Yet another computer museum
The Epson PX-4

I got this fine computer from Lino Lampers. Thanks Lino!

Top Epson Px-4 top The PX-4, introduced in 1985, is successor of the by then aging HX-20 (from 1981), but is based on the PX-8. The HX-20 was positioned as a portable terminal to be used in the field, the PX-8 was more geared for administrative uses on the road. The PX-4 is similar to the the PX-8 on the outside, but included features from the HX-20 like modular peripherials as printers, mini-cassette and extra RAM. As the internal architecture is quite different, most hardware related PX-8 programs do not work and vice versa (like FILINK.COM or CONFIG.COM). It runs generic CP/M programs and has a virtual 80x25 (configurable even to 80x50!). The actual screen is 40x8.
The PX-4 was followed by the PX-4+, which at least incorporated the 120 kByte RAM disk.
Side &
Epson PX-4 side Item keyboard The PX-4, being slightly newer than the PX-8, contained some features not found on the older model.
  • It used less energy (100mA vs. 200mA, allowing it to operate on four penlight batteries. Alternatively a rechargeble battery pack could be installed.
  • The option bay could contain a cassette drive, printer, or other option. The PX8 has a fixed cassette drive.
  • Its optional External RAM Disk contained apart from the 128 kByte RAM also an ROM socket.
  • A Centronics compatible printer port. In those days Epson was famous for the printers they made. The IBM Proprinter escape code set was very much based on the FX-80.
  • The keyboard is also a separate module. Shown are the 'Item keyboard' and the standard 'ASCII keyboard'.
  • The cassette drive eject button has a blocking mechanism. The cassette will only come out if it is unmounted. The PX-8 had no blocking, and has a warning on the case.
  • An external cassette port. This was a returning HX-20 feature.
  • Dates beyond 12/31/99 are accepted. The PX-8 is limited from 1977 to 1999.
  • Another capsule ROM format 'P', which allows execution of programs directly from the ROM. The original format 'M' which is the same as used with the PX-8, is a strict disk image. The PX-4 BASIC is an example of the 'P' format.

    The PX-4 at the left has an "Item keyboard" installed. Here is a hi-res scan of the overlay.

  • Family The complete Epson portable family Following the PX-4 Epson developed the PX-16, a portable PC compatible which by its modular construction could be used as field terminal (compact, sturdy & lots of options) or as portable (PC compatible complete with 80x25 monochrome LCD screen). In fact, the PX-16 is a recreation of the PX-4, but PC-XT compatible. The cartridge interface of the PX-4 and PX-16 is the same.
    PX-4 PINE board Epson PX-4 pine board The PX-4 (Pine) board is less populated than the PX-8 (Maple) board. The 6301 additional processor and the discrete 82C51 USART are gone. Epson probably realized a slow 6301 didn't add much functionality next to a 3.6 MHz Z80. This may be the main reason of the reduced power consumption.
    The disadvantage of having the Z80 processor suppory all functionality, is that about 10 kByte less is available for the TPA (and RAM disk):
    without external
    RAM disk
    29.5 kByte TPA
    (+25 kByte int.
    RAM disk)
    54.5 kByte TPA
    (+8 kByte int.
    RAM disk)
    with external
    RAM disk
    55.4 kByte TPA63.5 kByte TPA
    The internal RAM disk sizes are the defaults. TPA is 'Transient Program Area', CP/M speak for free memory size.

    A weak spot is the backup batttery, an Epson RB 301 4.8V/90mA NiCd back. The dimensions are 67.7 mm long, 14.4 mm diameter. 1/4AAA NiMh 1.2V, 80mA (11 mm long, 10 mm diameter) are still for sale and could be used to construct a replacement with 4 cells in series. The PX-4 will work without, but will not keep memory when switched off (same as backup switch off).
    RAM disk board Epson PX-4 External RAM disk board The External RAM Disk board is not "Intelligent" as the PX8 type was. No Z80 and ROM, but still dynamic RAM. There is a rechargable battery included.
    The ROM capsule socket is upward compatible with the internal ones, but larger sizes are not supported by the operating system. It appears as drive J:. User programs can access the larger than 32 kByte capacities, assuming you find the correct chips or equivalent (128 kByte masked PROMs existed in 28 pin packages). ROM capsule switches:
    SW2 (pin 27)8K,16K32K,64K
    SW3 (pin 1)8K,16K,32K64K
    PX-4+ PINE II
    Epson PX-4+ pine II board The PX-4+ is an upgrade of the PX-4. Both the 120 kByte RAM disk and the extra ROM socket is integrated. The extra socket maps to drive J:. Some related jumpers:
    J4 (Drive B, pin 27)8K,16K32K
    J5 (Drive C, pin 27)8K,16K32K
    J7 (Drive J, pin 1)64K8K,16K,32K
    J10 (Drive J, pin 22)?GND
    The display uses a different technology, making the background more yellow-greenish. The PX-4/8 had a more grey/blue-green.
    Between the battery compartment and cartridge-bay two connectors suggest an interesting, but unknown option. There is also a socket for a permanent OS ROM extension ROM.


    Local links:

    External links:

    Latest update: 2022-06-18