followed by the LOAD ADRS switch.
5. Press the EXAM switch to display the content of the CSR register in the data display; bits 0-15 should be unlit indicating that no error occurred and interrupts are disabled.
6. Toggle the ENABLE switch followed by CONT to resume execution.

The above procedure can also be accomplished using the SIMH console:

^E
sim> examine 17776006
17776006:       177777
sim> examine 17776014
17776014:       000000
sim> continue

... but where was the fun in that?

Contributing

Pull requests are welcome! If a problem is encountered using this repository, please file an issue on GitHub.

License

Modifications to SIMH are licensed under the same modified X-Windows license. See see the documentation included with each software package for more details.