Epson HX-20 - Expansion Port

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.

Expansion port pinout
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

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