80-Bus News |
July–August 1984 · Volume 3 · Issue 4 |
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(Or a better solution to Dr. Dark’s Ring of Rust.)
| i | A computer with an RS232 (V24) interface. (Nascom 2 Owners NB: this generally means including the control signals but do not despair, read on.) |
| ii | A Modem or acoustic coupler. |
| iii | A terminal program (with file up/down load facilities). |
| iv | Various bits of wire, plugs, your infinite patience, etc. |
| v | A telephone line. (Preferably someone else’s to keep your own bill down.) |
| i&v | Most readers will already have one. |
| ii | Surplus stores; I obtained an ex-GPO Modem 2b from Display Electronics for £30. The modem only does CCITT V21 300 bd but it is a well-built device in a very smart box (about the size of a Kenilworth case), complete with its own PSU. |
| iii | CP/M users have it easy here: Henry’s have both the original MODEM 7 program and an improved UK version called UKM715, so pick a time when they aren’t very busy, wander in with a disk, get down on your knees and grovel, and you might get a copy. Tape Nascomers, PolyDossers, etc: Not having tried this myself (you understand), start with a simple rewrite of one of the terminal programs already published, then progress to a terminal file saving program by dumping all characters typed into memory before saving. (Similar to the Prestel program.) |
| iv | The wire and plugs bit should be easy, as for the infinite patience, well, er … |
Assuming you haven’t fallen asleep yet, here’s how you get the thing working.
Connecting the modem to the computer (serial interfaces, here we go again, YAWN ee)
When the RS232 standard was conceived (7) it was assumed that communication would proceed thus.
Computer ––> Modem ––> (Telephone) ––> Modem ––> Computer
Computers are always thought of as
Data Terminal Equipment (DTE) or just plain Terminals
Modems are always thought of as
Data Communications Equipment (DCE) or Hosts
as they protect BT from its users by hosting the communication.
All computers, therefore, should be wired up as terminals and have a MALE plug. All modems are hosts and have a FEMALE socket. Thus, for Gemini serial connector wire a male DB25 plug onto it with transmit data out on pin 2 and receive data in on pin 3, bring all other control signals out to the plug as
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