Epson Serial Interface GQ-3500 User Manual page 11

1. When the input buffer becomes nearly full (within 128 bytes)
When the pinter goes off line (by pressing the ON LINE button)
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3. When the printer runs out of paper
4. When the printer runs out of toner
5. When an error is detected.
when the DTR signal goes low, the computer musts stop sending
data within 128 characters The signal goes back high when the buffer
has room for 255 Characters.
If an error conditionoccurs (conditions 2-5 above), the DTR signal
will only return to high error has been cleared and the ON
L I N E b u t t o n -
When DIP switch 53 is off the DIR pin is always high.
When you are using the Rs422 interface, the function of the DTR
pin is taken on by TR+ (pin 25) and TR- (pin 13).
RS-23X Pin Assignments
The RS-232C connector pin assignment and a description of the
respective interface signals are shown in Table 5.
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Serial Interface