Cable between PX-8 and modern PCs

There are several reasons to connect the PX-4 or PX-8 to a PC;

The standard cables in the User manual are for specific purposes like PX-4/8 to modem (#724), PX-4/8 to another generic computer or a printer (#725), PX-4/8 to another PX-4/8 (#725). The #725 should be right, but it doesn't take the quirks of the IBM PC/AT into account, like the gender of the connector and the handshake line state dependency. One can solve this with a gender changer and a loop-back. But when often used, a custom cable is a better solution. The cable that fixes the gender looks like the ASCII-art below. PC-9 pin DE9F and PC-25 pin DB-25F are two options for the PC end. Both are female connectors.

  PX-4/8 8 pin mini-DIN   PC-9 pin DE-9F   PC-25 pin DB-25F
  ---------------------   --------------   ----------------
S.GND      1 ----------------- 1 --------------- 7
TxD        2 ----------------- 2 --------------- 3
RxD        3 ----------------- 3 --------------- 2
RTS        4 ----------------- 4 --------------- 5
CTS        5 ----------------- 5 --------------- 4
DSR        6 ----------------- 6 -------------- 20
DTR        7 ----------------- 7 --------------- 6
DCD        8 ----------------- 8 --------------- 8 

Nothing short of remarkable is how Epson used the exact same numbering that several years later was used by IBM for the AT RS-232c 9-pin connector!

This will work wel if you are sure the PX-4/8 sets the handshake lines properly, or when the PC is set to ignore them. This latter case is probably the reason an USB-RS-232c converter works with this cable.

The handshake lines are a remnant from the times modems were mostly analog and signalled the condition of the line and signals received this way. Since the introduction of the Hayes AT command set, these lines were only there to be backward compatible. For the PX-4/8 - PC usage, certainly when you use a real RS-232 port, it is better to loop them back, certainly at the PC-side. The PX-4/8 doesn't really care for the handshake lines.

  PX-4/8 8 pin mini-DIN   PC-9 pin DE-9F   PC-25 pin DB-25F
  ---------------------   --------------   ----------------
S.GND      1 ----------------- 1 --------------- 7
TxD        2 ----------------- 2 --------------- 3
RxD        3 ----------------- 3 --------------- 2
RTS        4             +---- 4           +---- 5
CTS        5             +---- 5           +---- 4
DSR        6           +------ 6         +----- 20
DTR        7           +------ 7         +------ 6
DCD        8           +------ 8         +------ 8 


Latest update: 2012-02-03

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