Expansion Port
The expansion port allows hardware to connect directly to the master processor's memory bus, exposing the 16-bit address bus, 8-bit data bus and control signals. An external device can add ROMs, RAM or MMIO-devices to the master CPU's address space. The address range 0x4000-0x5FFF exists solely for this purpose but almost any address can be used.
There's an "expansion unit" which contains sockets for extra RAM and ROMs which can be mapped to various areas in the address space using a DIP switch, even replacing the BASIC interpreter ROMs. Some software distributed as ROMs were designed to be used in the expansion unit.
| Pin | Name | Direction | Description | Pin | Name | Direction | Description |
|---|---|---|---|---|---|---|---|
| 1 | Vb | - | +5V | 21 | ADDR6 | OUT | Address line 6 |
| 2 | NMI# | - | Master/Slave CPU NMI-signal | 22 | ADDR7 | OUT | Address line 7 |
| 3 | +5V | - | Logic voltage | 23 | ADDR8 | OUT | Address line 8 |
| 4 | +5V | - | Logic voltage | 24 | ADDR9 | OUT | Address line 9 |
| 5 | DATA7 | IN/OUT | Data line 7 | 25 | ADDR10 | OUT | Address line 10 |
| 6 | DATA6 | IN/OUT | Data line 6 | 26 | ADDR11 | OUT | Address line 11 |
| 7 | DATA5 | IN/OUT | Data line 5 | 27 | ADDR12 | OUT | Address line 12 |
| 8 | DATA4 | IN/OUT | Data line 4 | 28 | ADDR13 | OUT | Address line 13 |
| 9 | DATA3 | IN/OUT | Data line 3 | 29 | ADDR14 | OUT | Address line 14 |
| 10 | DATA2 | IN/OUT | Data line 2 | 30 | ADDR15 | OUT | Address line 15 |
| 11 | DATA1 | IN/OUT | Data line 1 | 31 | R | OUT | Reset |
| 12 | DATA0 | IN/OUT | Data line 0 | 32 | R/W | OUT | Read (low)/Write (high) |
| 13 | IOCS# | OUT | I/O chip select | 33 | R (RAM)# | OUT | RAM reset |
| 14 | Vc | - | RAM backup voltage (+3V) | 34 | E | OUT | ENABLE signal |
| 15 | ADDR0 | OUT | Address line 0 | 35 | ROM E | IN | ROM enable |
| 16 | ADDR1 | OUT | Address line 1 | 36 | INTEX# | IN | External interrupt signal |
| 17 | ADDR2 | OUT | Address line 2 | 37 | GND | - | Signal ground |
| 18 | ADDR3 | OUT | Address line 3 | 38 | GND | - | Signal ground |
| 19 | ADDR4 | OUT | Address line 4 | 39 | CG | - | Chassis ground |
| 20 | ADDR5 | OUT | Address line 5 | 40 | CG | - | Chassis ground |
Read more about this topic: Epson HX-20
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