User manual

DATA TRANSFER

Section 8

6-2

6.2 Flow Control

Flow Control refers to the method used to stop data flow whenever the terminal or the

modem needs time to retransmit blocks in error or prevent a buffer overflow. The DPX-

213 supports both RTS/CTS (Hardware or Out-of-Band) and Xon/Xoff (Software or In-

Band) flow control. Flow control is needed whenever the terminal and line speeds are

different, or whenever MNP/V42 is active.

When file transfer is taking place, data flows into the DPX-213 and is sent to line in

packets. When a line error occurs, the block is retransmitted. This takes a small amount of

time and during this time, more data is received from the terminal and must be buffered in

the modem RAM. If the line errors are substantial, or the data is continuous or faster than

the line can accommodate, the modem RAM will eventually fill and either CTS is dropped,

or Xoff is issued at 85% memory capacity to indicate to the terminal to stop sending while

the backlog of characters in the modem are cleared. When the buffers fall to 30% full, CTS

is asserted or the Xon character is sent allowing normal transfer of data to resume.

Any "error free" modem link can lose data if the terminal does not respond to Xoff quickly

and over-runs the buffer, or if the modem is interrupted or loses power at either end before

the data in RAM is transferred. The DPX-213 uses DC1 (11/91 HEX or

###

) as Xon

and DC3 (13/93 HEX or

###

) as Xoff. Flow control (AT&Kn and AT&Un) and error

control (AT&En) are separate tasks and need to be individually set within the DPX-213

according to your application needs.

6.3 Flow Control Buffers

There are eight modem buffers plus the buffers residing in your computer and terminal

which must all be correctly driven or data may be lost in some conditions.

Figure 6.1 Flow Control Buffers

Referring to the figure above, buffers 1, 3, 6 and 8 are the DTE buffers controlled by the

AT&K command. Buffers 2, 4, 5 and 7 are the modem (or line side) buffers controlled by

the AT&U command. If MNP/V42 is enabled and both terminals can accommodate the full

data rate without issuing Xoff to the modem then flow control issues are handled for you

and no data will be lost.