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EPP 3 - The hardware protocol

Here's a view of the DB25 printer connector coming from your PC.

The pins 2 to 9 are the 8-bits bus. In EPP mode, the 8-bits bus is bi-directional.
There other important pins are:

17address strobePC -> FPGAlowaddress transaction
14data strobePC -> FPGAlowdata transaction
11waitFPGA -> PClowresponse to strobe
1writePC -> FPGAlow0 for write transactions, 1 for read transactions

You can see that there are 2 "strobe" signals plus a "wait" signal. The "strobes" are coming from the PC, while the "wait" is going to the PC.

Here's how this works: for every transaction, the PC asserts one of the strobe and the FPGA responds with the wait.
Let's pick one strobe signal (only one is activated at a time) and let's look at an EPP transaction:


  1. The PC wants to start a transaction. It asserts one of the strobes (=low).
    If the transaction is a write, the PC also drives "write" low, and drives the 8-bits bus.
    Otherwise it drives "write" high and leaves the 8-bits bus floating.

  2. The FPGA detects that one of the strobe is asserted, and responds by de-asserting "wait" (=high).
    If the transaction is a read, the FPGA starts driving the 8-bits bus.

  3. The PC detects that wait is de-asserted, so it de-asserts the strobe.
    If the transaction is a write, the PC stops driving the 8-bits bus.

  4. The FPGA detects that the strobe is de-asserted, so it asserts "wait".
    If the transaction is a read, the FPGA stops driving the 8-bits bus.

All this is done in hardware; the PC software doesn't have to do anything besides starting the transaction.